[sdiy] Matrix 1000 repair, unexpected hack

[Colin f]

> On my schematics CTS is tied to 'pulse' which is derived from an 
> analog mux summing all of the voice outputs. It looks like the mux is 
> controlled by a 'mute'.  There is a comparator in there looking at 
> the audio levels. I don't understand it at all but would venture a 
> guess that it is part of some type of calibration or self-test where 
> they want to inhibit transmission of MIDI.  Why, I don't know, but 
> with the circuitry, CTS is disabled when there is audio output so I'd 
> guess you'd never see it in 'normal' operation.

I checked the datasheet for the 6850.
The CTS line disables the transmit data register empty bit, which could be
used to inhibit MIDI transmission, but why wouldn't you just do that in
software ?
Maybe this connection is being used to prioritise an interrupt from
somewhere else, by inhibiting transmit complete interrupts just for a short
period ?

Another possibility is that they simply ran out of input pins to the CPU,
and realised they could kludge the CTS input for the purpose - the state of
the CTS input is available to the CPU from the UART status register.
Curious...

Cheers,
Colin f