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'[EE] Clairfy RS232 signal directions'
2005\07\19@133356 by Vic Fraenckel

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Given an RS232 cable with just 3 wires thus:

Pin 2 - RXD
Pin3 - TXD
Pin5 - GND
connected to a DB9 on a computer and going to some sort of device.

1. Is this the proper way to name the wires?
2. Does RXD imply  a signal going TO a computer FROM the device?
3. Does TXD imply a signal coming FROM a computer TO the device?

This always confuses me but I seem to be able to muddle through.

Any enlightenment will be appreciated.

Regards,

Vic
________________________________________________________

Victor Fraenckel - The Windman         victorf ATSIGN windreader DOTcom
KC2GUI

2005\07\19@140609 by Dave VanHorn

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At 12:33 PM 7/19/2005, Vic Fraenckel wrote:
>Given an RS232 cable with just 3 wires thus:
>
>Pin 2 - RXD
>Pin3 - TXD
>Pin5 - GND
>connected to a DB9 on a computer and going to some sort of device.
>
>1. Is this the proper way to name the wires?
>2. Does RXD imply  a signal going TO a computer FROM the device?
>3. Does TXD imply a signal coming FROM a computer TO the device?

That's usually the convention.  When I do a schematic, the signal
names are "Me-centric".
RXD is coming to me, and TXD is coming from me.

Of course my RXD must connect with the other guy's TXD, and some
people see that as confusing, so they want to label a given signal
TXD, on both sides.


2005\07\19@141657 by Herbert Graf

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On Tue, 2005-07-19 at 13:33 -0400, Vic Fraenckel wrote:
> Given an RS232 cable with just 3 wires thus:
>
> Pin 2 - RXD
> Pin3 - TXD
> Pin5 - GND
> connected to a DB9 on a computer and going to some sort of device.
>
> 1. Is this the proper way to name the wires?
> 2. Does RXD imply  a signal going TO a computer FROM the device?
> 3. Does TXD imply a signal coming FROM a computer TO the device?
>
> This always confuses me but I seem to be able to muddle through.

Welcome to the club, I think everyone is confused be this all the time!

Personally, I connect a cable, get my meter out and measure to see
whether voltage is on pin 2 or pin 3, whichever has ~-10V on it is the
TX of the device in question. It's often happened that a device I'm
trying to interface to uses the pins contrary to the way they should be
used (hence the $30 special cable they charge for).

TTYL

-----------------------------
Herbert's PIC Stuff:
http://repatch.dyndns.org:8383/pic_stuff/

2005\07\19@143256 by Harold Hallikainen

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Back in the 25 pin days of EIA232, I'd remember it by saying "terminal
talks on two." A "terminal" (DTE) transmits on pin 2, while a modem (DCE)
talks on pin 3. When they moved to 9 pin, they reversed these. So, a
terminal (like your computer emulating one) talks on pin 3 of a 9 pin
connector. It listens on pin 2.

Good luck!

Harold

{Quote hidden}

> -

2005\07\19@143303 by Bill & Pookie

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The cable is just wires hooked to pins.  It's the device that matters.  I
think of the RXD pin on the device going to the receive pin on it's UART.
The TXD on one device goes to the RXD on the other device.  If both devices
use the same pin for RXD then you need to cross the RXD & TXD wires before
they get to the other device.  This is called a "Null Modem" cable..

This is not a complete answer but I am tired, hungry and Pookie says "It's
time for noon nap."

Bill

{Original Message removed}

2005\07\25@175216 by Wouter van Ooijen

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> Personally, I connect a cable, get my meter out and measure to see
> whether voltage is on pin 2 or pin 3, whichever has ~-10V on it is the
> TX of the device in question.

Unfortunately some receivers seem to put out a respectable negative
voltage unless loaded :(

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2005\07\25@175217 by Wouter van Ooijen

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> Pin 2 - RXD
> Pin3 - TXD
> Pin5 - GND
> 1. Is this the proper way to name the wires?

No, you can't name the wires. You are naming the pins!

> 2. Does RXD imply  a signal going TO a computer FROM the device?

Yes, if you are talking about the pin of the computer's connector.

> This always confuses me but I seem to be able to muddle through.

stop trying to name the wires, name the pins, connect TxD to RxD.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


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