Difference between RS485 and 4~20mA
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The instrument is equipped with multiple flow meters, all of which provide RS485 and 4–20 mA outputs. When connecting them to a DCS, which method is more appropriate? How is RS485 connected to the DCS?
It is recommended to use 4–20 mA for signal transmission; the signal can be connected directly to the corresponding channel of the AI device. RS485 is used for communication, and it requires a corresponding interface on the DCS system, although the number of such interfaces is usually limited.
The flow signal must be connected to the DCS system via RS485, and a communication protocol must be established between the flow meter and the DCS system. The flow signal is connected to the DCS system via 4-20mA; only configuration of the DCS system is required. Another point is that the communication distance of 485 is limited.
1. Where the AI channel of the DCS system permits it, it is preferable to use 4–20mA signals to connect to the DCS system, as analog signal transmission is currently the most reliable method. If there are insufficient system channels but a communication card that supports RS485 is available, the RS485 communication method can be used to connect to the DCS system. Based on practical operation experience, the failure rate of communication methods is higher than that of analog transmission, and the speed is slightly slower; therefore, analog transmission is recommended as the preferred option. 2. When using RS485 communication, Class V communication cables are used to connect to the RS485 communication interface on the flow meter side (special plugs are available on the market if needed), as well as to the corresponding interface or terminal block of the DCS communication card (in most cabinets, the card interfaces are already connected to the terminal blocks).
It depends on what your purpose is. If the flow data is used in situations that require precision, such as in trade transactions, it’s recommended to use a method with good frequency accuracy. Analog signals are mainly used for display and control purposes, while RS485 signals are typically used to read various parameters and diagnostic information from flow meters; any delay means that the data is not real-time. The RS485 signal is a bus signal; simply add a communication card to the DCS.
An analog signal, a digital signal
The reliability of both is more or less the same. After all, they were both invented by foreigners; you have no right to comment as if you were an expert
I’m not qualified; you are qualified, fine then
4-20ma is a standard current signal; the signal line can be connected directly to the DCS terminal board. RS485, on the other hand, is used for communication. There are only a few RS485 interfaces available on a single DCS unit, so it is used in cases where such communication is required. In most situations, terminal boards are used for connection, that is, via 4-20ma current signals