MODBUS RTU - Simulate multiple slave to response to a PLC

[jeedee78]

Hi! I'm new with the use of the Modbus communication. I would like to simulate 4 devices (I/O Extenders) to response to a PLC. It is possible to do this using only one com port with RS232 to RS485 converter. Do you have an exemple that do this?

[fromm8]

Hello Jeedee78

First I would like to confirm what software tool you are using to implement Modbus protocol, DSC module? NI Modbus Library?

You mentioned simulate 4 devices to response to a PLC, maybe your questions is if you could implement multiple modbus slaves to talk to the PLC, correct?

Additionally, check this link.

Regards

Frank R.

[jeedee78]

To you to reply!

I use Modbus Library.

I suppose that to implement multiple slave I need a NI-9871 or a multi port pci card. But I would like to use only one ComPort for 4 devices. To specify which devices I want to simulate (implement) it's 3 I/O extenders 900MB series and a wind sensor. It is possible to do that or I absolutely need a multiport RS485 card?

[jeedee78]

See the picture. There is what I want to do.

[unclebump]

It looks like the acromag device uses modbus rs485 communication. This is a multidrop connection that will allow several devices to be present on the network at the same time. Up to 32 or maybe 64. Each device on the network needs to have a unique ID numbers assigned to it. Part of the formatting of the modbus communication protocol is to identify what device the message is sent to. You will either need a serial port that can do RS485 or an RS232/485 adapter. There are also ethernet devices that can convert to modbus rs485 networks.