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Modbus Gateway - Transparent Mode

Modbus gateways that support agent mode communication and have a multiport density can accomplish these goals. Agent mode, designed to overcome the shortcomings of transparent mode (poll/response), allows each side of the gateway to conduct conversions independently from each other. On the one side, each serial port of the gateway acts as a Modbus RTU master actively polling the meters; on the other side, the gateway functions as a Modbus TCP server polled by the SCADA system. An all-important feature of gateways that support agent mode is an internal memory that facilitates seamless data retrievals.

Modbus Transparent Mode

Modbus Gateway - Intelligent Mode (Retrofitting)

If you are using transparent gateways and you are not happy with the scan times, then retrofitting your current system is your best solution. Although the market is flooded with an array of gateways, only one holds the key to reduced scan times through retrofitting. Employing intelligent mode, the Moxa MGate MB3660 will learn and remember the Modbus commands it receives from the SCADA system. Once a command has been learned, the gateway will act as though it is in agent mode and actively send Modbus requests to the relevant Modbus devices. Since the data is saved in a gateway’s memory, the SCADA system can retrieve Modbus response data directly from the gateway’s memory, instead of waiting for the data to pass through the Modbus devices, dramatically increasing communication performance. All you need to do is enable the innovative Command Learning function with one click on a web console. Again, from a SCADA system’s vantage point, the scan time will be much shorter, because the time spent on communication only occurs on the Ethernet side of the gateway. Users will see a dramatic reduction in the amount of time a Modbus device needs to wait to be accessed.

In large-scale applications, the results clearly show that transparent mode is not an ideal option due to the long scan times.  The retrofit option for existing SCADA systems, which requires fewer costs and less effort, points to intelligent mode as the optimal choice.