The RS-422 interface represents an excellent choice for communicating in noisy environments and over a distance. But when the distance exceeds the RS-422 capability for reliable data transfer, adding a repeater is mandatory. The repeater circuit of Figure 1, which must operate from batteries when no power supply is available, draws under 3 µA of current from a 3-V supply.
Low standby power and true fail-safe operation are the key features in this application. U1 and U2 drive their receiver outputs (RO) high when the RS-422 inputs are open-circuited, or terminated and undriven. An incoming data byte on the differential inputs A1-B1 forces a transition on RO of U1, and a state machine (right half of the schematic) is latched ON by the falling edge of RO. The state machine asserts a high level at U2's driver-enable pin, causing the incoming data byte to be retransmitted from U2 at full RS-422 levels.
The state machine watches for transitions on RO. When a data-byte transfer is complete, as indicated by no falling edges within a time delay internal to the state machine, the state machine resets itself in anticipation of the next data byte from either side of the interface.
An incoming data burst (Fig. 2, top trace) is retransmitted as outputs A2 (bottom trace) and B2 (middle trace). U2 de-asserts those outputs 700 µs after the final transition. You can implement other delays by adjusting R1/C1 and R2/C2 in Figure 1.
Schematic appears incomplete. U3B pin 5, U4A pin5 and U2 pin 3 appear unconnected, but should be, according to text. Should they be connected together?
Bruce Andrews -September 20, 2004
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