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The quartz crystal and the 74393 provide the heart of the device: the 31.25 kHz MIDIbitrate-signal.
The 4017 and 4015 form a serial-to-parallel-convertor, which places
incoming MIDI-bytes on the bus (b0..b7). If it receives a note-on- or note-off-statusbyte
on the selected channel, the bottom flip-flop (of leftmost 4013) is SET while the upper
flip-flop of that same (leftmost) 4013 remembers whether it was an ON- or an OFFmessage.
The bottom part of the rightmost 4013 just counts the number of databytes
received: 0, 1, 0, 1, 0, etc. (thus supporting running-status-protocol). Finally, the upper
flip-flop (of the rightmost 4013) accumulates all velocity bits to support switch-off with
null-velocity. The rest is just trivial latching, multiplexing and some logic-gates.
Note that quartz crystals with higher resonance frequencies are often easier to obtain
(and cheaper) than low frequency crystals. In the circuit above, one may use crystals up
to 8 MHz. As the market pushes us to higher and higher frequencies, it may even be
possible in the future to cut costs by building, let's say, a 32 Mhz oscillator and adding a
binary counter to obtain the 31.25 kHz MIDI-clock-frequency. For the opto-coupler,
CNY17-1, 6N138, or 6N139 may also be used but watch the pin-layout!
There might be errors in the diagrams above. I built 2 such devices (prototypes) which
operate properly, but I made these drawings afterwards and I cannot really guarantee
they are 100% correct. So if you don't understand basic building-blocks like shiftregisters,
latches, multiplexers, gates, etc., you'd better not start with it. I hope the timing
diagram is of some help though.
Anyway, if you encounter any mistakes in these drawings, please e-mail me. But please
do not ask me whether I can build this hardware for you, or whether I can sell it to you
(for I am a designer, not a Plumbum-sniffer:-). Well, happy soldering then!!
A4-3