factory talk ME interlocking

I just have a little task I’m trying to complete, and I wonder if anyone has any suggestions. I have a crushing operation running with 3 separate SLC 500 p.l.c.’s. I am controlling them all with one computer with a single hmi. I want to do a bit of simple interlocking (eg. Something quits on plc 1 and automatically kills something on plc 2). I’d like to do this without sending messages between plcs. I was hoping I could do this in factory talk using expressions perhaps. Or some sort of free running if, than, else statement. It seems to me that it should be possible but I just can’t find/think of the right object or instruction to make this happen.
Any help would be appreciated!!!

Thanks

Nathan

You could set up a heart beat with the HMI. In the HMI set a bit on for each of the PLC's being controlled. The PLC when seeing the on bit will reset the bit and start a timer. If the timer expires before the bit turns on again then stop.
In the HMI read the bit. It should be off, if it is not then the PLC has stopped working. If it is off then turn it on again. The whole process starts again.

Hope this helps you out.
Regards,

Just use messaging between the PLC's. Or plain wires.
Adding a third layer is adding two extra points of failure, and you really really really do not want your HMI's to be doing control.

rdrast said:

Just use messaging between the PLC's. Or plain wires.
Adding a third layer is adding two extra points of failure, and you really really really do not want your HMI's to be doing control.

Click to expand...

I agree. This is especially true for a Windows based HMI, and made even more risky when the HMI software is notoriously buggy.

I would get the PLCs do the thinking and interlocking. The HMI is just for fancy buttons and pretty pictures.

ok thanks guys, the reason I was trying to avoid messaging is that we are constantly rearranging equipment, for example we might have 20 small crushing plants with PLCs that will work independently or in random groups of 2, 3, or 4, in almost any combination and be changing monthly. So I am constantly creating new HMI files to match the current configuration anyway so I just didn't want to have to be constantly making changes to the plc programs as well, or hard wire changes. but it sounds like that is the best bet.

thanks for your input!