Section 1: Relay Control Panels

Section 1: Relay Control Panels

Section Objectives: Before the invention of the Programmable Logic Controller (PLC), most industrial control was done using relay control panels. Switches and relays can be arranged in circuits to make logical decisions. Output from these circuits can be used to drive “loads” such as motors, heaters, or electromagnetic coils. A relay control panel is comprised of a single to thousands of these circuits. In this Section, relay control panels will be presented.

Relay Control Panel Components : Switch
2 3 1 2 3 1 Off: contacts 1 and 2 connected On: contacts 1 and 3 connected Pins 1 and 2 are “normally closed” since they are connected when the switch is off. T Pins 1 and 2 are not connected when the switch is on. Pins 1 and 3 are “normally open” since they are not connected when the switch is off. Pins 1 and 3 are connected when the switch is on. (Note: Although this is a toggle switch, this switch can symbolize any type of input source such as push button switches, sensors, power supplies, etc. in this lecture.)

Relay Control Panel Components : Coil
Coil off Coil on (Note: Although this is really an electromagnetic coil, this can symbolize any “load” such as a pump, dc motor, heating element, light, etc. for this lecture.)

Relay Control Panel Components : Relay
1 3 2 1 3 2 Off: Coil off, contacts 1 and 2 connected ON: Coil on, contacts 1 and 3 connected A relay is a combination of coil and switch. With coil off, the switch goes to its normal position off. With coil on, the switch is pulled by electromagnetic force to its on position.

Relay Logic : NOT (Inverter)
Using one switch, a logical “NOT” operation can be constructed. An example is given below: “NOT” Switch 1 = Coil V+ Switch 1 Coil 2 3 1

Relay Logic : NOT (Continued)
Switch 1 Coil 1 “NOT” Switch 1 off = Coil on V+ Switch 1 Coil 2 1 3 Switch 1 Coil 1 “NOT” Switch 1 on = Coil off V+ Switch 1 Coil 2 1 3

Relay Logic : AND Using two switches, a logical “AND” operation can be constructed. An example is given below: Switch 1 “AND” Switch 2 = Coil V+ Switch 1 Switch 2 Coil 2 3 1

Relay Logic : AND (continued)
Switch 1 off “AND” Switch 2 off = Coil off V+ Switch 1 Switch 2 Coil 2 2 3 1 1 3 Switch 1 Switch 2 Coil Switch 1 on “AND” Switch 2 off = Coil off V+ Switch 1 Switch 2 Coil 2 2 1 1 3 3 Switch 1 Switch 2 Coil 1

Relay Logic : AND (continued)
Switch 1 off “AND” Switch 2 on = Coil off V+ Switch 1 Switch 2 Coil 2 2 1 1 3 3 Switch 1 Switch 2 Coil 1 Switch 1 on “AND” Switch 2 on = Coil on V+ Switch 1 Switch 2 Coil 2 2 1 1 3 3 Switch 1 Switch 2 Coil 1

Relay Logic : OR Using two switches, a logical “OR” operation can be constructed. Parallel structure because when either is on, current will flow through output coil Example is given below: Switch 1 “OR” Switch 2 = Coil 2 3 1 V+ Switch 1 Switch 2 Coil

Relay Logic : OR Switch 1 off “OR” Switch 2 off = Coil off V+ Switch 1
3 1 V+ Switch 1 Switch 2 Coil Switch 1 Switch 2 Coil

Relay Logic : OR Switch 1 on “OR” Switch 2 off = Coil on V+ Switch 1
3 1 V+ Switch 1 Switch 2 Coil Switch 1 Switch 2 Coil 1

Relay Logic : OR Switch 1 off “OR” Switch 2 on = Coil on V+ Switch 1

Relay Logic : OR Switch 1 on “OR” Switch 2 on = Coil on V+ Switch 1

Relay Logic : XOR Using two switches and four relays, a logical “XOR” operation can be constructed. An example is given below: Switch 1 “XOR” Switch 2 = Coil V+ Coil 2 3 1 Switch 1 Switch 2

Relay Logic : XOR (continued)
Switch 1 Switch 2 Coil Switch 1 off “XOR” Switch 2 off = Coil off V+ V+ V+ Switch 1 1 1 2 1 Coil 3 3 2 3 2 Switch 2 1 1 2 3 1 3 2 3 2

Switch 1 Switch 2 Coil 1 Switch 1 on “XOR” Switch 2 off = Coil on V+ V+ V+ Switch 1 1 1 2 1 Coil 3 3 2 3 2 Switch 2 1 1 2 3 1 3 2 3 2

Switch 1 Switch 2 Coil 1 Switch 1 off “XOR” Switch 2 on = Coil on V+ V+ V+ Switch 1 1 1 2 1 Coil 3 3 2 3 2 Switch 2 1 1 2 1 3 3 2 3 2

Switch 1 Switch 2 Coil 1 Switch 1 on “XOR” Switch 2 on = Coil off V+ V+ V+ Switch 1 1 1 2 1 Coil 3 3 2 3 2 Switch 2 1 1 2 1 3 3 2 3 2

Problems with relay control panels:
Mechanical Relays and switches failed regularly (coil failure, contact wear and contamination, etc.) Difficult to diagnose problems and replace relays and switches Difficult to change hardwired logic (example: changing an “OR” circuit to “XOR”) Consumed a lot of power To address these problems, Richard E. Morley of Bedford Associates invented the first PLC as a consulting project for General Electric in Bedford Associates is currently named Modicon and is a supplier of PLCs.

Section 2: Basic PLC Components needed to replace relay control panels

Section Objectives: Basic PLC Components needed to replace relay control panels will be presented. These include: Isolated Power Supply Micro-controller (Note: Advanced features such as Timers, Interrupts, Counters, etc. will not be discussed in this lecture) Digital Input and Output pins ( DI/0) Memory For this lecture, Siemens A&D S7 314C-2 PtP PLC installed in the Mechatronics Laboratory will be used as an example. Siemens 314C-2 PtP

Basic PLC: Isolated Power Supply
Every PLC has an external or internal Isolated Power Supply. Isolated Power Supplies can have more than one isolated outputs. One isolated output is reserved for the PLC micro-controller. The rest are reserved for other components such as DI/O. Normally Power supplies are high voltage. Typically 24 Volts for industrial PLCs. The S7 314C-2 PtP PLC uses the Siemens A&D PS307 5A power supply. The PS307 5A can source 5 amps of current at 24 volts. The PS307 5A has 3 isolated outputs. For the PLC that we are using. There are 3 isolated power supply. One isolated power supply is for the PLC micro-controller. And the other two can be used for input or output. Siemens PS307 5A

Basic PLC: Micro-controller
Every PLC has at least one micro-controller The S7 314C-2 PtP PLC uses a custom micro-controller. Designed by Siemens A&D and manufactured by Infineon Technologies AD. Part Number: Infineon Siemens A&D IBC 16 SXA1020A-E S7 Controller Specifications not given in documentation

Basic PLC: Digital Inputs and Outputs (DI/Os)
DI/Os are electrically isolated from the micro-controller The number of DI/Os can be increased by adding additional DI/O modules. Example: The S7 314C-2 PtP PLC has 16 digital outputs and 24 digital inputs. Can be expanded up to 1024 DI/Os by adding additional DI/O modules. SM232 DI/O module

Memory on a PLC is separated into 3 main areas:
Basic PLC: Memory Memory on a PLC is separated into 3 main areas: LOAD Memory Can be RAM (dynamic) or EEPROM (retentive) Used to store user programs For S7 314C-2 PtP PLC : LOAD Memory located on memory card WORK Memory Memory is RAM When PLC starts, Program is copied from LOAD memory to WORK memory. The program is then executed from Work memory. For S7 314C-2 PtP PLC: 48K bytes of WORK memory

Basic PLC: Memory ( Continued)
SYSTEM Memory Memory is RAM Is used by micro-controller to implement counters, timers, interrupt stacks, etc.. Contains a bit for each D I/0 Contains “Marker Memory”. Marker memory is a free area of RAM that can be used by the programmer. (In S7 314C-2 PtP, 258 bytes are available as Marker Memory) Contains “Process Input and Output Images.” Periodically the PLC will store the states of the inputs to the Process Input Image and Process Output Image to the output. (In S7 314C-2 PtP, this is limited to the first 128 bytes of input information and 128 bytes of output information.)

Section 3: Transistion from Relay Control Panels to PLCs

Section Objectives: Initially PLCs were used to directly replace relay control panels. To directly replace relay control panels based on mechanical relays with PLCs based on a micro-controller presented challenges. These challenges and solutions will be discussed.

Transition:A Simplified Programmer’s Model
In the simplified programmer’s model of relay logic, all inputs I1, I2, .., Im go into each relay logic section. Each relay logic section then produces an output Q. I1,I2, … ,Im Relay Logic Section 1 Q1 I1,I2, … ,Im Relay Logic Section 2 Q2 . Here is the simplified model for relay logic. All inputs i1~in go into each logic section Qn I1,I2, … ,Im Relay Logic Section n

Transition: Relay control panel execution of Model
A relay control panel will execute all relay logic sections in parallel since each switch is capable of powering many coils at a time. If any input changes at time t0 then all the relay logic sections will update the outputs at time t1. I1,I2, … ,Im changes at t0 Relay Logic Section 1 Q1 changes at t1 I1,I2, … ,Im changes at t0 Relay Logic Section 2 Q2 changes at t1 . Qn changes at t1 I1,I2, … ,Im changes at t0 Relay Logic Section n

Transition: PLC execution of Model
A PLC will execute all relay logic sections in series since a micro-controller can execute only one instruction at a time. If any input changes at time t0 then relay logic section 1 will update Q1 at t1, relay logic section 2 will update Q2 at t2, …. , and relay logic section n will update Qn at tn. I1,I2, … ,Im changes at t0 Relay Logic Section 1 Q1 changes at t1 I1,I2, … ,Im changes at t0 Relay Logic Section 2 Q2 changes at t2 . Qn changes at tn I1,I2, … ,Im changes at t0 Relay Logic Section n

Transition: Differences in Relay Control Panel vs
Transition: Differences in Relay Control Panel vs. PLC execution of Model Difference 1: Relay Control Panel – The maximum time any change in input is reflected in any output is t1. PLC – The maximum time any change in input is reflected in any output is t1+t2+…+tN. Difference 2: Relay Control Panel – Since this is made from analogue components. It is possible to replace a logic section without stopping execution of other logic sections if wired correctly. PLC – This is made with a digital micro-controller. The micro-controller must be halted to replace a logic section. All other logic sections will stop operation. Difference 3: Relay Control Panel – Since parallel execution of logic sections, all outputs are a function of one set of inputs. PLC – Since serial execution of logic sections, all outputs may not be a function of one set of inputs. (example: input I2 may change as the micro-controller is processing Logic section 2. Therefore Q1 and Q2 are based on different inputs)

Transition: PLC Operation
To minimize the effects of differences between the Relay Control Panel and PLC execution of the programming model, the PLC operates in the following manner: Steps: PLC Restarts (Warm Restart - contents of RAM are maintained) Reads Inputs and updates Process Input Image Executes User Program Once Writes Process Output Image to Outputs Take care of system processes ( such as communications with other PLCs, updating user program, checking for STOP condition, etc..) Loop Back to step 2 Steps 2 through 5 is called a scan cycle. (Note: some people may refer to a PLC as a Programmable “Loop” Controller because of the scan cycle loop.) Warm Restart Update Process Input Image User Program scan cycle Update Process Output Image PLC System Processes STOP

Transition: PLC Operation
To Minimize Difference 1: Time to complete a scan cycle can be set by user. If PLC violates the scan cycle, an interrupt routine can be run or the PLC will halt execution. (For S7 314C-2 PtP, maximum scan cycle allowed is 6 sec) (All outputs of PLC must be updated at the fixed time set by the user) To Minimize Difference 2: If a part of the user program is replaced, the new part is written first to LOAD memory. During step 5, PLC System Processes, the new part is copied into WORK memory from LOAD Memory. During the next scan cycle, the new part of the user program will be executed. To Minimize Difference 3: If the programmer uses the inputs stored in the Process Input Image, the user program will have access to the same inputs per scan cycle. Also if the programmer, writes outputs to the Process Output Image, all the outputs will be updated simultaneously during step 4 (Update Process Output Image)

Section 4: Ladder Logic Programming (The biggest transition)

Section Objectives: The biggest transition from relay control panels to PLCs was the transition from the hard wired relay logic to logic defined by user program. In order to allow established relay logic users to program the PLC, a visual programming language that looks like a relay control panel was created. This visual programming language is called “Ladder Logic”. In this section, basic Ladder Logic will be presented.

Ladder Logic: System Memory Addressing
To address a bit of memory ___ ___ . ___ Memory Area Notation Byte Address Bit Number To address a byte, word, or double word ___ ___ ___ Memory Area Notation Size of Addressed Memory Notation Byte Address

Ladder Logic: System Memory Addressing (continued)
Memory Area Notations: Notation Memory Area I Process Input Image Q Process Output Image M Marker Memory PI Peripheral Input ( Actual Input Pins) PQ Peripheral Output ( Actual Output Pins) T Timer Storage Area C Counter Storage Area L Local Memory of current Data Block DB Data Block Memory (Note: Advanced features such as Timers, Counters, Data Blocks will not be discussed in this lecture)

Size of Addressed Memory Notations: Notation Size of Addressed Memory B Byte (8 bits) W Word (16 bits) D Double Word (32 bits) Byte Address: Each Memory Area is addressed in one byte increments starting at byte 0. Bit Number: MSBit is 7 and LSBit is 0

Examples: Marker Area Byte 0 Byte 1 Byte 2 Byte 3 Byte 4 Byte 5 Byte 6 Byte 7 MB0 M1.3 (Note: only bit 3 of Marker Area byte 1) MW1 MD3 MD4

Examples: Peripheral Input Area Byte 0 Byte 1 Byte 2 Byte 3 Byte 4 Byte 5 Byte 6 Byte 7 PIB1 PI2.5 (Note: only bit 5 of Peripherial Input Area byte 2) PID4

Ladder Logic : The Ladder
A ladder logic program has a “ladder” look to it. The sides of the ladder are the power rail on the left and ground rail on the right. The rungs of the ladder consists of Virtual Relay Components. (Note: Rungs are called “Networks” in Step 7) (Currents always flow from power to ground) Virtual Relay Components Power Rail Ground Rail Virtual Relay Components Virtual Relay Components

Ladder Logic : Virtual Relays
Virtual Relay Components: Normally Open Switch ( equivalent to pins 1 and 3 of Mechanical Relay. If this switch is closed for a virtual digital output relay, the digital output pin is high. If this switch is open for a virtual digital output relay, the digital output pin is low ) Mechanical Relay 1 3 2 Normally Closed Switch ( equivalent to pins 1 and 2 of Mechanical Relay) Coil ( equivalent to coil of Mechanical Relay. Used only as output to control state of output bit.

Ladder Logic : Virtual Relays (continued)
Any Marker or Function Block memory bit can be used to turn coils on/off in order to control switches in one or more virtual relays. If memory bit is 0, the coils of virtual relays associated with the bit are off. If memory bit is 1, the coils of virtual relays associated with the bit are on. Examples: I0.0 bit controls both normally open and normally close switches in two virtual relays I0.1 bit controls both normally open and normally close switches in two virtual relays Any D I/O memory bit ( Peripheral or Process Image) is a virtual relay for a digital input or output pin of the PLC. I0.0 I0.1 Q0.0 I0.1 I0.0

Example: Ladder Logic : NOT (an inverter)
Ladder Logic: Rules for converting (transforming) Relay Logic to Ladder Logic Each external signal source (e.g. Sensors, switches, push buttons etc.) must be connected to an input pin of a PLC. Each external load (e.g. Motors, solenoids, etc.) must be connected to an output pin of a PLC. Relay logic must be recreated by using virtual input and output relays associated with input and output pins. (Virtual relay components at least consist of virtual relays associated with all input and output pins) Example: Ladder Logic : NOT (an inverter) I0.0 Q0.0 Virtual Relay Components Virtual Relay associated with input pin I0.0 Virtual Relay associated with output pin Q0.0

Ladder Logic: Rules for converting Relay Logic to Ladder Logic
Construct Ladder Logic based on each possible virtual path inside PLC from power to ground depending on connections between virtual relays (using Virtual Relay Components to replace switches and coils along the possible virtual paths): Use to replace normal closed switch Use to replace normal open switch Use to replace coil 1 3 2 1 3 2 1 3 2

Converting Relay Logic to Ladder Logic :NOT (an inverter)
“NOT” Switch 1 = Coil From Relay Logic: V+ Switch 1 (off state) Coil (on state) 2 3 1

Converting Relay Logic to Ladder Logic :NOT(continued)
Separate external input and output :(make sure the circuits are connected to the terminal 3 of the switch) Recreate circuit using relay without changing functionality of original circuit Switch 1 (off state) NOT Operator (using 1 Relay) V+ V+ 2 1 1 3 2 Coil 3 External input External output

NOT Operator using 2 virtual Relays
Converting Relay Logic to Ladder Logic :NOT(continued) Recreate Relay Logic to include Virtual Input and Output Relays associated with each input and output pins(1 input + 1 output = 2 Virtual Relays) : NOT Operator using 2 virtual Relays inside PLC V+ Switch 1 2 1 V+ V+ Coil 1 3 2 1 3 External input (Note: Wired to PLC Input Pin Associated with I0.0) 3 2 External output (Note: Wired to PLC Output Pin Associated with Q0.0) Virtual Relay associated with input pin I0.0 Virtual Relay associated with output pin Q0.0

Construct Ladder Logic using Virtual Relay Components to replace switches and coils along the possible virtual path inside PLC from power to ground : V+ Coil 3 Switch 1 2 1 NOT Operator using 2 virtual Relays inside PLC I0.0 Q0.0 Ladder Logic

Ladder Logic Equivalent: Switch 1 (push buttons, sensors, etc.) is wired to PLC input pin associated with I0.0 Coil (loads, motors, lights, etc.) is wired to PLC output pin associated with Q0.0 I0.0 Q0.0

Converting Relay Logic to Ladder Logic : AND
Switch 1 “AND” Switch 2 = Coil From Relay Logic: V+ Switch 1 Switch 2 Coil 2 3 1

Converting Relay Logic to Ladder Logic : AND(continued)
Separate external switches and coil and recreate circuit using relays: AND Operator using 2 Relays V+ Switch 1 V+ 1 3 2 2 3 1 Coil V+ Switch 2 2 3 1 1 3 2

AND Operator using 3 virtual Relays
Converting Relay Logic to Ladder Logic : AND(continued) Recreate Relay Logic to include Virtual Input and Output Relays associated with each input and output pins (2 inputs + 1 output= 3 Virtual Relays) : AND Operator using 3 virtual Relays inside PLC V+ Switch 1 2 1 V+ 1 3 2 3 (Note: Wired to PLC Input Pin Associated with I0.0) V+ Coil 1 3 2 Virtual Relay associated with input pin I0.0 V+ Switch 2 2 1 (Note: Wired to PLC Output Pin Associated with Q0.0) 1 3 2 Virtual Relay associated with output pin Q0.0 3 (Note: Wired to PLC Input Pin Associated with I0.1) Virtual Relay associated with input pin I0.1

Construct Ladder Logic using Virtual Relay Components to replace switches and coils along the possible virtual path inside PLC from power to ground : Coil 3 Switch 1 2 1 V+ Switch 2 Virtual Relay associated with input pin I0.0 Virtual Relay associated with output pin Q0.0 Virtual Relay associated with input pin I0.1 I0.0 Q0.0 I0.1 Ladder Logic

Ladder Logic Equivalent: Switch 1 is wired to PLC input pin associated with I0.0 Switch 2 is wired to PLC input pin associated with I0.1 Coil is wired to PLC output pin associated with Q0.0 I0.0 I0.1 Q0.0

Converting Relay Logic to Ladder Logic : OR
Switch 1 “OR” Switch 2 = Coil From Relay Logic: 2 3 1 V+ Switch 1 Switch 2 Coil

Converting Relay Logic to Ladder Logic : OR(continued)
Separate external switches and coil and recreate circuit using relays: OR Operator using 2 Relays V+ Switch 1 2 1 V+ 1 3 2 3 Coil V+ Switch 2 2 1 V+ 1 3 2 3

OR Operator using 3 virtual Relays
Converting Relay Logic to Ladder Logic : OR(continued) Recreate Relay Logic to include Virtual Input and Output Relays associated with each input and output pins(2 inputs +1 output =3 Virtual Relays) : OR Operator using 3 virtual Relays inside PLC V+ Switch 1 2 1 V+ 1 3 2 3 (Note: Wired to PLC Input Pin Associated with I0.0) V+ Coil Virtual Relay associated with input pin I0.0 1 3 2 V+ Switch 2 2 1 V+ (Note: Wired to PLC Output Pin Associated with Q0.0) 1 3 2 Virtual Relay associated with output pin Q0.0 (Note: Wired to PLC Input Pin Associated with I0.1) Virtual Relay associated with input pin I0.1

Construct Ladder Logic using Virtual Relay Components to replace switches and coils along each possible virtual path inside PLC from power to ground : Coil 3 Switch 1 2 1 V+ Switch 2 Virtual Relay associated with input pin I0.0 Virtual Relay associated with output pin Q0.0 Virtual Relay associated with input pin I0.1 I0.0 Q0.0 I0.1 Ladder Logic

Ladder Logic Equivalent: Switch 1 is wired to PLC input pin associated with I0.0 Switch 2 is wired to PLC input pin associated with I0.1 Coil is wired to PLC output pin associated with Q0.0 I0.0 Q0.0 I0.1 Parallel structure because when either switch is on, current will flow through output coil

Converting Relay Logic to Ladder Logic : XOR
Switch 1 “XOR” Switch 2 = Coil From Relay Logic: V+ V+ V+ Switch 1 1 1 2 1 Coil 3 3 2 3 2 Switch 2 1 1 2 1 3 3 2 3 2

Converting Relay Logic to Ladder Logic : XOR(continued)
Recreate Relay Logic to include Virtual Input and Output Relays associated with each input and output pins (2 inputs with each one controlling 2 Virtual Relays+ 1 output =5 Virtual Relays) : XOR Operator using 5 virtual Relays inside PLC Virtual Relay associated with input pin I0.0 V+ Switch 1 Virtual Relay associated with input pin I0.0 2 V+ 1 3 2 V+ 1 3 2 1 3 Virtual Output Relay at Q0.0 Coil (Note: Wired to PLC Input Pin Associated with I0.0) V+ 1 3 2 V+ Switch 2 2 1 1 1 (Note: Wired to PLC Output Pin Associated with Q0.0) (Note: Wired to PLC Input Pin Associated with I0.1) 3 2 3 2 Virtual Relay associated with input pin I0.1 Virtual Relay associated with input pin I0.1

Converting Relay Logic to Ladder Logic : XOR (continued)
Construct Ladder Logic using Virtual Relay Components to replace switches and coils along each possible virtual path inside PLC from power to ground : Coil 3 Switch 1 2 1 V+ Virtual Relay Associated with output pin Q0.0 Switch 2 Virtual Relay associated with input pin I0.0 Virtual Relay associated with input pin I0.1 I0.0 Q0.0 I0.1 Ladder Logic

Converting Relay Logic to Ladder Logic : XOR (continued)
Ladder Logic Equivalent: Switch 1 is wired to PLC input pin associated with I0.0 Switch 2 is wired to PLC input pin associated with I0.1 Coil is wired to PLC output pin associated with Q0.0 I0.0 I0.1 Q0.0 I0.1 I0.0

Section 5: Extending Ladder Logic
beyond Virtual Relays

Section Objectives: A micro-controller can be used for more than relay logic with virtual relays. Ladder logic has components that take advantage of the micro-controller. These components can be categorized as follows: bit logic,comparator, converter, counter, data base calls, jumps, integer functions, floating point functions, move, program control, shift/rotate, status bits, timers, and word logic. It is impossible to cover all of the components in one lecture. This lecture will first explain formatting of constants. Then, only a few categories and examples of components will be shown.

Constants

Bit Logic Available Bit logic components: Normally Closed Switch
Normally Open Switch Output Coil Midline Output Set Coil Reset Coil Invert Power Flow Save RLO into BR Memory Bit Exclusive OR Positive Edge Detection Negative Edge Detection Address Positive Edge Detection Address Negative Edge Detection Set-Reset Flip Flop Reset-Set Flip Flop Immediate Read Immediate Write

Bit Logic example: Set Coil and Reset Coil
Description: Set Coil is executed only if power flows to the coil. When executed, the specified <address> of the element is set to "1". It will remain set even if power is removed from the coil.

Description: Reset Coil is executed only if power flows to the coil. When executed, the specified <address> of the element is reset to "0". No power flow to the coil has no effect and the state of the element's specified address remains unchanged. (Note: can be used to reset timers and counters)

Switch 1 connected to Input 0.0 Switch 2 connected to Input 0.1 Coil connected to Output 0.0 If Switch 1 is turned on then turn on Coil and keep it on even if Switch 1 is released. If Switch 2 turns on then turn off the Coil. I0.0 Q0.0 S Q0.0 I0.1 R

Comparator Available Comparator components (Note: Integer is Word, Double Integer is Double Word) Integer: Equal to Integer: Greater than Integer: Less than Integer: Greater than or Equal to Integer: Less than or Equal to Double Integer: Equal to Double Integer: Greater than Double Integer: Less than Double Integer: Greater than or Equal to Double Integer: Less than or Equal to Real: Equal to Real: Greater than Real: Less than Real: Greater than or Equal to Real: Less than or Equal to

Comparator example: Integer Compares

Comparator example: Integer Compares
Coil (any load) connected to Output 0.0 If MW0 and MW2 are equal then turn on coil. Q0.0 CMP == I MW0 MW2 IN1 IN2

Jumps Available Jump components (Note: called Logic control in Step 7 Help) Label Unconditional Jump Conditional Jump Not conditional Jump

Jump example: Conditional Jump
Description Conditional Jump: The micro-controller will goto the specified Label if power flows into the JUMP. (Note: a label can be assigned to any Network)

Jump example: Label and conditional Jump
Switch 1 connected to Input 0.0 If Switch 1 turns on then jump to label “END” I0.0 “END” JMP Components Components END Q0.0 I0.1

Integer Math Available Integer Math components:
(Note: Integer is Word, Double Integer is Double Word) Integer: Add Integer: Subtract Integer: Multiply Integer: Divide Double Integer: Add Double Integer: Subtract Double Integer: Multiply Double Integer: Divide Double Integer: Modulus

Math example: Integer Add
Description: IN1 and IN2 are added and the result is stored in OUT when power is applied to EN . Power (current) flows out of EN0 when the addition is completed unless the result of the addition results in an overflow.

Math example: Integer Add
Add 5 and integer stored at MW0. Store the result in MW2. ADD_I EN EN0 5 MW0 IN1 IN2 OUT MW2

Move Available Move components: Move

Move example: Description:
When power is applied to EN, IN is moved to the variable connected to Out and power flows out of EN0

Move example: Example: Move 5 to MW2. MOVE EN EN0 5 IN1 OUT MW2

Timer Available Timer components: Pulse S5 Timer
Extended Pulse S5 Timer On-Delay S5 Timer Retentive On-Delay S5 Timer Off-Delay S5 Timer Pulse Timer Coil Extended Pusle Timer Coil On-Delay Timer Coil Retentive On-Delay Timer Coil Off-Delay Timer Coil

Timer example: Extended Pulse S5 Timer
Description: A power transition from OFF to ON on S will restart the timer. Power flows from Q while timer is running. The timer will run for a preset time TV. (Note: 256 timers allowed in S7 314C-PtP PLC. This gives the PLC the capability to control up to 256 different coils (loads, devices, etc.))

Timer example: Example: Switch 1 connected to Input 0.0
Coil is connected to Output 0.0 Turn on coil for 10 seconds if Switch 1 is turned on. T 0 I0.0 Q0.0 S_EXt S Q S5T#10s TV BI R BCD

Word Logic Available Word Logic components: “AND” Word
“OR” Word “XOR” Word “AND” Double Word “OR” Double Word “XOR” Double Word

Word Logic example: “AND” Word
Description: When power is applied to EN, IN1 and IN2 are “ANDed”, and the result is stored in the variable connected to OUT. Power always flows out of EN0

Word Logic example: Integer Add
“AND” MW0 and MW2. Store the result in MW4. WAND W EN EN0 MW0 MW2 IN1 IN2 OUT MW4

Section 6: Ladder Logic Examples

Section Objectives: In this section two example ladder logic programs will be given.

Example 1 : Switch 1 connected to Input pin 0.0
Load connected to Output pin 0.0 If Switch 1 is on then turn on and off a load at 2 second intervals (Note: 2 second interval means a period of 4 seconds and 50% Duty cycle).

Example 1 (Continued) Time: Scan cycle just before t = 0s
User Action : None Initial state: M0.0 = 0 (off); I0.0 = 0; Q0.0 = 0 Note: I0.0 is address for Input pin 0.0; Q0.0 is address for Output pin 0.0 Content of address I0.0 can be 0 or 1 depending on state of switch Content of address Q0.0 can be 0 or 1 depending on whether current flows through virtual coil, ,associated with it T 0 I0.0 M0.0 Q0.0 S_EXt S Q S5T#2s TV BI R BCD T 1 I0.0 Q0.0 M0.0 S_EXt S Q S5T#2s TV BI R BCD

Explanations for the previous slide
Time:Scan cycle just before t = 0 User Action: None Status of 1st Rung: Content of I0.0 is 0. Current cannot flow through the virtual switch associated with it. Content of M0.0 is 0 because on second rung the virtual coil associated with M0.0 is not energized. The virtual switch associated with M0.0 is closed. However, no current flows through it. Timer 0 is not started. No current flows out of Q. Virtual coil associated with Q0.0 is not energized. The content of Q0.0 is 0. Status of 2nd Rung: Content of Q0.0 is 0. The virtual switch associated with Q0.0 is closed. However, no current flows through it. Timer 1 is not started. No current flows out of Q. Virtual coil associated with M0.0 is not energized. The content of M0.0 is 0.

Example 1 : Continued Time:Scan cycle at t = 0
User Action: User turns Switch 1 on Virtual switches and coils associated with M0.0 and Q0.0 prevent current from flowing through two networks (rungs) at same time T 0 I0.0 M0.0 Q0.0 S_EXt S Q S5T#2s TV BI (Note: Time left: 2 s) R BCD T 1 I0.0 Q0.0 M0.0 S_EXt S Q S5T#2s TV BI R BCD

Time: Scan cycle at t = 0 User Action: User turns Switch 1 on Status of 1st Rung: Content of I0.0 is 1. Current flows through the virtual switch associated with it. Content of M0.0 is 0 because on second rung the virtual coil associated with M0.0 is not energized. The virtual switch associated with M0.0 is closed. Current flows through it. Timer 0 is started. It will run for 2 seconds. Current flows out of Q when timer is running. The virtual coil associated with Q0.0 is energized. The content of Q0.0 is 1. Status of 2nd Rung: Content of Q0.0 is 1. The virtual switch associated with it is open. No current flows through it. Timer 1 is not started. No current flows out of Q. Virtual coil associated with M0.0 is not energized. Content of M0.0 is 0.

Example 1 : Continued Time: Scan cycle just before t = 2s
User Action: Keeps pressing switch 1 T 0 I0.0 M0.0 Q0.0 S_EXt S Q S5T#2s TV BI (Note: Time left: ~0) R BCD T 1 I0.0 Q0.0 M0.0 S_EXt S Q S5T#2s TV BI R BCD

Time: Scan cycle just before t = 2 s User Action: keep pressing Switch 1 Status of 1st Rung: Content of I0.0 is 1. Current flows through the virtual switch associated with it. Content of M0.0 is 0 because on second rung the virtual coil associated with M0.0 is not energized. The virtual switch associated with M0.0 is closed. Current flows through it. Timer 0 is running. Current flows out of Q. BI is close to 0. The virtual coil associated with Q0.0 is energized. The content of Q0.0 is 1. Status of 2nd Rung: Content of Q0.0 is 1. The virtual switch associated with it is open. No current flows through it. Timer 1 is not started. No current flows out of Q. Virtual coil associated with M0.0 is not energized. Content of M0.0 is 0.

Example 1 : Continued Time: Scan cycle at t = 2 s
User Action: Keeps pressing switch 1 Note: M0.0 in first network is 0 but changes state to 1 in second network T 0 I0.0 M0.0 Q0.0 S_EXt S Q (Note: Time left 0 s) S5T#2s TV BI R BCD T 1 I0.0 Q0.0 M0.0 S_EXt S Q S5T#2s TV BI (Note: Time left: 2 s) R BCD

Time: Scan cycle at t = 2 s User Action: keep pressing Switch 1 Status of 1st Rung: Content of I0.0 is 1. Current flows through the virtual switch associated with it. Content of M0.0 is 0 because on second rung the virtual coil associated with M0.0 is not energized yet. The virtual switch associated with M0.0 is closed. Current flows through it. Timer 0 is stopped. No current flows out of Q. No current flows through the virtual coil associated with Q0.0. The content of Q0.0 changes from 1 to 0. Status of 2nd Rung: Content of Q0.0 is 0. The virtual switch associated with it is closed. Current flows through it. Timer 1 is started. Current flows out of Q. Virtual coil associated with M0.0 is energized. Content of M0.0 changes from 0 to 1.

Example 1 : Continued Time: Scan cycle just after t = 2 s
User Action: Keeps pressing switch 1 M0.0 contains 1, therefore, changes normally closed switch to open Current can not flow anymore through first network T 0 I0.0 M0.0 Q0.0 S_EXt S Q S5T#2s TV BI R BCD T 1 I0.0 Q0.0 M0.0 S_EXt S Q S5T#2s TV BI (Note: Time left: 2 s – 1 scan cycle time) R BCD

Time: Scan cycle just after t = 2 s User Action: keep pressing Switch 1 Status of 1st Rung: Content of I0.0 is 1. Current flows through the virtual switch associated with it. Content of M0.0 is 1 because on second rung the virtual coil associated with M0.0 is energized. The virtual switch associated with M0.0 is open. Current cannot flow through it. Timer 0 is stopped. No current flows out of Q. No current flows through the virtual coil associated with Q0.0. The content of Q0.0 remains 0. Status of 2nd Rung: Content of Q0.0 is 0. The virtual switch associated with it is closed. Current flows through it. Timer 1 is running. Current flows out of Q. Virtual coil associated with M0.0 is energized. Content of M0.0 remains 1.

Example 1 : Continued Time: Scan cycle just before t = 4 s
User Action: Keeps pressing switch 1 T 0 I0.0 M0.0 Q0.0 S_EXt S Q S5T#2s TV BI R BCD T 1 I0.0 Q0.0 M0.0 S_EXt S Q S5T#2s TV BI (Note: Time left: ~0 s) R BCD

Time: Scan cycle just before t = 4s User Action: keep pressing Switch 1 Status of 1st Rung: Content of I0.0 is 1. Current flows through the virtual switch associated with it. Content of M0.0 is 1. The virtual switch associated with M0.0 is open. No current can flow through it. Timer 0 is stopped. No current flows out of Q. The virtual coil associated with Q0.0 is not energized. The content of Q0.0 is 0. Status of 2nd Rung: Content of Q0.0 is 0. The virtual switch associated with it is closed. Current flows through it. Timer 1 is running. Current flows out of Q. BI is close to 0. Virtual coil associated with M0.0 is energized. Content of M0.0 is 1.

Example 1 : Continued Time: Scan cycle at t = 4 s
User Action: Keeps pressing switch 1 T 0 I0.0 M0.0 Q0.0 S_EXt S Q S5T#2s TV BI R BCD T 1 I0.0 Q0.0 M0.0 S_EXt S Q S5T#2s TV BI (Note: Time left: 0 s) R BCD

Time: Scan cycle at t = 4s User Action: keep pressing Switch 1 Status of 1st Rung: Content of I0.0 is 1. Current flows through the virtual switch associated with it. Content of M0.0 is 1. The virtual switch associated with M0.0 is open. No current can flow through it. Timer 0 is not started yet. No current flows out of Q. The virtual coil associated with Q0.0 is not energized. The content of Q0.0 is 0. Status of 2nd Rung: Content of Q0.0 is 0. The virtual switch associated with it is closed. Current flows through it. Timer 1 is stopped. No current flows out of Q. BI is equal to 0. Virtual coil associated with M0.0 is not energized any more. Content of M0.0 changes from 1 to 0.

(Note: A once scan cycle error has been introduced in the timing
(Note: A once scan cycle error has been introduced in the timing. The reason is that the coil of M0.0 on the second rung was turned off during the scan cycle at t = 4s. The normally closed switch of M0.0 is not evaluated again until the scan cycle after the scan cycle at t = 4 s. Therefore, Timer T0 starts one scan cycle after t = 4. This error will propagate and similar errors will accumulate. ) Example 1 : Continued Time: Scan cycle just after t = 4 s User Action: Keeps pressing switch 1 T 0 I0.0 M0.0 Q0.0 S_EXt S Q S5T#2s TV BI (Note: Time left: 2 s) R BCD T 1 I0.0 Q0.0 M0.0 S_EXt S Q S5T#2s TV BI R BCD

Time: Scan cycle just after t = 4s User Action: keep pressing Switch 1 Status of 1st Rung: Content of I0.0 is 1. Current flows through the virtual switch associated with it. Content of M0.0 is 0. The virtual switch associated with M0.0 is closed. Current flows through it. Timer 0 is started. Current flows out of Q. The virtual coil associated with Q0.0 is energized. The content of Q0.0 changes from 0 to 1. Status of 2nd Rung: Content of Q0.0 is 1. The virtual switch associated with it is open. Current cannot flow through it. Timer 1 is stopped. No current flows out of Q. Virtual coil associated with M0.0 is not energized. Content of M0.0 remains 0.

Example 1 : Continued Time: Some time later
User Action: User turns Switch 1 off T 0 I0.0 M0.0 Q0.0 S_EXt S Q S5T#2s TV BI R BCD T 1 I0.0 Q0.0 M0.0 S_EXt S Q S5T#2s TV BI R BCD

Time: Some time later User Action: turn off Switch 1 Status of 1st Rung: Content of I0.0 is 0. Current cannot flow through the virtual switch associated with it. Timer 0 is stopped. No current flows out of Q. The virtual coil associated with Q0.0 is not energized. The content of Q0.0 changes is 0. Status of 2nd Rung: Timer 1 is stopped. No current flows out of Q. Virtual coil associated with M0.0 is not energized. Content of M0.0 is 0.

Example 1 : Comments: As this example illustrates, consistent timing is difficult to achieve with a PLC due to the scan cycle. This is the reason why PLC’s are not used to control systems with very fast time constants such as CNC machines, chemical mixers, etc….

Example 2 : Switch 1 connected to Input pin 0.0
A Hall effect sensor (switch) is connected to Input pin 0.1 (Note: a Hall effect sensor will turn on when a magnetic object comes in close proximity) The motor for a conveyer belt is connected to Output pin 0.0 (Note: As previously mentioned, a coil can be any “load” such as a motor during these lectures.) If Switch 1 is turned on, the conveyer belt will transport 1000 magnetic rods to Georgia Tech Students. Switch 1 must be turned off then on to send another 1000 magnetic rods. The hall effect switch is positioned right under the conveyer belt and can be used to count the rods as they pass by. Once the hall effect sensor has counted 1000 rods, it turns the conveyor motor off.

Example 2 (Continued) Time: Scan cycle just before t = 0s
Actions : no rod near hall effect sensor (switch) Review of normally closed and open virtual switches is present in next slide Switch 1 Conveyor Belt Conveyor Belt I0.0 M0.0 Q0.0 Move Q0.0 EN EN0 S IN1 OUT MW1 M0.0 S Switch 1 I0.0 M0.0 R Conveyor Belt CMP == I Q0.0 R 1001 IN1 Hall Effect Switch MW1 IN2 I0.1 M0.1 ADD_I IN1 IN2 EN EN0 OUT M0.1 S 1 Hall Effect Switch MW1 MW1 I0.1 M0.1 R

Review of normally open and closed virtual switches
When switch 1 is not turned on, or equivalently, I0.0 contains 0: Switch 1 Conveyor Belt Conveyor Belt I0.0 M0.0 Q0.0 Move Q0.0 EN EN0 S IN1 OUT MW1 M0.0 S Switch 1 I0.0 M0.0 R Conveyor Belt Conveyor Belt V+ M0.0 Q0.0 Move Q0.0 1 3 2 EN EN0 S Switch 1 IN1 OUT MW1 M0.0 S M0.0 R

Time: scan cycle before t = 0s
Explanations for the previous slide Time: scan cycle before t = 0s Actions : No rod near hall effect switch Status of 2nd and 5th Rungs: M0.1 reset coil and M0.0 reset coil are turned on by the normally closed switch I0.0 and I0.1 respectively. Therefore, the corresponding normally closed switch M0.1 and M0.0 keep their initial state.

Example 2 (Continued) Time: Scan cycle at t = 0s
Actions : Switch 1 is turned on, no rod near hall effect switch Switch 1 Conveyor Belt Conveyor Belt I0.0 M0.0 Q0.0 Move Q0.0 EN EN0 S IN1 OUT MW1 M0.0 S Switch 1 I0.0 M0.0 R Conveyor Belt CMP == I Q0.0 R 1001 IN1 Hall Effect Switch MW1 IN2 I0.1 M0.1 ADD_I IN1 IN2 EN EN0 OUT M0.1 S 1 Hall Effect Switch MW1 MW1 I0.1 M0.1 R

Actions : No rod near hall effect switch Status of 1st Rung:
Explanations for the previous slide Scan cycle at t = 0s Actions : No rod near hall effect switch Status of 1st Rung: When switch 1 is turned on, the current flow from power to ground in the first rung, as shown in red. MW1 is initialized as 0 by Move. Set coils Q0.0 and M0.0 are turned on (shown in red). The conveyor belt starts to move because the set coil Q0.0 is turned on. Status of 2nd Rung: Normally closed switches I0.0, is turned on to prevent the M0.0 reset coil to be turned on.

Example 2 (Continued) Time: Scan cycle just after t = 0s
Actions : no rod near hall effect switch Switch 1 Conveyor Belt Conveyor Belt I0.0 M0.0 Q0.0 Move Q0.0 EN EN0 S IN1 OUT MW1 M0.0 S Switch 1 I0.0 M0.0 R Conveyor Belt CMP == I Q0.0 R 1001 IN1 Hall Effect Switch MW1 IN2 I0.1 M0.1 ADD_I IN1 IN2 EN EN0 OUT M0.1 S 1 Hall Effect Switch MW1 MW1 I0.1 M0.1 R

Time: scan cycle just after t = 0s
Explanations for the previous slide Time: scan cycle just after t = 0s Actions : No rod near hall effect switch Status of 1st Rung: Since set coils Q0.0 and M0.0 are turned on, the corresponding normally closed switches Q0.0 and M0.0 change their states to open (from red to black). The conveyor belt continues to move because of the set coil Q0.0 to be turned on.

Example 2 (Continued) Time: t = t1
Actions : Rod approaches hall effect switch, 1 is added to MW1 Switch 1 Conveyor Belt Conveyor Belt I0.0 M0.0 Q0.0 Move Q0.0 EN EN0 S IN1 OUT MW1 M0.0 S Switch 1 I0.0 M0.0 R Conveyor Belt CMP == I Q0.0 R 1001 IN1 Hall Effect Switch MW1 IN2 I0.1 M0.1 ADD_I M0.1 EN EN0 S 1 IN1 Hall Effect Switch MW1 IN2 OUT MW1 I0.1 M0.1 R

Actions : Rod approaches hall effect switch Status of 4th Rung:
Explanations for the previous slide Time: : t = t1 Actions : Rod approaches hall effect switch Status of 4th Rung: Normally open switch I0.1 is turned on because od approaches hall effect switch. Now current can flow on the 4th rung, and 1 is added to MW1 by ADD_I. Set coil M0.1 is turned on, as shown in red.

Example 2 (Continued) Time: t = t1 + 1 scan cycle
Actions : Rod passes over hall effect switch Switch 1 Conveyor Belt Conveyor Belt I0.0 M0.0 Q0.0 Move Q0.0 EN EN0 S IN1 OUT MW1 M0.0 S Switch 1 I0.0 M0.0 R Conveyor Belt CMP == I Q0.0 R 1001 IN1 Hall Effect Switch MW1 IN2 I0.1 M0.1 ADD_I IN1 IN2 EN EN0 OUT M0.1 S 1 Hall Effect Switch MW1 MW1 I0.1 M0.1 R

Actions : Rod passes over hall effect switch. Status of 5th Rung:
Explanations for the previous slide Time: : t = t1 + 1 scan cycle Actions : Rod passes over hall effect switch. Status of 5th Rung: Since set coil M0.1 is turned on, the corresponding normally closed switches M0.1 is turned on (as shown from red to black). Now the current cannot flow through this rung.

Actions : no rod near hall effect switch Switch 1 Conveyor Belt Conveyor Belt I0.0 M0.0 Q0.0 Move Q0.0 EN EN0 S IN1 OUT MW1 M0.0 S Switch 1 I0.0 M0.0 R Conveyor Belt CMP == I Q0.0 R 1001 IN1 Hall Effect Switch MW1 IN2 I0.1 M0.1 ADD_I IN1 IN2 EN EN0 OUT M0.1 S 1 Hall Effect Switch MW1 MW1 I0.1 M0.1 R

Actions : Rod passes far away from hall effect switch
Explanations for the previous slide Time: : t = t1 + 2 scan cycle Actions : Rod passes far away from hall effect switch Status of 4th Rung: Rod passes far away from hall effect switch, content in address I0.1 changes from 1 to 0. The corresponding normally open switches I0.1 is turned off (from red to black). Status of 5th Rung: The corresponding normally closed switches I0.1 is turned off (from black to red). Now current can flow on the last rung, and reset coil M0.1 is turned on.

Actions : no rod near hall effect switch Switch 1 Conveyor Belt Conveyor Belt I0.0 M0.0 Q0.0 Move Q0.0 EN EN0 S IN1 OUT MW1 M0.0 S Switch 1 I0.0 M0.0 R Conveyor Belt CMP == I Q0.0 Reset M0.1, M0.1 R 1001 IN1 Hall Effect Switch MW1 IN2 M0.1 ADD_I IN1 IN2 EN EN0 OUT M0.1 I0.1 S 1 Hall Effect Switch MW1 MW1 I0.1 M0.1 R

Actions : No rod near hall effect switch
Explanations for the previous slide Time: : t = t1 + 3 scan cycle Actions : No rod near hall effect switch Status of 5th Rung: Since reset coil M0.1 is turned on, the corresponding normally closed switches M0.1 in the 4th rung resets to its initial state. Now the network is waiting for the next rod.

Example 2 (Continued) Time: t = t2
Actions : the 1001st rod approaches hall effect switch (so 1000 have been delivered) Switch 1 Conveyor Belt Conveyor Belt I0.0 M0.0 Q0.0 Move Q0.0 EN EN0 S IN1 OUT MW1 M0.0 S Switch 1 I0.0 M0.0 R Conveyor Belt CMP == I Q0.0 R 1001 IN1 Hall Effect Switch MW1 IN2 I0.1 M0.1 ADD_I M0.1 EN EN0 S 1 IN1 Hall Effect Switch MW1 IN2 OUT MW1 I0.1 M0.1 R

Actions : the 1001st rod approaches hall effect switch
Explanations for the previous slide Time: t = t2+ 1 scan cycle Actions : the 1001st rod approaches hall effect switch Status of 4th Rung: When the 1001st rod approaches hall effect switch, MW1 is added by 1. Now MW1 equals to So far, 1000 shafts have been delivered.

Example 2 (Continued) Time: t = t2+ 1 scan cycle
Actions : the conveyer is stopped with 1001th rod over the Hall effect switch Switch 1 Conveyor Belt Conveyor Belt I0.0 M0.0 Q0.0 Move Q0.0 EN EN0 S IN1 OUT MW1 M0.0 S Switch 1 I0.0 M0.0 R Conveyor Belt CMP == I Q0.0 R 1001 IN1 Hall Effect Switch MW1 IN2 I0.1 M0.1 ADD_I M0.1 EN EN0 S 1 IN1 Hall Effect Switch MW1 IN2 OUT MW1 I0.1 M0.1 R

Time: t = t2+ 1 scan cycle Actions : The conveyer is stopped with 1001th rod over the Hall effect switch Status of 3rd Rung: Since MW1 equals to 1001, now power can flow out from the Comparator (Equal to), which turns on the reset coil Q0.0, (shown in red). Because reset coil Q0.0 is turned on, conveyer is stopped.

Example 2 (Continued) Time: t = t2+ 1 scan cycle
Actions : the conveyer is stopped. Switch 1 must be turned off and on to deliver 1000 more Switch 1 Conveyor Belt Conveyor Belt I0.0 M0.0 Q0.0 Move Q0.0 EN EN0 S IN1 OUT MW1 M0.0 S Switch 1 I0.0 M0.0 R Conveyor Belt CMP == I Q0.0 R 1001 IN1 Hall Effect Switch MW1 IN2 I0.1 M0.1 ADD_I M0.1 EN EN0 S 1 IN1 Hall Effect Switch MW1 IN2 OUT MW1 I0.1 M0.1 R

Status of 1st Rung: Explanations for the previous slide
Time: t = t2+ 1 scan cycle Actions : Conveyer is stopped. Switch 1 must be turned off and on to deliver 1000 more Status of 1st Rung: Because reset coil Q0.0 is turned on, set coil Q0.0 on 1st Rung is turned off, as shown in black. Since set coil M0.0 is still on, the corresponding normally closed M0.0 is also on, which prevents current flowing into the 1st rung. Thus, set coil Q0.0 cannot be turned on, and conveyer cannot move. How to deliver another 1000 rods If we turn off switch 1, the current can flow into 2nd rung, then reset coil M0.0 is turned on, which leads the corresponding normally closed M0.0 is turned off. Everything returns to their initial states. Now if we turn on switch 1, a new cycle begins, and another 1000 rods will be delivered.

Example 2 : Comments: This and the previous example illustrates that the serial nature of the PLC micro-controller can still affect program execution. Also, this program can be simplified using an positive edge detection coil. This was not done because the positive edge detection coil was not an example in Section 5.

So far we have looked at topics applicable to all PLC’s
So far we have looked at topics applicable to all PLC’s. Further Study Should focus on: Topics applicable to some but not all PLC’s: A/D Function Blocks Interrupts Counters Communication Protocol: Profibus How to use communications to communicate with other PLC’s, smart actuators and sensors, etc…

Section 1: Relay Control Panels

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Section 1: Relay Control Panels

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