PLC ARCHITECTURE – Memory 2 by Dr. Amin Danial Asham

References  Programmable Controllers- Theory and Implementation, 2nd Edition, L.A. Bryan and E.A. Bryan

 Application Memory.  The application memory stores programmed instructions and any data the processor will use to perform its control functions.  The maximum amount of application memory varies depending on the size of PLC.  The controller stores all data in the data table section of the application memory, while it stores programmed instructions in the user program section. Application memory map

 Application Memory (continue)  Data Table:  This section stores: i.The input table ii.The output table iii.The storage area

 Application Memory (continue)  Data Table (continue) i.The input table  This area of memory stores the status of the digital inputs of a PLC, such that each bit represents a digital input.  The maximum number of input table bits is equal to the maximum number of field inputs that can be connected to the PLC.  Each digital input is represented by a bit; that is, if the maximum number of digital inputs is 16 field inputs connected to a PLC then the input table must have a maximum of 16 bits (two bytes).  The address of an input to a PLC is consisting of the analogous bit position and the word address that contains this bit in the input table.

 Application Memory (continue)  Data Table (continue) i.The input table (continue)  Example: An input address means the bit number 07 8 in the word location in the input table area.  This bit is 1 when the input is ON and 0 when the input is OFF.  These input table changes take place during the input reading part of the scan cycle.

 Application Memory (continue)  Data Table (continue) ii.The Output table  This area stores the status of the digital outputs of a PLC, such that each bit represents a digital input.  The maximum number of output table bits is equal to the maximum number of field outputs that can be connected to the PLC.  Each digital output is represented by a bit; that is, if the maximum number of digital outputs is 16 field outputs connected to a PLC then the output table must have a maximum of 16 bits (two bytes).  The address of an output to a PLC is consisting of the analogous bit position and the word address that contains this bit in the output table.

 Application Memory (continue)  Data Table (continue) ii.The Output table (continue)  Example: An output address means the bit position 05 8 in the word location  If 0 is written by the program in this bit the corresponding output will be turned OFF.  If 1 is written by the program in this bit the corresponding output will be ON.  The turning ON and OFF of field devices via the output module occurs during the update of outputs after the end of the scan cycle.

 Application Memory (continue)  Data Table (continue) iii.Storage Area  The purpose of the storage area section of the data table is to store changeable data, whether it is one bit or a word.  This area has two sections; Internal Bit Storage and Register/Word storage. o Internal Bit Storage This section area contains storage bits that are referred to as either internal outputs, internal coils, internal (control) relays, or internals. These internals provide virtual outputs, for interlocking purposes, of ladder sequences in the control program. In other words these internal outputs are used as variables used throughout the program. Internal outputs are not reflected on the real physical devices connected to a PLC. o Register/Word storage This area is used to store groups of bits (bytes and words). This information is stored in binary format and represents quantities or code such as BCD code, ASCII, decimal values in binary form,..etc.

Storage area section of the data table  Application Memory (continue)  Data Table (continue) iii.Storage Area (continue)  Values placed in the register/word storage area represent input data from a variety of devices, such as thumbwheel switches, analog inputs, and other types of variables.  These registers can contain output values for interface modules connected to field devices, such as seven-segment LED indicators (BCD), and drive speed controllers.

 Application Memory (continue)  Data Table (continue) iii.Storage Area (continue)  Storage registers are also used to hold fixed constants, such as preset timer/counter values, and changing values, such as arithmetic results and accumulated timer/counter values  Depending on their use, the registers in the register/word storage area may also be referred to as input registers, output registers, or holding registers

 Example2: what happens to internal 2301 (word 23, bit 01) when the limit switch connected to input terminal 10 closes?  Solution:  When LS closes contact 10 will close,  Turning internal output 2301 ON (a 1 in bit 01 of word 23).  This will close contact 2301 (-||-)  and turn real output 20 ON  causing the light PL to turn ON at the end of the scan.

Example 3: For the shown memory map in illustrate how to represent the following numbers in the storage area: (Word 16bit) (a) the BCD number 9876, (b) the ASCII character A (octal 101) in one byte (use lower byte), and (c) the analog value 2257 ( binary). Represent these values starting at register 400.

Solution of Example 3

 Application Memory (continue)  User Control Program All of the PLC instructions that constitute a control program for a machine or a process are stored in this area. The processor’s executive software language, which represents each of the PLC instructions, stores its instructions in the user program memory. When a PLC executes its program, the processor interprets the information in the user program memory and controls the referenced bits in the data table that correspond to real or internal I/O. The processor’s execution of the executive program accomplishes this interpretation of the user program.

 An example of the organization of the Application memory map  For a PLC with the following configuration: total application memory of 4K words with 16 bits capability of connecting 256 I/O devices (128 inputs and 128 outputs) 128 available internal outputs capability of up to 256 storage registers, selectable in groups of 8- word locations, with 8 being the minimum number of registers possible (32 groups of 8 registers each) octal (base 8) numbering system with 2-byte (16-bit) word length

 An example of the organization of the Application memory map (continue) Input table words =128/16=8 words Addr. from to Output table words =128/16=8 words Addr. from to Internal Output storage =128/16=8 words Addr. from to Maximum register storage =256 words in 32 groups of 8 words

 Example 4: Show that how the following simple circuit is implemented using PLC, where the used input address is and output address The internal output bit is derived by the virtual contact  Solution:

Solution Example 4 (continue): o When the LS is closed bit turns into 1. o When bit becomes one the virtual contact closes. o When contact closes the output bit becomes 1 and hence the virtual coil is energized hence corresponding output the light PL is energized. The virtual contact closes. o When the virtual contact closes the internal output bit turns ON.

Steps in the evaluation of a PLC circuit

Thanks