WP34s Repurposing Journey PhotoJournal

[matthiaspaul]

(02-20-2015 11:37 PM)Marcus von Cube Wrote:  

(02-20-2015 11:02 PM)MarkHaysHarris777 Wrote:  I'm wondering if Marcus controls the RS232ON input to the Q2 BC856A transistor?

I don't think so (but may not remember correctly).

I haven't looked this up in the sources, but according to one of Harald's photos showing how he connected the IR LED,

http://www.hpmuseum.org/cgi-sys/cgiwrap/...186#223186

you are using PC7 (that is, the port pin driving the "RS232ON" signal) to control the IR LED. Fortunately, PC7 / pin 94 is one of the few of the microcontroller's "high-power" pins able to source up to 4 mA - most of the other pins available can source only a fraction of it and would not be able to reliably drive a LED in the first place.

Still, in my own experience with other designs, it is better to avoid drawing much current from port pins in designs with weak power planes like this coin-cell powered device (and in the case of units with USB board even over long thin wires).

I think it would be better to use a transistor to drive the IR LED. This, combined with a small resistor (perhaps 10R) and a 10uF low-leakage & low-ESR capacitor to decouple the IR LED's anode from the VBAT plane, would most probably fix those reset problems some have observed when trying to print with a somewhat exhausted battery.

See also:

http://www.hpmuseum.org/forum/thread-2214.html
http://www.hpmuseum.org/forum/thread-1773.html

Alternatively, trying to support the VBAT plane by adding a 10uF low-leakage & low-ESR capacitor close to the CPU in parallel to C15 or C16 might help as well.

Quote:The problem with adding the RS232 level converter is that the original cable no longer works (it will probably cause harm).

Definitely.

Quote:And who has an RS232 interface with that crude connector anyway? I think the provision for such a chip was for a reuse of the platform as an automation controller with custom firmware.

I also don't think it would be a good idea to retrofit the ADM3202AR for as long as the calculator remains powered by two coin-cells only. The level translator should better be powered by the other side, either explicitly as through USB, or parasitically through RS232 hardware handshake lines.

Greetings,

Matthias