On Wednesday 24 Sep 2003 6:30 pm, you wrote:
> Ian Bell wrote:
> > On Wednesday 24 Sep 2003 4:17 pm, you wrote:
> >> How do they get away with this?
> >>
> >> They have an RS232 line going into a digital I/O port using just a
> >> couple of resistors.
> >>
> >> Also, the 7404 as a line driver. How much cable and speed is that
> >> good for?
> >
> > That's bloody criminal.  Note they have two 7404s in parallel.
>
> Naw, it's called 'letrical injunearing.

Not is my book.  I call it very poor design.

>  Their circuit will work fine in
> probably >90% of the applications.

I am not so sure.  PC RS232 ports still output +-12V and feeding that straight
into a gate via a 10K resistor is going to take the gate inputs outside spec.
The 'normal' way to do a cheap RS232 receiver is to use something similar but
add an overvolt zener and reverse volts protection diode.  Even better there
are some very cheap RS232 receivers available that have these built in.

 > I'm sure that HEX inverter costs
> allot less than a MAX232, plus it doesn't need four expensive
> electrolytic caps.  It probably saves more than a US$1.00 per unit.

I applaud saving unit cost but the hex invertor does not produce a negative
RS232 voltage so does not come anywhere near the RS232 spec.  I can just
imaging all the posts to their message board about lost data.

Except the MAX232 isn't even a good choice.  There a plenty of cheaper devices
that will do the job just as well.

Ian

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads