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I want to virtually simulate a flow sensor with a second Arduino that is getting controls from a PC through Serial COM port. The flow sensor is a hall effect sensor so it either transmits LOW or HIGH level.

Is it safe to directly connect a digital output pin from #1 Arduino to digital input pin in #2 Arduino? Should a resistor or a diode put between the two pins?

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Yes, if you also connect the two grounds. That's the point of digital inputs and outputs.

There should be no need for a series resistor unless you are doing something unusual.

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  • \$\begingroup\$ If I connect the two grounds should they be on the same voltage source? \$\endgroup\$
    – papaiatis
    Commented Jul 14, 2013 at 17:02
  • \$\begingroup\$ @Papai: Huh? Connecting the two ground means running a wire from the ground of one unit to the ground of the other unit. \$\endgroup\$
    – Olin Lathrop
    Commented Jul 14, 2013 at 17:11
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    \$\begingroup\$ Since Arduino is usually a development environment (and things may go wrong), and there is no harm in a series resistor between output and input, I'd advise to use one. I usually use 220 or 330 ohm between microcontrollers/Arduino's signal pins. The maximum current will be limited to about 20mA when things accidentally go wrong. \$\endgroup\$
    – jippie
    Commented Jul 14, 2013 at 17:12
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    \$\begingroup\$ @papaiatis they can be on different voltage sources and share the ground only (+ signal of course). \$\endgroup\$
    – kenny
    Commented Jul 14, 2013 at 17:35

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