AVR-Chat

Peter Jakacki wrote:
> 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.
 >
 > 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).

If you know how BJTs work, you'll know about stored base charge in saturated
switching.

 > 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.

Well, old technology without a baker clamp or gold-doped switching
transistors (or a homebrew job) may use an NPN common-emitter input
transistor like a BC548 (Beta=300, ft=300MHz etc).

If this was driven with ~5V via a 10k resistor (with a base-emitter diode),
the stored charge would be atleast 68pC, excluding depletion
capacitances and excess storage in the base-collector epitaxial layer.
The input thresh-hold would be ~0.7V.

Now with 0V applied, the base charge can only be removed by 0.7V
across 10k. The switch-off delay-time is then ~1us. 115200 baud is 8.7us/bit.

Not all logic pulls down to ground or to supply, so the RS232 drive
could be 0.8V->3.3V (TTL: http://www.interfacebus.com/voltage_threshold.html)
which makes it impossible at any speed. If the pull-down voltage was more
typically 0.5V, that would be a delay time of 3.4us. The noise margin is
ratshit too.

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

Only after the stored base charge is rapidly removed.

 > 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.

It would be advisable to use a 1k shunt if you were attempting unipolar drive
anyway.

 > 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.

I never "tinker" things into existance, which is *not* engineering.
I write electronics simulation software and did a masters research
degree that included radio-frequency transistor measurement, modelling, and
circuit simulation on both BJTs and mosfets. After that, i designed and
built radio-link fading simulators. I'm an analog/radio-frequency designer,
and only got into software and computer hardware by necessity.

-- 
Russell Shaw, B.Eng, M.Eng(Research)