The simple RS-485 repeater (Fig 1) provides full-duplex communications--simultaneous transmitting and receiving--with only two ICs. Its balanced and differential data lines battle high-noise environments and drive long lines. Single-ended RS-232C schemes cannot equal this circuit's performance.

The RS-485 standard allows for bidirectional, multipoint, party-line communications at data rates up to 10 Mbps (150 kbytes/sec) and line lengths to 1200m. To achieve data rates up to 2.5 Mbytes/sec, substitute the components in Table 1.

IC₁, a half-duplex interface, includes transceivers, optocouplers, a power driver, and a transformer. The transformer couples power across the device's isolation barrier from its logic (nonisolated) side to its isolated side.

IC₂, powered by the isolated V_CC, upgrades the half-duplex operation of IC₁ to full duplex by using IC₁'s dedicated optocouplers. You must tie IC₂'s pin 3 low to disable IC₁'s driver and leave pin 4 floating. The driver outputs of IC₁ and IC₂ exhibit high impedance when D_E is low; bringing D_E high enables the outputs to function as line drivers.

Any TTL/CMOS logic family can drive IC₁'s digital inputs through a series resistor. With the aid of resistor pullups, the outputs can drive such loads as well. IC₁'s isolated outputs meet all RS-485 specs. (DI #1613)