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When a push button is pressed the HMI writes a value to the PLC(usually 1).
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All the HMI Nodes see the change of state. It then reupdates the IO Database along with all the other HMI nodes to the new value. When you push the push button it writes a preconfigured value to the plc(1,0,Toggle). The HMI reads the value from the PLC register and stores it in it's local IO Database every poll interval. The other HMIs will see the state change and be able to toggle the same bit. I figured it out relatively quickly by just comparing the programming, but before I got there, the plant technicians messed with that for weeks trying to figure out what was going on.Ī Maintained Push Button does not latch the bit in the HMI. The last one I worked on where there were 3 HMIs talking to one PLC had separate alterations done in each HMI by different technicians trying to tweak the process to solve different problems, not realizing that changes made in one were not showing up in the others. Not a big deal if you trust yourself and everyone else who may someday make contact with the operating program, I just have too much experience troubleshooting nightmares like this. So you can only make changes in one place, and any changes made there show up on any others. In a Thin Client topology, only one HMI is actually running the program, the other is a mirrored image of it. Sure, it's not supposed to happen, but Murphy rules. Otherwise you end up with a mess if someone changes only one. Just by the way, the reason I recommend using a Thin Client topology is that, if you have separate HMIs running independently all talking to the same PLC, you MUST remember to change ALL of the HMI programs if you change one of them. For the simple kind of thing you seem to be talking about, thats almost certainly the best approach. Sometimes they do not work quite like you think they should, and it takes time to debug them.
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I would also stay away from the more complicated templates in the HMI. I also do not like using NC PBs in the HMI. It simplifies things to use the same addresses, and if it works on one HMI, you only have to test it once. I don't have a problem with reusing the PB bits in multiple HMIs.
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It also makes it easier to test because you can just set the bit with your software package and the program will shut it off. Once in a blue moon you will get a PB bit set and it does not reset, which will cause you grief.
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You should also add a rung of code in your PLC that resets all the PB bits if any stay on for more than some period of time (like 2 seconds). I agree with never using maintained PBs in the HMI.