RS-485 line length

The EIA/TIA RS-485 communications standard, an upgrade of RS-422, supports 32 devices (driver/receiver pairs) in a party line or multi-drop mode, on a cable of up to 4,000 feet.

The standard specifies that each device has a “unit load” of not more than 12k ohm. It does not specify cable type or data rate.

The MD485 has a 1/8-unit-load receiver input impedance (96k ohm) that allows up to 256 transceivers on the bus. Practical network design will be more of a limitation than the electrical load limit of 256 nodes for the MD485.

The RS-485 data transceivers used in the MD485 feature fail-safe circuitry, which guarantees a logic-high receiver output when the receiver inputs are open or shorted. Because of this no “biasing resistors” need to be used. They also feature reduced slew-rate drivers that minimize EMI and reduce reflections. Because of this, termination resistors do not need to be used for most applications.