• Remember: The PLC is a computer. Computers have basic components, as shown below.
• In fact the computer above looks more like the one below,
• 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 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.