The RS-485 interface is a standard for serial data transmission widely used in industry and automation. It is a balanced interface that enables data transfer over long distances (up to 1200 meters) and allows multiple devices to be connected to a single communication line.
The RS-485 standard, also known as EIA-485 (Electronic Industries Alliance), is a specification for a serial interface used to transmit data between devices via differential communication lines.
Features of the RS-485 interface:
1. Bidirectionality: RS-485 supports bidirectional data transmission, meaning devices can both send and receive data over the same communication line.
2. Multiple devices: Multiple devices can be connected to a single RS-485 line, each having its own unique address, enabling targeted communication with specific devices on the line.
3. Balanced transmission: RS-485 uses differential data transmission, which improves noise immunity and enables long-distance data transfer without significant signal degradation.
4. Data transfer rate: RS-485 supports various data rates, making it flexible for different applications, from low speeds of several kbps (up to 10 kbps) to high speeds of several Mbps (up to 10 Mbps).
5. Line characteristics: RS-485 uses a balanced communication line, making it resistant to interference and allowing the use of various cable types, including twisted pair.
6. Signal voltage: The standard defines the voltage range for logic levels, typically from –7 V to +12 V. However, implementations may vary, with some using different voltage ranges.
7. Low power consumption: RS-485 is a relatively energy-efficient interface, making it suitable for applications where energy saving is important.
These features make RS-485 a popular choice for industrial automation systems, access control, security systems, building management systems, and other applications requiring reliable long-distance data transmission.
This standard is the most common communication interface in automated process control systems (APCS). The connection is implemented using a two-wire scheme (A and B lines).
Transmission is carried out using differential signals:
- Logic “1”: (A–B) > +200 mV;
- Logic “0”: (A–B) < –200 mV.
The operating principle follows a master/slave model.
The interface capabilities include:
- Connecting up to 32 transceivers in one network segment;
- Maximum segment length of up to 1200 meters (at a transmission speed of 62.5 kbps);
- Only one transmitter active at any given time;
- Possibility to create a network with up to 256 nodes using repeaters.
Oscillogram of actual data exchange
The oscillogram below shows the actual process of data exchange (request and start of response) between two controllers via the RS-485 interface (A line relative to B line). Key points are indicated:
- Activation of the master controller’s transmitter. A pause follows activation.
- Start of data transmission – start bit of the first frame (frames are similar to RS-232: start bit, data bits, parity bit, stop bit).
- End of data transmission – stop bit of the last frame.
- Deactivation of the transmitter.
- Activation of the slave controller’s transmitter.
Signal bias
As shown in the oscillogram, in the absence of a transmitted signal, a slight bias is observed on the signal lines. This bias is necessary to prevent false triggering of receivers.
Main disadvantages of the interface:
- Limited bandwidth;
- Relatively short communication range;
- Even when using repeaters, connection stability is not always guaranteed.
Related articles: RS-232 serial interface, RS-485 serial interface