9.1.2 Bus Design

An RS-485 bus consists of multiple transceivers connected in parallel to a bus cable. To eliminate line reflections, each cable end is terminated with a termination resistor, R_T, whose value matches the characteristic impedance, Z₀, of the cable. This method, known as parallel termination, allows for relatively high data rates over long cable length.

Common cables used are unshielded twisted pair (UTP), such as low-cost CAT-5 cable with Z₀ = 100 Ω, and RS-485 cable with Z₀ = 120 Ω. Typical cable sizes are AWG 22 and AWG 24.

The maximum bus length is typically given as 4000 ft or 1200 m, and represents the length of an AWG 24 cable whose cable resistance approaches the value of the termination resistance, thus reducing the bus signal by half or 6 dB. Actual maximum usable cable length depends on the signaling rate, cable characteristics, and environmental conditions.