How confident are you that a compromised receiving machine able to send arbitrary voltages on 8 of the 9 pins wouldn't be able to trigger some unexpected behavior on the airgapped machine?
Pretty confident, if the recieving machine is compromised, it is only connected to the opto coupler, that ensures only one way traffic, on one pin.
Somehow you would have to get a receive pin to transmit, and then get through the opto coupler and then it just hits a pin that's designed to only send data.
Ohhh that's a good point, you can set the Rx pin to Tx, then it still has to get through the opto isolator, and then it's talking to a port that's set to Tx.
Somehow you would have to get a receive pin to transmit, and then get through the opto coupler and then it just hits a pin that's designed to only send data.