Hello everybody

Is it possible to address multiple slaves unit on a single IP address over Modbus/TCP?

Some modbus driver include a field for the unit ID at the end of the IP address.

I plan to use a Modbus/RTU network of slave unit over a Modbus TCP/RTU gateway.

You'll find the manual of the modbus gateway in attachment. (see page 20)

Thanks for your help.

Max

Hi Lynx,

I've never tried this myself. In the Modbus_Module-object there is an attribute \"UnitId\". Set this to the proper id and I guess it will work.

I have worked with Modbus slaves where it is needed to set the UnitId correctly, though the slave only has one unit.

By the way, I'm missing your attachment so I can't check what is said on page 20.

/Robert

Hello Robert

Thanks for the hint!

Here is the attachment:

Modbus/TCP includes a Unit ID field, which is used to address multiple Modbus slaves at a single IP address. Unfortunately, some first generation software drivers assumed a single slave at each IP and always set the Unit ID field to 0. This causes the Device server problems because it requires the Unit ID for the Modbus/RTU “Slave Address”. To support these older applications, the Device Server allows you to force a fixed address for Modbus/RTU and Modbus/ASCII, but note that this restricts you to a single serial slave
device per Device Server. Setting this value to 0 causes the Device Server to use the Modbus/TCP Unit ID as received. Setting it to any other address causes the Device Server to always use the set value as a fixed address.