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How do I determine if communication is successfully occurring?
There are a number of ways to determine if successful communication is occurring between devices:
- Visual Inspection: The PLC’s serial port LEDs can be visually inspected. From the master end, the TX (Transmit Data) LED flashes first as a master message is sent out of the serial port and the RX (Receive Data) LED flashes almost immediately afterwards as the slave’s response is received. From the slave end, the RX LED flashes first as the master’s command message arrives in the serial port and then the TX LED flashes as the slave’s response is returned. Failed communication at the master end could manifest itself as only the TX LED flashing, indicating that the response was never returned or the TX LED never flashing indicating that the message was never sent. Conversely, failed communication at the slave end could manifest itself as the RX LED never flashing, indicating that the master message never arrived or the RX LED flashes but the TX never flashes (or flashes intermittently) indicating that the received master message was incomprehensible.
- Software Monitoring: There are a number of serial and TCP port diagnostic modules available for use by the control logic application. These modules, typically found in the register assignment in TelePACE or as custom function blocks in ISaGRAF, allow the logic to autonomously monitor the counts of protocol commands and errors. Successful communication at the master end is manifested as the Protocol Master Responses Received count equaling the Protocol Master Commands Sent count as it increases. At the slave end, successful communication is manifested as the Protocol Slave Responses Sent count equaling the Protocol Slave Commands Received count as it increases.
