Regarding the communication methods between DCS and SIS systems, what are the differences between RS485 and MODBUS, and what other communication methods exist?

RS485 is a physical standard for signals, while Modbus is a communication protocol; the two are completely different.

RS485 is a wired transmission method, so it requires a hardware transmission medium; in fact, just two wires are sufficient. The same signal is transmitted over these two wires – the sender divides this signal into two parts, but the receiver reassembles it back into its original form. The advantages of this approach need to be compared with those of RS232； RS232 also requires two wires; more often, a ground wire is added as well, resulting in three wires in total. We can ignore the ground wire, and of the remaining two wires, one transmits the data signal while the other transmits the clock signal. In other words, the signal is sent in its original form by the sender, and the receiver handles it in the same way. Therefore, the receiver for 485 can cancel out the interference introduced during signal transmission, whereas the receiver for 232 cannot do so. As a result, 485 has stronger interference resistance and can transmit signals over kilometers, while 232 can only transmit them for about ten meters. As for MODBUS, it is just one type of communication protocol; it’s like Chinese and English – a language used for communication, a language through which machines can communicate with each other. So before communication can take place, there must be a bridge for communication, and that is the transmission medium – 485 or 232, or other electrical standards. The same protocol can be used with different transmission media such as 485 or 232, but two different protocols cannot coexist on the same transmission line. . . . Generally, we use the RS485 interface for Modbus communication. The typical distance for RS485 communication is 1200 meters; when this distance is not sufficient to meet the requirements, fiber optics are used as the transmission medium, with an optoelectronic converter required in between. However, this type of connection seems difficult to establish. The best approach is to transmit Modbus data over an Ethernet network and then use that Ethernet connection to transmit the data via fiber optics. RS-485 has the following characteristics: 1) The RS-485 interface utilizes a combination of balanced drivers and differential receivers, which enhances its resistance to common-mode interference, meaning it has good noise immunity. 2. The standard maximum transmission distance for the RS-485 interface is 4000 feet; in practice, it can reach 3000 meters. Additionally, the RS-232-C interface allows only one transceiver to be connected to the bus, meaning it has single-station capability. The RS-485 interface allows up to 128 transceivers to be connected on the bus. That is, it has multi-station capability, allowing users to easily establish a device network using a single RS-485 interface. Due to its advantages such as good noise resistance, long transmission distances, and the ability to support multiple stations, the RS-485 interface makes it the preferred serial interface. Since a half-duplex network using RS485 interfaces generally requires only two wires, RS485 interfaces employ shielded twisted pair for transmission. The RS485 interface connector uses a 9-pin DB-9 plug socket; the RS485 interface of intelligent terminals uses DB-9 (female), while the RS485 keyboard interface connected to keyboards uses DB-9 (male). 3. Electrical characteristics of RS-485: A logic \"1\" is represented by a voltage difference between the two wires of + (2 – 6) V ； The logical \"0\" is represented by a voltage difference between the two lines of - (2 – 6) V. The interface signal level is lower than that of RS-232-C, which makes it less likely to damage the chips in the interface circuit. Moreover, this level is compatible with TTL levels, allowing for easy connection to TTL circuits. 4. The maximum data transmission rate for RS-485 is 10 Mbps.

RS485 is a communication interface, while Modbus is a communication protocol