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TPIC6B595: TPIC6B595 as a series shift register for relay signal multiplexing

Part Number: TPIC6B595
Other Parts Discussed in Thread: SN74HC245


We have strung 50 of these TPIC6B595's together in series to form a large multiplexer with relays. The schematic may be seen here:

There appears to be some bit errors on occasion. I was wondering if there is anything obviously wrong or not optimal about our design.

We are using a 5V FTDI USB to SPI cable to drive the shift registers, which act as drivers for the Littlefuse HE3600 series reed relays.

Joe Gorse

  • My first guess would be that the loading on each clock signal line to the TPIC6B595s is different, and the timing is just slightly off because of this.  I would recommend adding a buffer to fanout the incoming clock signals and ensure that the traces (or wires) are at least close to equal in length/capacitance.  If you have to use long traces for some parts and short traces for others, another solution (instead of matching trace lengths) is to artificially increase the input capacitance at the closer devices by adding a small capacitor (probably 5 to 30pF, depending on the other traces).

    There are many buffers that would work for the fanout circuit, however I would recommend the SN74HC245. The pinout makes it very easy to route through-signals, and it is optimized for 5V operation.