What factors affect the maximum transmission distance of RS232?

RS232 (also known as EIA RS-232) is one of the commonly used serial communication interface standards. It uses full-duplex communication and requires three lines: ground, transmit, and receive. RS-232 is only suitable for point-to-point communication between devices. Due to its single-ended signal, it has poor anti-interference capabilities. Therefore, RS232 improves signal anti-interference and increases transmission distance by increasing the voltage level. RS232 uses negative logic levels, with a logic 0 level of [3, 15]V and a logic 1 level of [-15, -3]V.

The maximum transmission distance of the RS232 standard is affected by many factors, mainly depending on the transmission rate, cable characteristics, chip driving capability, and signal transmission characteristics.

1. Transmission Rate

Transmission distance is inversely proportional to the baud rate. The higher the baud rate, the more severe the signal attenuation and distortion.

At 19.2 kbps, the distance usually does not exceed 15 meters.

At 9.6 kbps, the distance can reach 30-50 meters.

At rates as low as 1.2 kbps, it may reach 300 meters (with other conditions optimized).

2. Cable Characteristics

The RS232C standard specifies that the driver is allowed to have a capacitive load of 2500 picofarads. When using a communication cable with a capacitance of 150 picofarads per meter, the maximum communication distance can reach 15 meters. If the capacitance per meter of the cable is reduced, the communication distance can theoretically be increased accordingly.

3. Chip Driving Capability

Early chips had weaker driving capabilities, while modern chips (such as MAX232) can support longer distances with driving currents reaching 1mA.

4. Signal Transmission Characteristics

• RS232 uses a single-ended signal transmission method, which has problems such as common-ground noise and the inability to effectively suppress common-mode interference, further limiting its transmission distance. Using shielded cables to reduce external electromagnetic interference can also increase the transmission distance.

In practical applications, RS232 communication lines are usually used for short-distance communication within 20 meters.

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