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I am using a Pi compute module and wish to make two of them to communicate to each other.

I have hooked up some slave I2C devices to it (RTC, Temperature sensor). So I don't think making Pi as slave using on-board I2C is an option

And I read Understanding SPI on the Raspberry Pi, on board SPI only function in Master mode.

I unable use Ethernet/Wifi/BT (design requirement not allow involving a LAN switch in between AND no USB wifi/bluetooth).

Is there any other way (maybe use the existing GPIO implementing my own software I2C/SPI/1wire slave?) to make both Pi communicate to each other? data speed is not important

Updates
Serial link TXD/RXD also already used up for communication with PC

  • A serial link perhaps? – joan Apr 29 '15 at 11:18
  • sorry @joan, serial link already used up for PC-Pi communication – Dennis Apr 29 '15 at 11:19
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    Doesn't the compute module have another serial link, e.g. on gpios 32/33. You might have to search to find a Linux driver (I think one has been written) if you don't fancy writing a userland program to access the hardware. – joan Apr 29 '15 at 11:23
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You could use the GPIO ports on the Pi and the RPi.GPIO API for Python to communicate and convert the number to binary, although this would take quite a bit of coding.

Another option would be to connect the two pis' directly to each other with just 1 ethernet cable, for which you would need to configure a default, static IP address to use on each. Then you could use the socket API from Python to communicate.

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    If you use an Ethernet cable to connect the Pis directly, I would think it should be a crossover cable - so that P1.Send -> P2.Receive (and vice versa) - at least that's the way it was done in the old days. – condiosluzverde May 7 '15 at 17:23

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