2 transistor interface

[Dig]

> -----> Peter Writes:
> 
> >I've had a few questions about the two transistor
> >ALDL
>
>http://www.andywhittaker.com/ecu/images/Simple_ALDL_Interface.gif
> >I'm sure this circuit will work on certain PCs. The
> >problem
> >is that the voltage levels on pin 2/9 (RxD) vary
> >between 0
> >and +12 Volts. Unfortunately, RS232 is not
> guaranteed
> >to
> >work reliably at a lowest voltage level of 0.
> 
> I've been running a self-powered circuit for some
> time
> now, Based on a 7805 and a MAX 232. I use DTR and
> RTS
> for power (can turn the cable on and off via
> software),
> and it works fabulously. In fact, I've yet to find
> a PC that it didn't work on, and I've tried old 
> laptops to the latest and greatest desktop CPUs. 
> 
> I've looked at the 2-transistor circuit, and I
> actually
> think it will work better than my current design
> when
> self-powered. It has the advantage of having less 
> parasitic current draw than the MAX and the 7805, so
> it should be able to drive even the most stubborn
> 232
> port. I have yet to test one, but I hope get to it
> shortly.
> 
> > OTOH, most
> >laptops seem to use CMOS transceivers that will
> record >a logic
> >LO at a voltage level of 0 Volts.
> >The two transistor circuit may also be more
> >susceptible to noise pickup.
> 
> I don't think it's that picky, as long as you keep
> your cable length sane. Good grounding and sheilding
> practices should keep noise problems to a minimum.
> If anything has a problem, it will be a system with
> multiple listeners/chatterers on the ALDL bus.
> 
> Dig
> turbodig at yahoo.com
> 
> 

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