Programmable Logic Controllers: I/O Juan David Bastidas Rodríguez Escuela de Ingenierías Eléctrica, Electrónica y de Telecomunicaciones Universidad Industrial de Santander

Typical digital inputs

Typical analog inputs

Typical analog inputs

Typical analog inputs Consider a thermocouple used as a sensor with a PLC and giving an output of 0.5 mV per °C. What will be the accuracy with which the PLC will activate the output device if the thermocouple is connected to an analog input with a range of 0 to 10 V DC and using a 10-bit analog-to-digital converter?

Typical analog inputs Consider a thermocouple used as a sensor with a PLC and giving an output of 0.5 mV per °C. What will be the accuracy with which the PLC will activate the output device if the thermocouple is connected to an analog input with a range of 0 to 10 V DC and using a 10-bit analog-to-digital converter? 10-bit converter, there are 210=1024 bits 0 to 10 V range => 10/1023 V approx. 0.01 V (10 mV). PLC recognizes the input is ±5 mV or ± 10°C. PLC only takes “snapshot” samples of input signals.

Typical digital outputs

Typical digital outputs

Typical digital outputs

Typical analog outputs

Typical analog outputs Options: 4 to 20 mA, 0 to +5 V DC 0 to +10 V DC

Signal Conditioning External signal conditioning => a standard form of analog input channel. Convert current to voltage Change voltage level Comparison

Signal Conditioning Convert current to voltage

Signal Conditioning Change voltage level

Signal Conditioning Change voltage level

Signal Conditioning Comparison

Processing inputs Scan all the inputs and copy into RAM. Fetch, decode, and execute all program instructions in sequence, copying output instructions to RAM. Update all outputs. Repeat the sequence. Time to execute 1 cycle: cycle time (e.g. 10 to 50 ms)

Processing inputs Example: Consider a PLC with a cycle time of 40 ms. What is the maximum frequency of digital impulses that can be detected?

Processing inputs Example: Consider a PLC with a cycle time of 40 ms. What is the maximum frequency of digital impulses that can be detected? The maximum frequency will be if one pulse occurs every 40 ms, that is, a frequency of 1/0.04 = 25 Hz.

Processing inputs The cycle or scanning time for a PLC, i.e. its response speed, is determined by: The CPU used. The size of the program to be scanned. The number of inputs/outputs to be read. The system functions that are in use; the greater the number, the slower the scanning time.

I/O addresses With a small PLC this is likely to be just a number, prefixed by a letter to indicate whether it is an input or an output: I0.2, O0.1, X400, Y430, etc. With larger PLCs that have several racks of input and output channels, the racks are numbered. With the Allen-Bradley PLC-5: processor rack 0 address 0, the other racks: 1 address 1, 2 address, and so on. Each rack can have a number of modules, and each one deals with a number of inputs and/or outputs.

I/O addresses Allen-Bradley: Address I:012/03: an input, rack 01, module 2, and terminal 03. SIEMENS: Address I0.1 is an input at bit 1 in byte 0, and Q2.0 is an output at bit 0 in byte 2. GEM: A3.02 is an input in module 3 at terminal 02, and B5.12 is an output in module 5 at terminal 12.

Remote connections

Remote connections Serial communication is used for transmitting data over long distances. It is much cheaper to run the single core cable needed Internally however, PLCs work with parallel communications (speed) Universal asynchronous receivers/ transmitters (UARTs) convert serial to parallel Parallel communication might be used when connecting laboratory instruments to the system

Remote connections Reed the basics of serial and parallel standards in: Section 4.3.2 to 4.3.5 Programmable Logic Controllers, Bolton 6th Edition.

Networks

Networks Distributed systems SCADA: Supervisory Control and Data Acquisition System

Networks Network standards: International Standards Organization (ISO) 1979 => Open systems interconnection (OSI) ISO OSI model Layer 1: Physical médium Layer 2: Data link Layer 3: Network Layer 4: Transport Layer 5: Session Layer 6: Presentation Layer 7: Application A communication link between items of digital equipment is defined in terms of physical, electrical, protocol, and user standards => ISO OSI model

Networks ISO OSI model: Layer 1: Physical médium: include synchronizing data transfer and transferring bits of data Layer 2: Data link: include assembling bits from the physical layer into blocks and transferring them, controlling the sequence of data blocks, and detecting and correcting errors Layer 3: Network: defines the switching that routes data between systems in the network. Layer 4: Transport: defines the protocols responsible for sending messages from one end of the network to the other Layer 5: Session: provides the function to set up communications between users at separate locations. Layer 6: Presentation: ensures that information is delivered in an understandable form. Layer 7: Application: linkis the user program into the communication process and is concerned with the meaning of the transmitted information

Networks

Networks IEC 61131 standard for programmable logic controllers defines a number of standard communication blocks: CONNECT: to establish a channel between a calling PLC and a remote PLC (network address). STATUS and USTATUS: STATUS request for remote status information from a remote PLC USTATUS enables a PLC to receive status information from a remote PLC READ, USEND and URCV: READ requests a remote PLC for values of one of more variables URCV enables a remote PLC to receive variables USEND transmits data to a particular instance of a URCV

Networks IEC 61131 standard for programmable logic controllers defines a number of standard communication blocks: WRITE, SEND and RCV: WRITE writes one or more values variables within a remote PLC SEND block is for a request to a remote PLC to send data RCV block is the answer of to SEND NOTIFY and ALARM: NOTIFY message reports an alarm to to a remote PLC ALARM is the acknowledgment message of a remote PLC to NOTIFY message

Networks Examples of commercial systems: Manufacturing Automation Protocol (MAP): one of the first protocols, not widely used now. Ethernet ControlNet DeviceNet Allen-Bradley Data Highway PROFIBUS Factory-Floor Network