Combining ports for Modbus slaves


Thread Starter


I have a gateway which has only 2 ports used. I want to add add third device but not having port. Is it possible to combine into one port? One device is master and one is slave, whereas my new device will be Modbus Master. Any advise / suggestion please

Lynn August Linse

So today, is this what you have:
- an Ethernet gateway
- port 1 = Master which polls the slave on port 2
- port 2 = Slave shared by port 1 and remote Modbus/TCP

And you want to add a third device - a second Modbus serial master?

There will be no predictable way to have the 2 masters share port #1, nor can you combine the new master with the slave on port #2.

Your two options will be:
1) buy a bigger gateway with 3 or more serial ports (there are units out there with 4, 8 or even 16 ports)

2) use a simple, low-cost single-port 'Ethernet-to-serial' converter to make the second master into a Modbus/TCP master.
Gateway has 2 ports and both of them are used. One for Master and Other for Slave. I want to add 3 more slave device into existing port (where slave is used) by connecting in daisy chain. Is it something possible? Protocol Modbus RTU (Not Modbus TCP)

Jerry Miille


Not understanding your request completely but you might want to look at our Omnii-Comm module It can do a lot of different connections between Modbus systems, both masters and slaves.

Please contact me off list if you have any questions.

Jerry Miille

If the gateway has RS485 there is no problem. You can add the slaves with different addresses. If the gateway has RS232 you need RS232/RS485 converter.

How do we connect? Gateway has 2 ports. Port 1 is used to communicate with a device - Modbus Master.
Port 2 is used to communicate with a device - Modbus Slave.

I have 2 new devices with Modbus Interface and one of them is Modbus Master and other one is Modbus Slave. I want to use my existing gateway.

How should I configure? Both Modbus Slave & Modbus Master has RS232/RS485 converter.

IF I do not have RS232/RS485 Converter, will it work?

Please advise and Thanks for your input as well as other fellows.