cheap RS-232 conversion

Questions on UARTs, and serial communication
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YahooArchive
Posts: 1463
Joined: Fri Oct 19, 2012 5:11 am

cheap RS-232 conversion

Post by YahooArchive » Fri Jan 18, 2013 4:52 pm

OMG, I loathe the lack of edit capabilities here in the Yahoo Groups...

The link below is broke, as it picked up the trailing comma as part of the URL.

You can visit Dale's page here:

http://www.botkin.org/dale/rs232_interface.htm

--- In ARMexpress@yahoogroups.com, "Tod A. Wulff" wrote:
>
> I have seen some recent dialog here and elsewhere about interfacing
Microcontroller's GPIO to a Host PC's RS-232 Serial Port.
>
> I came across this site a couple of years ago. It is ran by a Ham Radio
operator (an ingenious bunch, they are...).
>
> If you visit http://www.botkin.org/dale/rs232_interface.htm, I think that you
will find that the guidance therein makes complete sense. I have successfully
used the 2-Resistor/2-MOSFET IO solution on numerous occassions. I haven't yet
had it not work for me.
>
> Frankly, is is the simplest, cheapest, and easiest interface I have came
across yet.
>
> Enjoy.
>
> -t



YahooArchive
Posts: 1463
Joined: Fri Oct 19, 2012 5:11 am

Re: cheap RS-232 conversion

Post by YahooArchive » Fri Jan 18, 2013 4:52 pm

This is pretty similar to the ckt on pins 1,2 of the ARMexpress, except its
pulling to GND. On the ARMexpress there is a diode ORing of the status and RXD
line to to generate the negative "supply"

Why pull below GND?

I can't speak for today, but in the days of the BASIC stamp, many RS-232 devices
needed the input to go below GND to be recognized. In fact there were some
laptops of that day that did not like Stamps because the Stamp did not always
pull below GND.

Now its quite possible with the integration of the PC, meaning everybody is
using the same handful of IO chips, that it might not need the RS-232 line to go
below GND anymore.

This is basically just a warning, if you find the ckt is flakey with your PC,
and also realize you probably can't push this much beyond 19.2K especially if
you use a long cable.

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