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I'd like to read a single digital output with two separate systems, each of which has a digital input. Both systems have a common ground.

In principle, since the digital output is low-impedance and since digital inputs are high-impedance, can I just tie these two inputs together at the same output without any contention or crosstalk, or must I add in some form of signal conditioning? Does this concept scale? Can I wire N digital inputs together to read a single digital output?

If not, what kind of buffering should I try to implement?

My use-case for this scenario is to wire a quadrature encoder output to both an off-the shelf PID controller and a separate system for logging ticks.

Cheers--and thanks for the..eh input.

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  • \$\begingroup\$ Your question is a bit confusing (to me) you say "I'd like to read a single digital output" then you say "can I just tie these two outputs together", did you mean Inputs? \$\endgroup\$ – Tyler May 23 '16 at 17:43
  • \$\begingroup\$ @Tyler, you're right. I've fixed my wording. \$\endgroup\$ – poofjunior May 23 '16 at 17:54
  • \$\begingroup\$ You still refer to an output. Replace this by 'signal source' and it will make more sense. \$\endgroup\$ – Sean Houlihane May 23 '16 at 18:08
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Yes, in principle you can. In digital circuits, people tend to call this 'fan-out' - single output to multiple parallel inputs.

If general, when you're trying to work out how big 'N' can be, you need to consider both the static case - how much will the DC load of all the inputs drag down/up the output - and the dynamic case - how fast can the output driver charge/discharge the parallel capacitance of all the inputs. In fast CMOS systems, the latter tends to be the main concern.

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