HMI (Panel) & PLC Logic

[volksian 0]

Hi, What should be the preferred location of the ladder logic using HMI panel (as Master) and PLC controller (as slave)? Should it be in HMI or in the PLC? I was advise that the main logic should be in the PLC by one of the support. By the way, in case i made my logic in the PLC, do i need to make a ladder similar with the PLC? Or just utilize the tasks and tags? Kindly spare your ideas. Chritian

[RussB 22]

Sometimes that depends on who's HMI you're using, the new GEIP QuickPanel Plus units can have the Ladder Logic run in them.

[ControlsGirl 2]

Ever since I have started, it always is best for the customer to keep as much logic on the plc as possible. This goes for any perephrial device. Robots, screw guns, etc. It is much more convenient to only have to look in one place and only have to train maintenance to know how to use one piece of software.

[Crossbow 295]

My basic rule of thumb has always been keep the machine logic in the PLC, and any logic required to make the HMI work could be done in the HMI. I had a few HMI programs way back when where I needed about 500 steps of code in the PLC to make them work according to my design. I then started doing that in the HMI, and stopped putting it in the PLC. But actual control should be in the ladder so the maintenance guys can monitor it and change it. I have worked with some HMIs which would allow me to set a switch to only work if 8 other bits were in the proper states. That's really nice, as it takes ladder to do that in PLC. But if the customer goes to press the switch and it doesn't work, how does he know which bit isn't in the right state to make the switch work? Easily viewed in ladder, but not so easy to determine in the HMI program.

[RussB 22]

I agree, provided the OEM wants this. There are also times where it is detrimental to the machine to have someone messing with it. Of course these need superior diagnostics built in. But isn't that always a goal anyway? Eliminate or minimize down time is most efficient. This is generally not what happens when more than one person is making changes to a "working" system. Edited 11 Jul 2014 by RussB