What do the terms “unit load” and “fractional unit load” mean?
The RS-485 specification defines a “Unit Load (UL)” as the impedance presented to the RS-485 bus by a device that sinks 1mA with 12V, and sources 0.8mA with -7V, applied to the bus receiver or driver pins. The RS-485 spec allows 32 ULs to be connected to a bus. “Fractional UL” devices present higher impedances to the bus, thus they are a fraction of the unit load. A 1/8 UL device sources or sinks 1/8 the current of a 1 UL device, thereby allowing up to 256 devices on the bus.
