When troubleshooting communication issues, it can be helpful to isolate the Mars sensor and verify communication independently of your datalogger or SCADA system.
Application Note 880084, available at the link below, provides a guide to testing Mars communication via Modbus TCP (Ethernet) or RTU (RS485) using the third-party software Modbus Poll. Use this guide to verify Mars is working properly and as an example for setting up your datalogger or SCADA communication.
Note: The attached guide 880084 states that the recommended polling interval (scan rate) for Mars is >= 5 seconds. A firmware update released after this guide was written now supports polling intervals of >=1 seconds. See Mars Firmware Release Notes.