Documenting a PLC Program with Software

Most PLC systems have programming software that's available from the company that made the PLC or from second-party software writers. This software will reside on a personal computer or portable computer and it will provide both on-line and off-line programming capabilities. This software will also provide a means of documenting the program by giving names (tags) to switches and outputs as well as their address numbers. The tags can be displayed while the program is being used to troubleshoot the machine. This means the troubleshooting technician will have the name of each switch displayed so that it's easy to identify.

Another feature provided by the software is the ability to add comments to each rung of the program. The comments allow the programmer to add a description of what the line of logic is doing and it can also explain the operation of the rung while various inputs and outputs in that rung are energized. The size of the comment can be as large as memory permits. It is important to understand that the tags and comments don't reside in the PLC; rather they are stored in the portable computer on the hard drive or on a floppy disk. ill. 39 (below) shows an example of a program that has both tags and comments.

ill.39 : Example PLC program with tags and comments that are useful for understanding the ladder logic and for troubleshooting.

Some programming comments from above screen capture:

THIS RUNG IS USED TO START THE PUMP MOTOR. DEPRESS THE START PUSH BUTTION TO START THE PUMP MOTOR.

THIS RUNG CONTROLS THE HIGH-PRESSURE ALARM. IF THE PRESSURE AT THE PUMP MANIFOLD EXCEEDS 2000 PSI, THE SYSTEM IS IN A HIGH-PRESSURE CONDITION AND PRESSURE SWITCH P51 (003) WILL CLOSE AND SET THE ALARM.

THIS RUNG CONTROLS THE EXTEND CYLINDER SOLENOID. WHEN THE EXTEND SOLENOID PUSH BUTTON 006 IS DEPRESSED THE EXTEND SOLENOID IS ENERGIZED.