Difference between RS232 and RS485 interfaces - Database & Sql Blog Articles

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1. RS-232-C is a serial physical interface standard developed by the EIA (Electronic Industry Association). The term "RS" stands for "Recommended Standard," while "232" is the identification number, and "C" denotes the revision level. The RS-232-C standard defines 25 signal lines, which include one main channel and one auxiliary channel.

Typically, only the main channel is used in most applications. For basic full-duplex communication, just a few signal lines are required—such as one transmit line, one receive line, and one ground line.

The data transmission rate defined by the RS-232-C standard ranges from 50 to 19200 baud per second, including values like 50, 75, 100, 150, 300, 600, 1200, 2400, 4800, 9600, and 19200 bps.

According to the RS-232-C specification, the driver can support a capacitive load of up to 2500 pF, which limits the communication distance. For example, with a cable that has a capacitance of 150 pF/m, the maximum distance is about 15 meters. If the capacitance per meter decreases, the communication range can be extended. Another limitation is that RS-232 uses a single-ended signal transmission method, making it susceptible to common-mode noise and unable to effectively suppress interference. As a result, it's typically used for short-distance communication, usually under 20 meters.

2. RS-485

RS-485 is a widely used serial bus standard when longer communication distances are required—ranging from tens of meters to several kilometers. It uses differential signaling, where balanced transmission and differential reception help reject common-mode interference.

In addition to its high sensitivity, the RS-485 transceiver can detect voltages as low as 200 mV, allowing signals to be reliably recovered even over long distances. This makes it ideal for industrial environments where noise and interference are common.

RS-485 operates in half-duplex mode, meaning only one device can transmit at a time. Therefore, the transmitting circuit must be controlled using an enable signal to prevent conflicts on the bus.

One of the key advantages of RS-485 is its ability to support multi-point connections, which reduces the number of required signal lines. This makes it highly efficient for networking distributed systems, supporting up to 32 drivers and 32 receivers connected in parallel.