Supervisory Control and Data Acquisition (SCADA) systems can remotely monitor electronic parameters and are widely used in power monitoring applications. SCADA systems typically use industrial Ethernet protocols such as Modbus TCP, while the serial meters/terminal devices that generate crucial data during monitoring support the Modbus RTU protocol.
To address interoperability issues, some users perform the necessary protocol conversion via embedded computers, but this solution requires additional resources in programming. Therefore, users often choose to deploy gateways to easily handle the conversion between Modbus TCP and Modbus RTU protocols. So, what are the different ways gateways can be used for data collection? Which method collects data the fastest?
1. Communication via transparent transmission method
Transparent transmission communication is typically standard on Modbus gateways. Deploying a gateway capable of transparent transmission conversion is relatively simple and convenient; however, transparent transmission communication has a significant drawback: each serial port can only perform one protocol request-response operation at a time, therefore the SCADA system must sequentially wait for each response.
If a gateway connects only a few serial port meters/terminal devices, scan time might not be a major issue. However, in large-scale applications, it can lead to system failure. In this scenario, a large number of devices connect to the gateway's serial port via RS-485 interfaces in a daisy-chain topology. The gateway can only respond to multiple synchronization requests sequentially, causing the SCADA system to consume excessive time to collect data. For example, if a gateway connects to 128 devices, there will be a significant time difference between the SCADA system reading data from the first and last device. Therefore, reducing scan time is a key factor in optimizing network communication.
2. Agency-based communication
The proxy method aims to overcome the shortcomings of transparent transmission methods, enabling independent protocol conversion at both ends of the gateway. On one hand, each serial port of the gateway can act as an RTUmaster, automatically polling various interfaces; on the other hand, the gateway can also function as a Modbus TCP server, polled by the SCADA system. A key feature of gateways supporting the proxy method is their built-in memory, which facilitates seamless data retrieval and increases the flexibility of SCADA programming.
For system integrators, ease of setup is always paramount. In a proxy mode, the gateway acting as the RTUmaster can be configured with numerous Modbus commands. To save setup time, some gateway brands support CSV (comma-separated value) files, allowing for easy gateway setup using Excel.
3. Intelligent communication
If you are dissatisfied with the scanning time consumed by the transparent transmission gateway, then optimizing the current system is your best option. The intelligent mode of the MoxaMGateMB3660 industrial gateway can learn and remember every Modbus command sent from the SCADA system. Once it learns a command, the gateway acts as a proxy, automatically sending Modbus requests to the relevant Modbus devices.
Because the data is stored in the gateway's memory, the SCADA system can directly obtain Modbus response data from the memory without waiting for the Modbus device to transmit data, thus significantly improving communication capabilities. You can initiate gateway commands to learn new functions with just a single button on the WebConsole. For the SCADA system, scanning time will be greatly reduced, as data transmission now only needs to be done from the gateway to the host computer via Ethernet. The waiting time for Modbus devices to be visited will also be significantly shortened.
Whether a plan is good or bad will be revealed after one test.
We conducted protocol conversion speed tests on the 16-port industrial gateway from the MGateMB3660-8/16 series, which supports transparent, proxy, and intelligent modes. Each of the gateway's 16 serial ports connects to eight interfaces, and each interface transmits 16 bytes of data, so a total of 2048 bytes of data needed to be collected for each test.
Test results show that the gateway in proxy mode has the fastest transmission speed, with a time of only 41 milliseconds to retrieve all parameters. The intelligent mode and transparent transmission mode took 312 milliseconds and 7502 milliseconds, respectively.
Furthermore, compared to gateway polling mechanisms using proxy or intelligent methods, the data collected by gateways via traditional transparent transmission methods is not necessarily more accurate. As shown in the figure below, due to the slow scanning speed, the number of data points collected via transparent transmission methods is far less than that collected using proxy and intelligent methods.
Clearly, the test results indicate that transparent transmission methods are unsuitable for large-scale deployment due to their long scan times. For building a completely new SCADA system, a proxy method is the superior choice. Its advantages are obvious: it can scan quickly and transmit data in data packets. However, if the goal is to optimize an existing SCADA system with less cost and effort, an intelligent method is the best option.
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