eNotes: Mechatronics and Controls

24.1 ADDRESSING

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To use advanced data functions in a PLC, we must first understand the structure of the data in the PLC memory.

There are two types of memory used in a PLC-5.

Program Files - these are a collection of 1000 slots to store up to 1000 programs. The main program will be stored in program file 2. SFC programs must be in file 1, and file 0 is used for program and password information. All other program files from 3 to 999 can be used for `subroutines'.

Data Files - This is where the variable data is stored that the PLC programs operate on. This is quite complicated, so a detailed explanation follows.

24.1.1 Data Files

In brief PLC memory works like the memories in a pocket calculator. The values below are for a PLC-5, although most Allen-Bradley PLCs have a similar structure.

These memory locations are typically word oriented (16 bits, or 2 bytes). This includes the bit memory. But the T4, C5, R6 data files are all three words long.

All values are stored and used as integers (except when specified, eg. floating point). When integers are stored in binary format 2's complements are used to allow negative numbers. BCD values are also used.

There are a number of ways the PLC memory can be addressed,

bit - individual bits in memory can be accessed - this is like addressing a single output as a data bit

word/integer - 16 bits can be manipulated as a group

data value - an actual data value can be provided

file level - an array of data values can be manipulated and operated on as a group

indirect - another memory location can be used in the description of a location.

expression - a text string that describes a complex operation

For the user assigned data files from 9 to 999 different data types can be assigned. These can be one of the data types already discussed, or another data type.

A - ASCII

B - bit

BT - block transfer

C - counter

D - BCD

F - floating point

MG - message

N - integer (signed, unsigned, 2s compliment, BCD)

PD - PID controller

R - control

SC - SFC status

ST - ASCII string

T - timer

24.1.1.1 - Inputs and Outputs

Recall that the inputs and outputs use octal for specific bits. This means that the sequence of output bits is 00, 01, 02, 03, 04, 05, 06, 07, 10, 11, 12, 13, 14, 15, 16, 17

24.1.1.2 - User Bit Memory

Bit data file B3 is well suited to use of single bits. the data is stored as words and this allows two different ways to access the same bit.

B3:0/0 = B3/0

B3:0/10 = B3/10

B3:1/0 = B3/16

B3:1/5 = B3/21

B3:2/0 = B3/32

etc...

The integer file N7 stores words in 2's complement form. This allows values from -32768 to 32767. These values can be addressed as whole words, and individual bits can also be changed.

The floating point file F8 will store floating point numbers that can only be used by floating point functions. The structure of these numbers does not allow bit access.

24.1.1.3 - Timer Counter Memory

Timer T4 values are addressed using the number of the timers, and an associated data type. For example the accumulator value of timer 3 is T4:3.ACC or T4:3/ACC.

EN - timer enabled bit (bit 15)

TT - timer timing bit (bit 14)

DN - timer done bit (bit 13)

PRE - preset word

ACC - accumulated time word

Counter C5 values are addressed using the number of the counters, and an associated data type. For example the accumulator value of counter 3 is C5:3.ACC or C5:3/ACC.

CU - count up bit (bit 15)

CD - count down bit (bit 14)

DN - counter done bit (bit 13)

OV - overflow bit (bit 12)

UN - underflow bit (bit 11)

PRE - preset word

ACC - accumulated count word

The values for the timers and counters can be accesses directly.

Consider the example below,

24.1.1.4 - PLC Status Bits (for PLC-5s and Micrologix)

Some of the more commonly useful status bits in data file S2 are given below. Full listings are given in the manuals.

S2:0/0 carry in math operation

S2:0/1 overflow in math operation

S2:0/2 zero in math operation

S2:0/3 sign in math operation

S2:1/15 first scan of program file

S2:8 the scan time (ms)

S2:18 year

S2:19 month

S2:20 day

S2:21 hour

S2:22 minute

S2:23 second

S2:28 watchdog setpoint

S2:29 fault routine file umber

S2:30 STI (selectable timed interrupt) setpoint

S2:31 STI file number

S2:46-S2:54,S2:55-S2:56 PII (Programmable Input Interrupt) settings

S2:55 STI last scan time (ms)

S2:77 communication scan time (ms)

24.1.1.5 - User Function Control Memory

Control file R6 is used by various functions to track progress. Values that are available are, listed below. The use of these bits is specific to the function using the control location.

EN - enable bit (bit 15)

EU - enable unload (bit 14)

DN - done bit (bit 13)

EM - empty bit (bit 12)

ER - error bit (bit 11)

UL - unload bit (bit 10)

IN - inhibit bit (bit 9)

FD - found bit (bit 8)

LEN - length word

POS - position word

Different bits will use these memory locations differently. It will be rare for any instruction to use all of these memory bits and words.

24.1.1.6 - Integer Memory

This memory can hold values from -32768 to +32767 - These values cannot be exceeded.

Decimal fractions are not allowed.

The values are stored as 2's compliment.

These values are normally stored in N7:xx by default, but new blocks of integer memory are often created in other locations.

24.1.1.7 - Floating Point Memory

This can hold real values with fraction in the range of +/-1.1754944e-38 to +/-3.4028237e38.

These values are always limited to 7 digits of accuracy.

Floating point memoy is stored in F8:xx by default.