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RS232 and RS485 are common interfaces in low-voltage electronics. Many friends have mentioned the differences between them, so today we will explore them together.
1. Physical Structure of Interfaces
1. RS232 Interface:
One of the computer communication interfaces, the RS-232 interface usually appears in the form of 9 pins (DB-9) or 25 pins (DB-25). Generally, personal computers have two sets of RS-232 interfaces, referred to as COM1 and COM2.
The connector uses a DB-25 type 25-pin socket. Some devices connected to a PC’s RS-232 interface do not use the transmission control signals from the other side; they only require three interface lines: “send data”, “receive data”, and “signal ground”. Therefore, a DB-9 9-pin socket is used, and the transmission line employs shielded twisted pair cable.
2. RS485
RS485 has no specific physical shape and the interface is determined based on the actual project requirements.
2. Electronic Characteristics of Interfaces1. RS232: Transmits level signalsThe signal level values of the interface are relatively high (signal “1” is “-3V to -15V”, signal “0” is “3 to 15V”), which can easily damage the interface circuit’s chips, and because it is not compatible with TTL levels (0~”<0.8v”, 1~”>2.0V”), a level conversion circuit is needed to connect with TTL circuits.Additionally, it has poor anti-interference capability.2. RS485: Transmits differential signals Logic “1” is represented by the voltage difference between the two wires as + (2—6) V; logic “0” is represented by the voltage difference as – (2—6)V.The interface signal level is lower than that of RS-232, making it less likely to damage the interface circuit’s chips, and this level is compatible with TTL levels, allowing for easy connection with TTL circuits.3. Communication Distance1. RS232:RS232 has a limited transmission distance, with the maximum standard transmission distance being 15 meters, and it can only communicate point-to-point, with a maximum transmission rate of 20kB/s.2. RS485:RS485 has a maximum wireless transmission distance of 1200 meters.The maximum transmission rate is 10Mbps, and only at a transmission rate of 100Kb/S can the maximum communication distance be achieved. Using impedance-matched, low-loss dedicated cables can extend this to 1800 meters! Beyond 1200 meters, repeaters can be added (up to 8), allowing for a transmission distance of nearly 10Km.4. Support for Multipoint CommunicationRS232:RS232 interfaces only allow one transceiver to be connected on the bus, which does not support multipoint communication.RS485:RS485 interfaces allow for connection of up to 128 transceivers. This means it has multipoint communication capability, allowing users to easily establish a device network using a single RS485 interface.5. Differences in Communication LinesRS232: Can use three-core twisted pair, three-core shielded cables, etc.RS485: Can use two-core twisted pair, two-core shielded cables, etc. In low-speed, short-distance, non-interference scenarios, ordinary twisted pairs can be used; conversely, in high-speed, long-distance transmission, impedance-matched (generally 120Ω) RS485 dedicated cables (STP-120Ω (for RS485 & CAN) one pair of 18AWG) must be used, and in environments with severe interference, armored twisted shielded cables (ASTP-120Ω (for RS485 & CAN) one pair of 18AWG) should be used.Supplement: Since the transmission distance of RS232 is only 15 meters, what is its purpose? In fact, its application is very broad; it can connect various devices, such as monitoring, upgrading, or debugging other devices. Its function is quite similar to USB, and as USB ports become more common, more devices that convert USB to RS-232 or other interfaces will emerge. Through the USB interface, more RS-232 devices can be connected, achieving higher transmission speeds, realizing true plug-and-play, while also solving the drawback of USB interfaces not being able to transmit over long distances (USB communication distance is within 5 meters).
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