What does the RS-485 term “fractional unit load” mean?
The “unit load” concept is an idea put forth in the RS-485 specification. The spec defines a “single unit load” (1UL) device as one where the RS-485 pins have a defined current-voltage relationship that represents an approximate 12kohm load (input resistance) on the RS-485 bus. The spec allows up to 32 unit loads on an RS-485 bus, or therefore 32 1UL devices. Fractional unit load devices present a bus load that is a fraction of a “unit load”, and therefore have an input resistance that is a multiple of 12kohms. For example, a 1/4UL device has an input resistance of approximately 48kohms, and thus there can be 128 1/UL devices on an RS-485 bus before reaching the 32 unit load limit. Typical fractional unit load versions are 1/4 and 1/8UL , which allow 128 or 256 devices on the bus, respectively.
