There are some control system applications where system outputs must hold last state in the event they become physically disconnected from the controller, or if the controller stops controlling those outputs for any reason.
These applications include some heating, cooling, and pumping applications where the safest failure state is for the outputs to continue in their last state as opposed to turning off when control has been lost. Examples include:
- A material may need to stay heated to keep it from solidifying and clogging up tanks and pipes.
- A freezer may need to be kept on to ensure it’s contents don’t spoil.
- An exhaust fan may need to continue to run to insure unsafe levels of gases don’t build up.
Reasons why control systems outputs could become disconnected from their controller vary greatly. They include everything from a technician accidentally disconnecting the wrong cable, to construction workers inadvertently damaging network conduits.
In additional to human mistakes, actual hardware failures are also a possibility. One of the weakest links is the power system. Everything from a power outage, to a power supply component failure, could result in the controller going offline. And though rare, occasionally controllers do also fail.
Aside from becoming disconnected, there are other situations in which a control system can loose control of it’s outputs. For instance, a control program may contain a bug which causes the processor to fault. Or, a new control program may need to be downloaded to the controller, temporality taking it offline.
That’s not to say there aren’t means to avoid these scenarios. Uninterruptible Power Supplies (UPS) and redundant power supplies can be used to insure Controllers receive continuous 
power. And redundant controllers can be used to insure a controller failure doesn’t take down a system.
Fault routines can be used to insure programming errors are captured and addressed. And most of today’s control systems allow for editing or importing code while the control system continues to run.
But even using the above precautions, in many applications the danger of control system outputs not remaining in last state is so great there is just no good reason for any control system manufacturer to release output modules in which this feature is not rock solid.
I hope the above article on the importance of control system output having the capability to hold last state was helpful. If you have your own thoughts you would like to add to the conversation, please feel free to click on the “post a comment or question” link below.
Until next time, Peace ✌️
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Shawn M Tierney
Technology Enthusiast & Content Creator
Eliminate commercials and gain access to my weekly full length hands-on, news, and Q&A sessions by becoming a member at The Automation Blog or on YouTube. You'll also find all of my affordable PLC, HMI, and SCADA courses at TheAutomationSchool.com.
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