ModbusBridge
A Modbus “bridge” (or gateway) is basically a Modbus server forwarding requests to other servers, that are in a different Modbus. The bridge will control and translate requests from a TCP Modbus into a RTU Modbus and vice versa.
Some typical applications:
- TCP bridge gives access to all servers from a connected RTU Modbus
- RTU bridge brings a TCP server into a RS485 Modbus
- bridge collects data from several (TCP and RTU) Modbuses under a single interface
As the bridge is based on a ModbusServer, it also can serve locally in addition to the external servers connected.
Basic Use
First you will have to include the matching header file for the type of bridge you are going to set up (talking to Ethernet in this case):
#include "ModbusBridgeEthernet.h"
Next the bridge needs to be defined:
ModbusBridgeEthernet MBbridge;
We will need at least one ModbusClient for the bridge to contact external servers:
#include "ModbusClientRTU.h"
… and set that up:
ModbusClientRTU MB(Serial1);
Then, most probably in setup(), the client is started and the bridge needs to be configured:
MB.setTimeout(2000);
MB.begin();
...
// ServerID 4: RTU Server with remote serverID 1, accessed through RTU client MB - FC 03 accepted only
MBbridge.attachServer(4, 1, READ_HOLD_REGISTER, &MB);
// Add FC 04 to it
MBbridge.addFunctionCode(4, READ_INPUT_REGISTER);
// ServerID 5: RTU Server with remote serverID 4, accessed through RTU client MB - all FCs accepted
MBbridge.attachServer(5, 4, ANY_FUNCTION_CODE, &MB);
// Remove FC 04 from it
MBbridge.denyFunctionCode(5, READ_INPUT_REGISTER);
And finally the bridge is started:
MBbridge.start(port, 4, 600);
That’s it!
From now on all requests for serverID 4, function codes 3 and 4 are forwarded to the RTU-based server 1, its responses are given back. Same applies to server ID 5 and all possible function codes - with the exception of FC 4.
All else requests will be answered by the bridge with either ILLEGAL_FUNCTION or INVALID_SERVER.