Optocoupler isolates shift registers

Jim Hartmann, Silent Knight LLC, Maple Grove, MN -- EDN, 1/20/2000

Conventional shift registers, such as the 74HC595, require data-, clock-, and strobe-logic signals. The circuit in Figure 1 needs only two logic signals to isolate and control shift-register devices. For each transmitted bit and one of the two optocouplers receives a short drive pulse: one optocoupler for a high transmitted bit and the other for a low bit and After pulsing all the bits, the circuit a final concurrent 1 and 0 pulse strobes the data into the output registers. Two logic-gate packages on the isolated side of the circuit decode the two negative pulse signals back into data, clock, and strobe. Two NAND gates form an RS latch that captures the data state for the serial input (SERIN). Two more NAND gates form an AND to combine the two pulse sources into the SRCK shift clock. Finally, a NOR gate (or four more NAND gates) produces the RCK strobe. You can cascade the shift-register devices as necessary.

You have no timing constraints on the signals other than observing the maximum data rate of the optocouplers and ensuring an off period between pulses. The final latch pulse also generates an extra rising SRCK edge that you can use to load the first bit of the next sequence. In this case, the optocoupler that turns off last determines the RS latch state for the first bit. You can also ignore the extra clock; it has no effect on the output. Low power consumption is possible by keeping the pulses as short as possible by limiting the LED current and the updating rate. For example, with 40-µsec pulses and 1-msec period, the average drive current is 80 µA. (DI #2470)