Good morning JR,

Thanks for your question. I may be misunderstanding, but are you saying you assigned your objects directly to I/O Addresses?

I've had students do that before when testing systems out, and I like to take these opportunities to suggest never writing from the HMI directly to I/O.

Reading directly from I/O is fine, but I always stress the HIM should not write directly to I/O, or to I/O Aliases.

Why? Well, typically you want your PLC code to control outputs, and for that reason (and to make the system easier to troubleshoot) if we need to turn an output on from the HMI we'll let the HMI turn on an internal bit or Boolean tag, and then in PLC logic have the bit turn on the output.

As far as inputs, the PLC typically scans the Input modules many times a second. And while some PLCs will allow you to change/write to an input's value, if the I/O card is present and providing data each I/O scan, the value you (or your HMI) writes will be overwritten by the values coming from the input module within milliseconds, making it appear that your write never took place.

Hope this helps – please let me know if I misunderstood,

Shawn Tierney,
Instructor, The Automation School