2) Each system has 3 zones.
3) For redundancy the two PLCs handle everything for each cooling/heating system. (one goes down the other half runs normally)
4) I have 3 zone VFDs to be controlled via their 4-20mA inputs.
5) Each VFD runs a zone blower.
6) The blowers are the only common element between the two separate refrigeration systems.
7) The blowers are to run at slow speed unless cooling is active.
8) Either or both PLCs could go to cooling.
Yes, there is a question here...:
Since either system could call for cooling while the other isn't what's a workable technique for tying the PLC blower 4-20mA signals from these PLCs together so if either is in cooling it can override the VFD blower signal with the higher speed? That includes if one PLC dies completely.
kcress - http://www.flaminsystems.com