What are the cable specifications and recommendations for RS-485?

Modified on Mon, 27 Jan at 12:40 PM

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What are the cable specifications and

recommendations for RS-485?

Definition

RS-485 is a standard that defines the electrical characteristics of a serial data transmission system. It is commonly used in industrial and commercial applications for transmitting data over long distances in a balanced and differential manner.

RS-485 transmits data using differential signalling, where two complementary signals, A and B, are sent over twisted-pair cables. This helps reduce the effects of electromagnetic interference (EMI) and noise, making it suitable for noisy industrial environments.

Conductor Specifications

RS-485 cable typically consists of two twisted copper conductors, often shielded to protect the signal from external interference. The use of twisted cables helps reduce electromagnetic noise and improves the reliability of data transmission.

The specific cable can be replaced with a LAN cable.

Specifications for using RS-485 with inverters:

Cable length: Maximum cable length over the entire RS485 bus: 1200 m (3937 ft), recommended no splices in the cable. Important to note that it is maximum distance that could be achieved using RS485 but it heavily depends on the external conditions and not always practically acceptable due to weakness of the signal over the long distance, package losses etc.

Cable impedance: impedance close to 120Ohm

Shielding: consider using a shielded cable to protect the RS-485 signal from external interference, especially if the cable is installed in noisy environments or subject to electromagnetic interference

Conductors: use two wires forming a twisted pair, min. 2 x 2 x 0.22 mm² (2 x 2 x 24 AWG)

Recommendations:

Use CAT6 cable.
Ensure that the housing of the devices to which the bus is connected is earthed.
To reinforce the signal in the case of very long buses, use the 8 conductors of the cable, joining the 4 solid colours (solid colour) and the 4 twisted ones (solid colour + white) together
in case you use 4 cable for RS485A and other 4 for RS485B and you still have problem we suggest to increase the CAT cable to a higher one