Standby PLC and RS485

[lhrbromleyp]

hi all,

I have a GE 9030 with a PCM301 module that is using the RS485 from this card. I've been asked about switching this RS485 to another PLC as a standby/bypass.

Question is, what sort of device would enable the RS485 to be switched from the main GE to a bypass PLC? I'm think a 4 pole relay, but it must be industrial standard, SIL2 preferably. Am I barking up the wrong tree here or is there another way to do this?

Any advice or ideas would be appreciated.

Thanks

 

[Steve Bailey]

What is connected to the RS485 port and what is the program in the PCM301 module doing?

I don't completely understand what you're asking. RS485 permits multiple connections with proper termination of the physical ends.

 

[lhrbromleyp]

Sorry, I'll be a bit clearer.

Question is around multiple masters on the 485 bus. What have people used in the past to switch between them.

I know you can't have multiple masters online at the same time, so what is the best way to achieve the switchover? Token passing? Relays?

Any help appreciated.

 

[GaryS]

I assume that it is RS485 multi prop network you should be able to connect the new PLC on the same network just make sur you don't duplicate addresses
but if both PLC's are running the same program you will have find a way for to change the master when you need the backup both can't control the any devices on the network at the same time.

 

[lhrbromleyp]

Hi Gary

yes it is multi drop, I'm aware of duplication of addresses and duplicate masters etc. Question is what is the best way to change the master over to the standby PLC?

Is it to put the RS485 through relays which then switch between either PLC when commanded or some other way that I'm not aware of?

Any help would be appreciated. thanks.

 

[AlfredoQuintero]

Hi lhrbromleyp. I think a pole relay solution is not necessary as the bus supports multi-drop wiring. But you need to figure out a way to communicate the two PLCs with each other so the stand-by PLC can know the status of the main PLC and vice-versa, so you can make sure only one master will be communicating through the RS485 bus. What do you have in mind to communicate these two PLCs? Are they in close proximity? do you have spare DI/DO lines in both PLCs so as to program some simple heartbeats between the two? Or are you planning using some communication between the two? Also, please note the switch over may cause temporary communication loss. Does your process tolerate a short communication loss when the main PLC fails? Or does it need to be seamless?

 

[Steve Bailey]

Assuming the communications master routine is a Mega Basic program in the PCM301 module. If that's all Mega Basic program does, then run it only when the PLC is in control. If the Mega Basic program needs to run all the time put the communications master in a subroutine that only gets called when the PLC is in control.
There is sample ladder logic to start/stop the Mega Basic program in the manual for the PCM module GFK-0255.