AVR-Chat

Peter Jakacki wrote:
> Ok, this is good, it's getting meaty and I like some of your figures. 
> This only takes into account a discrete BC548 and not the typical input 
> transistor of the receiver which also happens to be buffered by two 
> other transistors in typical inverter follower fashion.
> 
> Anyway, I enjoyed you reply and I will get back to the group with some 
> observations. I question a lot of the standards in the same way I 
> questions rules that require they be followed although the reasons for 
> them have long since faded. Observe the principle first and the rules 
> take care of themselves. Principles are hard to enforce which is why 
> people make rules. Principles don't change but rules do, although they 
> can be real slow to catch-up at times.
> 
> Tinkering things into existence is not the name of the game but 
> tinkering them out is. This way you know where the fall-off area is and 
> allow a comfortable margin. Ideally this could be designed and simulated 
> but you know for the same reason that you can design and manufacture a 
> BC548 that there are also BC547s and 549s, although they are the same 
> transistor. Due to manufacturing tolerances these parts would fail the 
> desired spec yet are "tinkered" as alternate types.
> 
> Sometimes it's not about margins but about knowing what is redundant and 
> clearing away the clutter. There are a lot of things about 232 including 
> the standard itself that are redundant and I get very tired of people 
> quoting the decades old rules that applied back then without taking 
> current usage into account. Sub 120K low-speed short-range coms to a PC 
> needs high plus and minus volts??? ...come on, nobody but nobody designs 
> like this except to keep the voices happy.

Unless the RS232 line is driven with an open collector/drain output stage
which guarantees close to 0V, i'd not trust even the fastest most modern
RS232 receiver to be reliable. The other problem is that the fastest most
modern RS232 receiver is extra sensitive to line noise when you have a
low switching margin.

Unless you can guarantee that the input transistor has an adequate shunt
base-emitter resistor instead of a diode, unipolar drive will mean the
speed performance of the RS232 receiver will always be in an all warrantees
void area of the data sheet, which should never be allowed.