AVR-Chat

That was funny Russ, this may be more true of a mosfet's gate charge, 
not a bipolar which works quite a bit differently (minority carrier 
charge, electron holes, etc). Anyway, what speed are we talking about? 
100Mhz? 10Mhz? 1Mhz? The speeds that RS-232 operate at can be handled 
easily by the slowest of the slow transistors.

Have you seen the reverse diode across the base emitter? This would 
clamp the voltage to -0.3V internally anyway.

Engineers are people that get their hands dirty, right? Ok then, how 
about hooking-up a scope to the receiver and driving it at say 115.2K 
via a quick and dirty 100K series pot so that you can vary the input 
voltage. This will confirm the reliable turn-on threshold and by placing 
a series diode you can cutout the negative transition (although this is 
not as good as 0-5V drive) to confirm that part too. If a diode is used 
then it really needs a 1K pull-down resistor after it.

Since I suggested this and since I design all kinds of coms gear anyway, 
I will run some real tests and post the results to settle this matter in 
a less academic but in a more "engineer" like manner.

*Peter*


Russell Shaw wrote:
> Peter Gargano wrote:
>> stevech@san.rr.com wrote:
>>
>>> *RS232C is such that if the voltage swings +/- 6V it is in compliance.*
>  >
>  > But may receivers will work at a swing of 0 to 5 volts.
>  >
>
> Maybe, but what about max speed reliability? Negative drive removes transistor
> base charge faster.