Timothy J. Weber wrote:
>> I have a breakout box from B&B Electronics, which is designed exactly 
>> this
>> way. However, first of all -- good luck drawing 30 mA from a serial 
>> port...
>> maybe at 0V, and only with *some* ports. Second, and I should have 
>> probably
>> mentioned it, I would like to affect the lines as little as possible. We
>> have designs which are powered from the port, and it's not a good idea to
>> add more load to the port (voltage drops below threshold).
>
> I've done this before with a PIC12F629 watching the lines and driving
> the LEDs.  You can also easily add a minimum on-time, if appropriate, to
> ensure that even brief activity is visible - I used 100 ms.

Yeah, but how do you determine that the line is not connected?

>From what I understand, there's no way to do what we want, unless we use a 
negative supply of some sort (I'd like to stick to +5V, for a number of 
reasons). For now, we've decided to use the INVALID pin of MAX221 to detect 
the presence of voltage on each of the lines. We happen to be using this IC 
in a production unit, and parts cost is not an issue for this in-house 
project -- but the whole thing is so hacky, it's disgusting...

-Vitaliy

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist