Simulating multiple modbus slave devices using node red

Question

I've managed to simulate a single slave device on my raspberry pi using node-red using functions to send data random values to the Modbus flex server. However, now I want to be able to simulate multiple Modbus slave devices on the port number and I'm unsure how to do this.

I've tried creating another Modbus flex server with the same port number, but this causes the whole node-red application to crash when it's deployed. Secondly, I've tried using different Modbus flex-write nodes to simulate different slave devices, but I'm unsure whether this is correct and if so, how I'd configure them to appear as different slave devices. This is because so far, my raspberry pi appears as slave 1, but I'm unsure where this comes from. I'm guessing it's to do with the unit-id of the Modbus flex-server but when I change the unit-id to a different number and type that number as the address in the master, it says no connection.

In conclusion, is it possible to use a single raspberry pi to simulate multiple slave devices on node-red using node-red-contrib-modbus and if so how do you do it?

Answer 1

The concept of Slaves in Modbus TCP differ somewhat from RTP as set out in the Modbus TCP Spec:

The MODBUS ‘slave address’ field usually used on MODBUS Serial Line is replaced by a single byte ‘Unit Identifier’ within the MBAP Header. The ‘Unit Identifier’ is used to communicate via devices such as bridges, routers and gateways that use a single IP address to support multiple independent MODBUS end units.

So there is a difference in termanalogy between Modbus RTP and TCP as well as a difference in the intended use of this field. The solution suggested by the spec would be to setup multiple servers on different ports (you cannot run multiple servers on a single port).

Having said that some TCP->RTP gateways (and other devices) use the unitid as the slave ID so I'm assuming you are trying to simulate something like this?

The first issue is that there appears to be a bug in Modbus Flex Server (reported) in that when you change the unit-id it is being stored as a string rather than a number. If you export the flow you will see something like "unitId": "3",; changing this to "unitId": 3, (no quotes around the 3) and importing fixes the issue (so that probably explains why you could not get this working).

Having said that changing the unit-id like this does not help you because it only supports one ID. However if you set the unit-id to 255 then it will listen on all unit-ids (this is a feature of the modbus-serial module used internally). Remember that you will currently need to manually fix the config to get this to work due to the bug.

Having done that you can do something like the following to respond to requests to different unit ids (the example will return the unit id (1 or 2) for all addresses so is not useful but shows the concept):