> If you just put a pullup
> PNP on the RS-232 Tx line, with a 4.7K pulldown resistor to a half-wave
> rectifier powered off the host PC Rx line, the RS-232 would be fine.
Yeah, but that would add a few components.....

> I haven't hashed out the exact code for a charge pump like you have
> there, but it seems like one could use a 28-pin PIC, which has 2
> hardware PWM outputs, set them as complements of each other going to
> the intermediate diode tap points, and have a really nice stable
> and adjustable Vpp generator.
Vpp does not need to be adjustable (at least not for a 16x84), it must be
(from memory) between 12 and 14V, and you can get away with somewhat lower
once programming has been enabled by ~ > 12V. The Vpp generator just
outputs a square wave, with the two pins at opposite polarities. The
frequency must be high enough, and some extra pumping was needed after
sending or receiving a char (which is 'busy wait', hence no pumping).

regards,
Wouter