corresponding unit number from the map table. The gateway will then transmit the
request on the TCP network and then wait for a response.
If the TCP device fails to connect or times out, the gateway will return exception code
11 to the RTU master. Exception code 11 is defined by Modbus protocol to mean
"Gateway target device failed to respond". This is how a gateway reports a timeout or
no-response error back to the requesting device.
If a good response is received from the TCP server, then the TCP header is removed,
the original RTU address is restored, a checksum is added, and the response is
transmitted back to the RTU master.
5. Accessing TCP Devices from RTU
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