Power consumption is a concern in many applications. Power supply current is delivered to the bus load and to the transceiver circuitry. For a typical RS-485 bus configuration, the load that an active driver must drive consists of all of the receiving nodes plus the termination resistors at each end of the bus.

The load presented by the receiving nodes depends on the input impedance of the receiver. The TIA/EIA-485-A standard defines a unit load as allowing up to 1 mA. With up to 32 unit loads allowed on the bus, the total current supplied to all receivers can be as high as 32 mA. The SN65HVD485E device is rated as a ½ unit load device, so up to 64 can be connected on one bus.

The current in the termination resistors depends on the differential bus voltage. The standard requires active drivers to produce at least 1.5 V of differential signal. For a bus terminated with one standard 120-Ω resistor at each end, this sums to 25-mA differential output current whenever the bus is active. Typically, the SN65HVD485E device can drive more than 25 mA to a 60-Ω load, which results in a differential output voltage higher than the minimum required by the standard (see Figure 7-2).

Supply current increases with signaling rate primarily because of the totem pole outputs of the driver. When these outputs change state, there is a moment when both the high-side and low-side output transistors are conducting, which creates a short spike in the supply current. As the frequency of state changes increases, more power is used.