Modbus to Modbus – A Simple Way to Convert Serial (RTU) to TCP/IP and TCP/IP to Serial (RTU)

In an assembly where the Fieldserver device is reading from a Modbus device on Modbus RTU (Serial) or Modbus TCP/IP (Ethernet), and then serving to another Modbus device on Modbus TCP/IP or Modbus RTU, i.e., if the Fieldserver is integrating from Modbus to Modbus – RTU to TCP/IP or TCP/IP to RTU, then in such cases, the assembly can be configured in a very easy way for Fieldserver device with just three main sections included in the config file and without including Client side and Server side Map Descriptors in the config.

 

The configuration file (config.csv) for such an assembly contains three main sections after  ‘Bridge’ section – ‘Client Side Connections’, Client Side Nodes (If Client side is Serial – Modbus RTU) and Server Side Connections. Please note that if Server Side is Serial (Modbus RTU), then ‘Server Side Nodes’ section is to be included.

 

An example of such a configuration file is shown here;

 

//=====================================================================

//

//    Client Side Connections

//

 

Ports

Port  , Baud   , Parity  , Data_Bits , Stop_Bits , Protocol           , Handshaking , Poll_Delay

R1    , 19200 , None   , 8               , 1              , Modbus_RTU  , None             , 0.100s

 

 

//=====================================================================

//

//    Client Side Nodes

//

 

Nodes

Node_Name  , Node_ID , Protocol           , Port

DIRIS_A20    , 01           , Modbus_RTU  , R1

 

 

//=====================================================================

//

//    Server Side Connections

//

 

Connections

Adapter , Protocol         , Poll_Delay

N1         , Modbus/TCP , 0.100s

 

In this example, when Modbus TCP/IP Master device polls for a Modbus Register on Ethernet Adapter N1 on Fieldserver, that request is passed on to the Node – 01 on the Client side and then from there – ‘Client Side Nodes’ it is recognised that Node – 01 is on Serial Port R1 and that is how the Modbus Register is ultimately polled from Node / Device – 01 on Serial Port R1. And that is how the overall data communication takes place.

 

And therefore, Client Side and Server Side Map Descriptors are not required.

 

This also applies to JBUS Serial protocol, because the only difference between the JBUS and Modbus Protocols is the capacity to handle the number of points. Where Modbus can handle up to 9999 Object Instances (points), JBUS can accommodate more than 9999 points.