I'm interested in the possibility of connecting two ZX Spectrum computers together using as simple an interface as possible.
The ZX Spectrum has two mono audio sockets at the back labelled EAR and MIC. To my knowledge these are labelled to match the labelling of the ports on a typical external tape player of the era. There is a single pin on the ULA labelled MIC/TAPE which connects to both of these sockets, meaning that it's a single bidirectional channel with respect to loading and saving.
If I were to write a simple multiplayer game and wanted to allow two Spectrums be connected together, it seems (at first) that it's possible to write a time dividing system where the two systems share the wire and establish a non-simultaneous, interleaved bidirectional link.
My understanding of the designed use of the two sockets is:
MIC is the 'data output' of the Spectrum, and the input to the tape deck. The MIC/TAPE pin on the ULA toggles between low and high logic levels, and is capacitively coupled. The result (combined with the load at the tape deck side) will be a 1V peak-to-peak signal around zero. Unloaded (well, lightly loaded?), it will be inspected as a 4V or 5V peak-to-peak signal around zero.
EAR is the output of the tape deck, and the 'data input' to the Spectrum. It is a 3V or 4V peak-to-peak signal (depending on volume) around zero designed to drive headphones correctly. In the Spectrum schematic, this is capacitively coupled, and the diode and 1K resistor network provide a system that biases the incoming AC signal a little higher so that the troughs are closer to zero and the peaks are raised above logic high, resulting in a signal that can be read by the ULA.
As far as I know there's no issue with using either EAR or MIC as the audio output. The Spectrum manual Chapter 19 suggests using EAR output for headphones or MIC output for amplifiers. I don't see any electrical dangers in connecting EAR to MIC on a single Spectrum (though it wouldn't be very useful - game instructions would explicitly suggest connecting EAR and disconnecting MIC when loading games (and vice versa), but I imagine for convenience having a four-headed EAR-MIC-MIC-EAR cable for loading and saving to a tape deck would be fine).
Therefore, it seems possible that EAR to EAR can be used to possibly transfer data from one Spectrum to another, if the timing of sharing the channel is established to prevent collisions and (as is normal for Spectrum tape loading) rapid waveforms are sent to represent the data rather than logic levels.
Is my understanding of the electrical properties of the two sockets correct? and Can EAR to EAR be connected between two systems or would this fail? (in terms of damage or just not work)
An example of a possible detrimental or even damaging effect would be ground loop hum for example. Is that a risk?
Here is the relevant portion of the Spectrum schematic, which is common to all the different Issues.