984/micro network


Thread Starter

Doug Craftchick

I'd like to network the two for an automation project, from what I've read there doesn't seem to be anything preventing me from running the micro as a slave to the 984. But what if I want to add a Panel mate later or add a third plc can I do this with Modbus?

Thanks Doug
First - 984 is not enough information. An example of a processor would be a 984-785E, 984A, or even Compact or Quantum. This is important as early
generation devices can not be Modbus "masters".

The Modbus protocol can only support one master communicating at a time. Yes, you could create a network of (1) master PLC and several slaves, but as soon as you added an HMI there would be problems. HMIs are always Modbus masters.

For multi-master networks you need to look at Modbus Plus or Modbus on Ethernet TCP/IP. Or have your HMI talk to the master PLC on a separate network.
Doug - Depending on your programming experience, the Micro can become the master to the 984 using the COMM function. The micro has 2 RS-232 ports of which one can be used for the 984 and the other can be used for the panelmate or HMI. The COMM function uses ASCII and can be confusing but will provide a solution.
My 984 is a compact A984-145, the Micro is a 411-00(third PLC may be addaed later also a 984-145). You folks say I can have more than 1
slave, is there a manual for Modbus networks that explains how to program(hopefuly my Modsoft light supporst this) and wire this?
Believe it or not, this is for a home brewery...

Thanks Doug

Chris Deschesnes


I understand that you will be using the 984-145 and micro 411 PLC and are trying to network them together using modbus. Although you can network PLC's over modbus, your situation will be difficult.

Modbus is a master-slave protocol with only the master being allowed to initiate communication. By default, all modbus ports on PLC's are
slave ports and cannot inititate any communication. Modicon does have a way of making them a master (i.e. XMIT loadable) but this is not supported by your PLC's and thus you cannot network them using modbus.

You may be able to use the COMM instruction to send ASCII messages between the PLC's but this would require considerable programming

When you introduce the panelmate, you add another level of complexity. You could use modbus to network everything if you did not need to have the PLC's communicate with one another as the panelmate would need to be the master device and both PLC's slaves. In this configuration, you still need to deal with the point-to-point nature of RS-232. To work around this, you need a multi-drop network such as RS-422 or RS-485. Converters are available to do this from many sources (i.e. Black Box).

If you use the COMM instruction above to provide PLC to PLC communication, you will not be able to network the panelmate on the same network
as the PLC's. You could use the MB+ port on the 984-145 but you will need a MB+ option on your panelmate too.

Hope this helps.


Bradley G. Hite

Hello Doug,

Home brewery Huh. Cool. I do some micro brewing myself. For my day job I teach people how to use PLC including Modicon PLCs.

Please feel free to e-mail me directly with any questions you have on you Modicon PLCs. I guarantee I can help.

Bradley G. Hite
Intertech Inc.
mailto:[email protected]
http://www.myplc.comTeaching Practical Skills for a Technological World