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19. PLC OPERATION

19.1 PLC ORGANIZATION

• Remember: The PLC is a computer. Computers have basic components, as shown below.

Keyboard

SVGA Screen

Input

Pentium

Output

Serial

CPU

Mouse

200/333 MHz

Input

Light output

32MB Memory

2 GB Disk

Storage

Storage

• In fact the computer above looks more like the one below,

output memory

inputs

input memory

computer

outputs

Keyboard

Input Chip

monitor

CPU Pentium

Screen memory

chips

Mouse

Serial Input Chip

digital output chip

LED display

Disk Controller

Memory Chips Disk

storage

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•The thing to notice is that in this computer outputs aren’t connected to the CPU directly.

•A PLC will scan a copy of all inputs from the various inputs, into memory. After this the ladder

logic program is run, and it creates a working table of all outputs in memory also. This table is written to the outputs only after the ladder logic program is done. This continues indefinitely while the PLC is on.

**The important point is that the inputs and outputs are “snap-shots” before and after the program runs.

• PLC operation can be shown with a time-line.

Self

input

logic

output

Self

input

logic

output

Self

input

logic

test

scan

solve

scan

test

scan

solve

scan

test

scan

solve

0

PLC turns on

ranges from 1 to 100 ms typically

time

-SELF TEST - Checks to see if all cards error free, reset watch-dog timer, etc. (A watchdog timer will cause an error, and shut down the PLC if not reset within a short period of time - this would indicate that the ladder logic is not being scanned normally).

-INPUT SCAN - Reads input values from the chips in the input cards, and copies their values to memory. This makes the PLC operation faster, and avoids cases where an input changes from the start to the end of the program (e.g., an emergency stop). There are special PLC functions that read the inputs directly, and avoid the input tables.

-LOGIC SOLVE/SCAN - Based on the input table in memory, the program is executed 1 step at a time, and outputs are updated. This is the focus of the later sections.

-OUTPUT SCAN - The output table is copied from memory to the output chips. These chips then drive the output devices.

19.2 PLC STATUS

•The PLC keeps track of its status, for internal use, as well as allowing use by the ladder logic.

•Some of the status flags available in PLC’s are,

-Communication errors

-PLC power on, first scan

-Data retention -- PLC restarts with same outputs stopped with

-Battery low

-Scan time too long error