So I need to make two Arduinos communicate. Though, I can't use any clock related system (so no Serial, no I2C...) and so my idea was basically to wire one from another so they could read electrical signals of the other.
So, I plugged a resistor to each (digital) pin from 2 to 6 (that makes five different signals, I don't want to use any signal combination to lower the number of pins needed, it's a thing i'll do later). And, on the other side, I plugged the wires to the same pins, but configured as inputs. To avoid short circuit, I added a 150 ohm resistor between each pin, so I don't destroy anything. Now, my question are :

  1. Is a 150 ohm enough ? I think yeah, but I'm not so sure so I want to verify.
  2. Will my second Arduino read the input as HIGH, since there is a resistor ? Once again, I'd say since the resistance is very low. I of course the Ohm's law (U = RI) but I don't know the amperage going out, so I can't calculate wether or not the voltage will be over 3 volts on the input (3 volts is the minimum to be considered as HIGH by the Arduino).

If anyone could help me on those questions, I'd be extremely recognising.
Thank you in advance, and have a good day

  • 5
    \$\begingroup\$ Without a schematic this is just a bunch of hand waving that needs to be closed. Also, what is "clock related"? Serial sends no clock signal, but IIC and SPI do, for example. \$\endgroup\$ Apr 23, 2015 at 18:33
  • \$\begingroup\$ Inputs are very high impedance. There is no "short circuit" issue here. The only way it could be short is if you had one pin set to output and HIGH, and the other board's pin set to output and LOW, and then connected them. \$\endgroup\$
    – KyranF
    Apr 23, 2015 at 18:35
  • \$\begingroup\$ So basically, a parallel port \$\endgroup\$
    – Passerby
    Apr 23, 2015 at 19:04
  • \$\begingroup\$ What do you mean "no clock system"? without a clock your devices will not know when data is ready. Any tipe of signal to flag "data ready" is at the end a clock. Maybe you mean "without hardware clocks"? Then you can implement a bitbang driver for I2C. \$\endgroup\$
    – Gusman
    Apr 24, 2015 at 1:14

1 Answer 1


The input impedance of an Arduino is very high, so there will be very little current flowing through your resistors, so no significant voltage drop. I'd suggest 510 ohms or so to protect the Arduino outputs in case you program both sides as outputs.

You must also connect the grounds of the two arduinos together.

  • \$\begingroup\$ So you'd say 510 rather than 150 and the two grounds together ? Ok, I'll do that. \$\endgroup\$ Apr 23, 2015 at 18:46

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