Programming PLCs: Keep the documentation clear and simple

Poor programmable logic controller documentation and housekeeping can lead to unnecessary troubleshooting and downtime. Keep it simple in order to avoid the possible risks and confusion.


Different rules and programming methods are needed based on the hardware and software being used. Programming in a Siemens programmable logic controller (PLC) is not identical to programming in an Allen-Bradley PLC. There are general good practices that should be followed in any programming. The obvious is to have the program function and control as desired. Maintaining good documentation and housekeeping are good general practices in programming. Code should be easily read and understood by programming colleagues and customers. Poor documentation and housekeeping can add unnecessary time to troubleshooting, downtime, and programming. Programming structure and how complicated it may or may not be is another variable that will affect how easily the code is deciphered. I suggest following the KISS principle: “Keep it simple, stupid.” Avoid unnecessary complexity and keep it simple and straightforward.

Easy living through documentation

Good documentation will make your programming life easier. Adding rung comments and descriptions to tags greatly helps understand the code and the process that the code is controlling. Typically, if a programmer has an allotted schedule to program, documentation is not a problem. It might become a problem when time is running out and code needs to be cranked out, with the mind-set that the programmer will add the documentation at a later time—but that later time may or may not come.

Keeping up with documentation while commissioning code can prove difficult; at the time of commissioning, the priority is ensuring the code is functional and meets customer processing needs. Having multiple programmers on the same PLC program might also cause some distress with documentation. Communication and coordination between the programmers will help minimize the loss of documentation.

Document unto others, as...

Documentation is critical in that it helps the customer/programmer understand more easily what is going on or the code’s intent. I suggest to program with the mind-set that you, the programmer, will need to maintain it. Typically, we program and walk away from a site and might not return to the site for a long period of time. It will be nice if you return to the site to add code for a new process or enhancing their process and you can easily understand what you wrote several years before with little to no effort. The same goes with other programmers within your company or other subcontractors that the customer might hire. Sure, we can easily determine that we can write the code in the most complex way and have little to no documentation to make the other guy depend on you to make any programming updates or changes. While this might sound good for job security, it is not good practice. Customers will not appreciate relying on the one guy. Customers typically want off-the-shelf parts to ease maintenance and downtime; the same concept can be applied to programming. Customers typically steer away from custom software and would rather move to something that has an open source code and good software support—whatever makes sense to minimize possible future downtime and future upgrades, enhancements, and additions.

Code: Keep it clean

With documentation, another good practice is housekeeping: keeping the code clean and removing tags and code that simply go nowhere and are not needed. As programmers we typically take our best practices to new projects, especially if the new project has no predetermined code structure standard. After all, why reinvent the wheel? But while doing so, we need to remove whatever code we carried over from our copy-and-paste that is not needed—cut the fat. As a programmer, it’s not easy going into a program that has little or no documentation; and it doesn’t help that after reverse engineering the code, you discover that a percentage of the code is bogus and unneeded. After a few choice words, the first question you ask yourself is, who wrote this? Don’t be that programmer.


Good documentation and housekeeping are good programming practices. I’m sure we all have run into a program that had little to no documentation; many hours were required to reverse engineer what the program is doing. Even if the program documentation is written in a language foreign to your own, at least there is documentation. Google Translate to the rescue.

This post was written by Miguel Guiterrez. Miguel is a Senior Control System Specialist at MAVERICK Technologies, a leading automation solutions provider offering industrial automation, strategic manufacturing, and enterprise integration services for the process industries. MAVERICK delivers expertise and consulting in a wide variety of areas including industrial automation controls, distributed control systems, manufacturing execution systems, operational strategy, business process optimization and more. 

David , TX, United States, 05/21/14 02:45 PM:

Good article. Very relevant.
ED , OH, United States, 06/03/14 12:24 PM:

This is preached over and over, yet some still don't get it. Undocumented code is unfinished work! Programmers need to be held accountable. Thank you for this article. I hope some will change because they read this.
ZYGMUNT , CA, United States, 06/03/14 04:52 PM:

Let us use PLC programming patterns
The flow chart doc can be converted into the PLC program (Ladder logic + Function Blocks + Structured Text).
ALAN , NY, United States, 06/03/14 04:58 PM:

What's the easiest program to learn for a PLC?
DINESH , NY, India, 06/21/14 04:20 AM:

Good article. Very relevant. some time observed that some of vendor-programmer make programme so complicated that client is unable to understand.
Don - PLC Training , MO, United States, 07/11/14 12:39 PM:

Excellent advice Miguel. this is the best practices we teach in our PLC training.
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