Transparent forward mode work with scada

Use Transparent forward mode to work with SCADA

How does Scada system read and write Modbus slave over 4G?

Usually, SCADA cannot directly send Modbus message to 4G gateway, because 4G network does not provide static public IP address for simcard (except VPN), For deal with this issue, CWT provide a Modbus TCP to RTU tool to work with Scada in server，it converts SCADA Modbus TCP to RTU message and send to 4g gateway, then 4g gateway forward Modbus RTU message to Modbus slave on RS485, So that realizes the connectivity of SCADA and remote Modbus slave over 4G.

Communication process:

1. CWT 4g gateway build TCP connection with Modbus TCP to RTU tool according to server IP address (in this case, static public IP address or domain is necessary for server)
2. Scada system send Modbus TCP command to Modbus TCP to RTU tool according to salve ID (in this case, remote RS485 slave is treat as a TCP slave)
3. Modbus TCP to RTU tool convert Modbus TCP to RTU message and send to 4g gateway
4. CWT 4g gateway forward the Modbus RTU message to Modbus slave on RS485.
5. The responds message of Modbus slave is sent to Scada in same way.

Video Tutorial

How to set gateway and test

1. Set gateway to Two-way data transparent forward mode

First, disable Modbus master mode in “Modbus” page

In “GPRS” page

Note:

1) the register message must be setup, it’s approval for connection between gateway and “Modbus TCP to RTU” tool, the format is RTU&123456&[ID], the [ID] is what we setup in step 2.
So, register message is RTU&123456&00000001 in this demo.

2) The heartbeat time and packet are necessary for holding connection between gateway and server. any characters can be heartbeat packet, and they don’t affect Modbus communication.

In “RS485” page

After setup, start the device to go to working mode.

2. Check connection in Modbus TCP to RTU tool
Run the tool on server

After 4G gateway connect up, Modbus TCP to RTU tool show the connection.

3. Use Modbus simulator to test
For example, 4G gateway connect a temperature humidity sensor on RS485, slave id is 1, below is register map

Run Modbus Poll

Setup reading register

Modbus poll connection is shown in Modbus TCP to RTU tool