I want to simultaneously trigger 4 DAQ nodes from a 5th control node (each node is a Raspberry Pi) see below. I have a number of solutions in mind with issues for each,

  1. Naïve solution, just connect the GPIO pins in parallel to the control GPIO pin, they all share the same ground, will their be fan out issues?

  2. Buffer each parallel connection to mitigate fan out, is there something missing here?

  3. Use a 4-bit Universal Shift Register eg 74LS194 with the parallel input register all 1s & clock this via the control node, is this overkill or is there another reason this shouldn't be done?

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1 Answer 1


The Pi is basically a CMOS device which have high impedance voltage controlled interfaces.

Fan out is concept related to TTL (or RTL for those old enough to remember) current controlled devices which is irrelevant to CMOS. CMOS does have limitations but these are totally different.

  • So there would be no problem with my naïve approach of just wiring up the 4 DAQ nodes to the same GPIO pin on my controller?
    – DrBwts
    Jun 26, 2023 at 10:26
  • @DrBwts I can see no problems with 4 connections. CMOS is limited by wiring length and capacitance (which depend on required speed).
    – Milliways
    Jun 26, 2023 at 10:30

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