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, RT, 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.

An external failsafe-resistor network must be used to ensure failsafe operation during an idle bus state. When the bus is not actively driven, the differential receiver inputs could float allowing the receiver output to assume a random output. A proper failsafe network forces the receiver inputs to exceed the V_IT threshold, thus forcing the SN65HVD888 receiver output into the failsafe (high) state. Table 3 shows the differential input voltage (V_ID) for various failsafe networks with a 54-Ω differential bus termination.