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8. Trouble Shooting
The communication status LEDs can be used to verify correct operation of the Modbus
module. During normal operation, the LED indicators flash as follows:
• RXD LED: Flashes when the Modbus master sends a message to the EX10
-MML11. On a multi slave network, this can also flash when when the
master polls other slaves, or when other slaves respond to the master. In
general, it indicates activity on the network.
• TXD LED: Flashes when that specific EX10-MML11 responds to the master.
This LED is only relevant to the module on which it is being viewed,
regardless of how many slaves are on the network. It indicates that the
module has received a message that is uniquely addressed to it, and the
EX10-MML11 is responding. This LED is usually on longer than the RXD
LED because the responses have more data than the requests.
If the LED indicators are not flashing, check the following:
1) Wiring connections
2) Resistance at terminals
3) Baud rate setting
4) Station Number
5) HAD Switch
6) Parity selection
7) Modbus master is using standard Modbus RTU format
If the LED indicators are on solid:
The polarity has been reversed on the network wiring. For example, the “A” terminals
are wired to the “B” lines, the “+” terminals are wired to the “-“ lines, etc. Correct the
connections to the EX10-MML11.