DESIGN SHOWCASE Standby current for RS-422 repeater is less than 3µA The RS-422 interface is an excellent choice for communicating in noisy environments and over a distance. However, when the distance exceeds the RS-422 capability for reliable data transfer, you must add a repeater. The repeater circuit in Figure 1, which must operate from batteries when no power supply is available, draws less than 3µA of current from a 3V supply. 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. 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 and B1 forces a transition on RO of U1, and a state machine (right half of the Figure 1 schematic) is latched ON by the falling edge of RO. The state An incoming data burst (Figure 2, top) is retransmitted as outputs A2 (bottom trace) and B2 (middle trace). U2 de-asserts those outputs 700µs after the final transition. Other delays can be implemented by adjusting R1/C1 and R2/C2 as shown in Figure 1. VCC D1 1N4148 R1 4.7MΩ C1 220pF 8 B1 R3 120Ω A1 VCC 7 B 6 5 U1 RO 1 RE 2 A DE 3 GND DI 4 MAX3471 6 5 1 1 2 3 U3A 74HC00 3 2 CLK CLR C3 100nF U3B 74HC00 4 Q D PRE VCC Q VCC 4 6 5 U4A 74HC74 GND1 C4 100nF B2 R4 120Ω A2 8 VCC 7 B 6 5 U2 RO 1 RE 2 A DE 3 GND DI 4 MAX3471 8 12 D PRE GND2 9 10 U3C 74HC00 Q CLK CLR 10 VCC Q 11 13 VCC D2 1N4148 R2 12 4.7MΩ 13 C2 220pF Figure 1. This RS-422 repeater draws only 3µA of standby current. 16 U3D 74HC00 11 9 8 U4B 74HC74 (dB) Figure 2. An input data byte causes the repeater to transmit the byte, then go to standby. Vertical scale is 1V per division; horizontal scale is 400µs per division. 17