Multidrop modbus network


Thread Starter

Eric Bossennec

In our automation project we need to link 7 modbus slaves which work only in an RS232 configuration. The modbus master is a GE FANUC plc module that can be configured in either RS232 or RS485 but the slaves are RS232 only. Is it possible to build a multidrop network in RS232 or do we have to build an RS485 network and add an RS485/RS232 coverter for each slave? any other suggestion?
There are a couple of alternatives to using RS-485 converters, but there is no reliable way to simply wire them all together.

One is to use an active port splitter/combiner. I think B&B electronics has one.

A second way is to use a Digi terminal server and configure the ports to send to each other. This can all be done in a single terminal server or in multiple terminal servers if it provides some other convenience. The Digi's can also convert serial Modbus to ethernet Modbus.

Fiber optic converters are yet another option.
Hi Eric,

You should know better than that - TIA/EIA 232 (formerly RS 232) defines the interface between precisely TWO devices (DTE and DCE). In a "null modem" configuration, it can be used to connect between two DTE devices. Any attempt to shoehorn 232 into a multidrop topology is a strech too far.

Your choice - RS485.


Jerry Miille

In my world, multi-dropping means that you can connect a single MASTER device (the one that originates a communication session) to multiple receiving devices or SLAVES. The MASTER transmitter connects to ALL the SLAVE receivers. This is permissible in RS232 connections (within loading limits).

ALL the SLAVE Transmitters must be connected to the MASTERS receive circuit in a multi-drop application. This is where the problem arises. Typically, there is no way to "disable" an RS232 driver. If you have more than one trying to transmit at the same time, or one trying to transmit and one just MARKing in place, then you can not communicate successfully.

By the specification, you cannot multi-drop RS232 devices.

In practice, you can, IF the equipment you use has RS232 DRIVER circuits that can be disabled. Some devices use this kind of driver but MOST EQUIPMENT does not. You can study the users manual for your MODBUS slaves to determine if your SLAVES can support this.

The solution is to use RS232 to RS422 or RS485 converters to eliminate this problem. This "solution" has potential problems, several of which are beyond your control. You do not specify if your Modbus network is using Modbus ASCII or Modbus RTU protocol. This can be a significant question because message framing is defined differently for each version of the protocol and the converters can affect this timing. Also, do your devices use the modem handshake line RTS to enable communication? Some converters use this line to enable their transmitters, others do not. Those that do not use the RTS signal have some sort of time out scheme to determine when to enable their transmitters and when to turn them off. This can be a major problem with Modbus RTU protocol.

You can contract me directly if you have additional questions at [email protected]

Jerry Miille
You have to build a RS485 network. So yes, you'll need a RS485 to RS232 converter for each Modbus slave.

B&B Electronics has them for about $70, and they work great.

- CH
It is possible to use a modem sharing device such as the one made by black baox. in this case you need to use a null modem connector for each modbus slave to connect to the network but it will work. the other method I have had success with is using radio modems in a master slave mode.
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