How This Works - VSCOM ModGate Plus User Manual

8 Example Configurations
And also enable Allow unknown clients for simplicity.
For each serial Slave device this system requires a Mapping entry. This is to inform the ModGate
Plus about the Modbus IDs existing on the second serial port. Open section Devices to Serial or
TCP (Direct Mapping) in figure 24.
Configure
Modbus Device ID The ID of one Slave device, a value of 1 to 247.
Destination Type This must be Serial to access the serial port.
Destination ID This is 2 for the second serial port.

How this works:

The serial Master sends requests on the serial line on port 1 to the Slave
devices. These do no longer connect to this cable, but the requests are received by the ModGate
Plus. It extracts the Modbus ID from the request, and as per configuration this Destination ID is
defined. So the ModGate Plus forwards the request to the serial port 2, where the Slave devices
receive the data. The device with this Modbus ID sends the response, which in turn is forwarded
back to port 1. Finally the Master receives the response from the Slave, and the communication
cycle is completed. This is just as before without the ModGate Plus.
While this communication is performed, any other TCP-Master can send a request via Mod-
bus/TCP to the ModGate Plus. If this request carries one of the configured Modbus IDs for a
Slave device, this will be forwarded to the second serial port. But only if there is no current com-
munication between Master and Slave. Otherwise the request is placed in a queue and sent to the
Slave when the current communication is finished. The new response from the Slave is then re-
turned via the TCP connection, and not to serial port 1. The setup as shown in figure 35 illustrates
how a TCP-Master can be integrated to a serial Modbus installation.
June 2018 ModGate Plus User Manual 42