What is RS485 cable (EIA-485 or RS-485)?

The RS485 cable is a type of communication cable used in serial communication systems, particularly in industrial and commercial applications. It is not an application or protocol but a balanced transmission interconnect standard that specifies the electrical characteristics of drivers, receivers and transceivers.

The RS485 standard defines that devices must transmit data using differential signaling over a twisted pair of impedance-matched signal wires A and B. The communication partners encode bits as the voltage difference between the two wires rather than a single wire and a common ground. The use of differential signaling in combination with physically twisted wires increases the robustness against electrical interference, which in turn facilitates reliable long-distance transmissions in electrically noisy environments such as factories.

The RS485 cables typically consist of one twisted pairs、two twisted pairs、four twisted pairs of wires. The twisting helps to reduce electromagnetic interference (EMI) and crosstalk, ensuring reliable data transmission over long distances.

To represent a logical high state (called mark state), the voltage on wire A must exceed the level on line B. Conversely, logical low states can be encoded by pulling the voltage on wire B higher than the voltage present on wire A. Effectively, the voltage levels on lines A and B must be inverted, and the exact voltage levels are not defined. Instead, the focus is on the voltage difference between the signal transmission wires.

A typical RS485 bus can support as many as 256 communication partners and cable lengths of up to 4,000 ft(1200 m) at a data rate up to 100k bps. However, reaching the maximum transmission rate of around 10 MBit/s usually requires shorter wire lengths of up to 40 ft(12 m).

RS485 is a physical-layer standard that describes the electrical characteristics of a multi-point serial communication bus. Devices on the bus usually have a unique address, and they exchange information via differential signaling over a stranded pair of wires, A and B. Differential signaling encodes the transmitted bits using the voltage difference between the two wires.

RS485 allows data exchange in two-wire system (one twisted pair) for half-duplex communication and a four-wire system (two twisted pairs) for full-duplex communication. In half-duplex communication, while all devices can transmit and receive data, they must take turns when using the bus. In full-duplex data exchange, multiple devices can send and receive information simultaneously. A second stranded pair of wires is required, however, when aiming for full-duplex communication in RS485.

In order to assure maximum system performance, a well shielded, low capacitance, paired cable with matched impedance is recommended to limit signal loss (attenuation) and distortion. Engineering studies have shown that characteristic impedance cable of 120 Ohms,, which matches the impedance of the RS485 transceivers to minimize signal reflections.low capacitance (less than 15pF/ft) and conductors of 24 AWG、22 AWG、20 AWG、18 AWG or larger will deliver optimal distance with low signal reflection. The AL/PET foil and braided shielding of the pairs will protect the signal from outside noise interference (EMI and RFI) and minimize distortion.

RS485 communication cable

Suitable Applications
1. Industrial Automation: RS485 is widely used in industrial environments for connecting programmable logic controllers (PLCs), sensors, and other devices in a network.
2. Building Automation: It is used in building management systems for controlling HVAC, lighting, and security systems.
3. Telecommunications: RS485 is used in some telecommunications equipment for reliable data transmission over long distances.
4. Networking: It is used in various networking applications, including point-of-sale systems, and in some cases, for connecting computers and peripherals.