On Wed, 24 Sep 2003 19:38:40 +0100, you wrote:

>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.

There is no such thing as 'poor design'.=20
"Inappropriate design", yes.=20
All designs have diferent criteria. Not using an RS232 driver on a $1000 =
product would be
inapproprate.=20
Using an expensive driver for a very low-cost product where cost is the =
top priority would be
equally inappropriate. =20
Similarly in an ultra-low power battery operated device which needs =
occasional connection to RS232
for configuration etc., use of any external drivers may be inappropriate =
due to extra power
consumption. =20


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

Make that 99.99%

>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.

CMOS inputs have clamp diodes which will limit the voltage. Using =
extermal inverters limits any
possible minor ill-effects of clamp diode current to the inverter chip =
(doing it direct into a PIC
can do things like shift ADC results).

> > 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.

I highly doubt it - I've never come across a single RS232 port that =
doesn't work fine with 5V levels
(many thousand products in the field).
Has anyone ...?
=20

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