SLC 5/05 Duty/Standby with Prosoft 3150 MCM Modbus


Thread Starter


We have a duty/standby system with modbus cards that need to read & write to the slaves.

The standby will write if the duty fails.
Does anyone have some advice on setting up the modbus cards to:
1) Only have 1 master on the RS485 network at any time (the second master needs to be attached in case Duty PLC fails)?
2) or is it possible to setup with two master but one having priority over the other to ensure that only the duty master can write to the slave?

If anyone has any advice or comments it would be appreciated.


Alan Whitfield

Hi Brendon,
I don't know how the Modbus cards work, but I have done this successfully with two SLC5/04 processors, each with a Devicenet scanner, so that if the master PLC fails, the standby picks up the Devicenet network and runs the system.It made a bumpless transfer, with a period of about 1.5 seconds without control, which was acceptable in the system concerned. Send me your email address and i will give you the details of how it works.
Alan Whitfield

The 3150 card requires three types of data from the SLC; configuration data (port config, RTU vs. ASCII, timing, etc), a command or to-do list, and the actual slave I/O data. You should be able to have the MCM in the standby SLC configured, but with an empty command list, or a command list where all the tasks are disabled. This will keep it from trying to access the slaves while the main PLC is running. You can even leave it wired onto the network. As soon as the standby SLC is notified that it is in charge, load the command list with the correct and enabled slave communication commands.

One thing to remember about the MCM card is that it has its own processor running independently of the SLC, so it doesn't know to stop polling the network just because the PLC is inactive, so you may want to have the standby SLC or some arbitrator actually remove power from the rack containing the primary SLC and MCM, or use a relay to disconnect the primary MCM from the network, until the problem concerning the primary has been resolved.

The guys at Prosoft are excellent and you shouldn't hesitate to give them a call. I've done quite a bit of work with the MCM's as well, and you can contact me offlist if you'd like.