RS-485 specifies communications for distances up to 4000 ft. This limit is the consequence of losses in the twisted pair used to carry the data. Beyond 4000 ft, skin effect and dielectric losses take their toll, attenuating the signal beyond use.

Fig 1 shows a simple RS-485 repeater. Two RS-485 transceivers connected back-to-back relay incoming data from either side. A pair of cross-coupled one shots control the data flow so that only one transmitter turns on at a time.

A 1-to-0 transition at the output of either idling receiver signifies incoming data. The first receiver to spot such a transition triggers its associated one shot, which, in turn, activates the opposite transmitter to ensure smooth data flow from one side of the repeater to the other. At the same time, the one shot locks out the other receiver/transmitter/one-shot combination, so that only one data path is open.

Successive 1-to-0 transitions and start bits retrigger the one shot, holding the data path in its present configuration. Set the one shots' time constants slightly greater than the interval between any two start bits.

When received data stops arriving, the previously active line idles high, producing a 1 at the receiver's output. The one shot resets, returning the opposite transceiver to the receive mode—ready for any subsequent data flow in either direction.

The software protocol must wait one word length after the end of any data transmission before responding to a call or initiating a new conversation to allow adequate time for the one shots to reset. The repeater in Fig 1 handles 100-kbps data rates and a 8-bit word length, plus start and stop bits. (DI #1421)