Modbus in a Plug And Play Enviroment

I do not believe your going to like the consequences that this is going to produce.

Every device in a network has an address. You plug-in #3 and monitor then plug-in #4 removing #3. If the network does not know about the address it will alarm when #4 is installed. If it does know about the address then it will alarm cause 4 is not there in the beginning. If you use the same addresses then your data could get confused if there is a problem. Unless 485 has changed a lot in the last year....

There should be no real problems, as long as all of the devices you are plugging into the network are pre-programmed with a unique Modbus address. Keep in mind that the Modbus address is assigned to the unit, not to the socket.

You will need to tell your PLC Modbus Master when you add or remove a device from the network in order for it to know whether to poll that device or not.

Although we recommend our customers power down the bus before adding new devices, we hot plug items here in the lab all the time on our RS485 multidrop network test systems. When you plug or unplug a device, it may cause a short glitch on the bus, leading to one or two communication polling messages to be corrupted and dropped.

If your cables to the sockets are very long or you are operating the bus at high speeds, I would recommend adding a terminating resistor right to the cable's socket connection, to ensure you will not get reflections from open sockets.

Hope this helps!

Tim Kirk Rogue Engineering Inc

Terry

We plug devices into hot RS485 networks frequently and have never had any problems.

Your PLC make be kind of sluggish though, if it has to spend a lot of time polling devices that aren't on the network.

Mark Walsh Rogue Engineering Inc.

...

RS-485 applies to the physical layer and is not Modbus per se. Translation: Modbus uses RS-485. Other systems do too.

Jerry

Thanks all for your comments the portable devices will of course be "pre programmed" with unique modbus addresses.

Terry