Re: Mk14 programming
The PC TxD line will swing -ve -- in fact it will be negative (-12V on many machines) most of the time. So shouldn't you add an inverse-parallel diode to the LEDs in the 'to MK14' section (the red indicator LED and the optoisolator) to protect them against reverse bias?
I also wonder (without trying it) if you could connect the red indicator LED in series with the optoisolator LED and have one series resistor and inverse-parallel diode.
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