RS232 and RS485 have always been common interfaces in low-voltage applications. Many people mention the differences between them, so today let’s explore them together.
1. Physical Structure of the Interfaces
1. RS232 Interface:
One of the computer communication interfaces, the RS-232 interface typically 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 via the RS-232 interface do not use the transmission control signals from the other party and 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 does not have a specific physical shape and is adopted based on the actual engineering situation.
2. Electronic Characteristics of the Interfaces
1. RS232: Transmits Level Signals
The signal level values are relatively high (signal “1” is “-3V to -15V”, signal “0” is “3V 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 required to connect with TTL circuits.
Additionally, it has poor anti-interference capability.
2. RS485: Transmits Differential Signals
Logic “1” is represented by a voltage difference of + (2—6) V between the two wires; logic “0” is represented by a voltage difference of – (2—6) V between the two wires. The signal level of the interface 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 Distance
1. RS232:
RS232 has a limited transmission distance, with a maximum standard transmission distance of 15 meters, and can only support point-to-point communication, 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 to achieve the maximum communication distance, a transmission rate of 100Kb/S must be used.
Using impedance-matched, low-loss dedicated cables can achieve distances of up to 1800 meters! Beyond 1200 meters, repeaters can be added (up to 8), allowing transmission distances to approach 10Km.
4. Support for Multi-Point Communication
RS232: The RS232 interface only allows one transceiver to be connected on the bus, and cannot support multi-station transceiver capabilities, so it can only communicate point-to-point and does not support multi-point communication.
RS485: The RS485 interface allows for the connection of up to 128 transceivers on the bus. This provides multi-station communication capabilities, allowing users to easily establish a device network using a single RS485 interface.
5. Differences in Communication Lines
RS232:
Can use three-core twisted pair, three-core shielded wire, etc.
RS485:
Can use two-core twisted pair, two-core shielded wire, etc.
In low-speed, short-distance, and non-interference situations, ordinary twisted pair cables can be used; conversely, in high-speed, long-distance transmissions, dedicated RS485 cables with impedance matching (generally 120Ω) must be used (STP-120Ω (for RS485 & CAN) one pair of 18AWG), 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 function?
In fact, its applications are very widespread, as it can connect various devices, such as monitoring, upgrading or debugging other devices, etc. It is quite similar in function 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, not only achieving higher transmission speeds and true plug-and-play functionality but also solving the drawback of USB interfaces not being able to transmit over long distances (USB communication distance is within 5 meters).