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