having trouble receiving serial data using my USB->serial adaptor

[Werth, Volker]

Volker Werth schrieb am 01.07.2008 13:16
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Am Dienstag, 1. Juli 2008 12:01 schrieb Rohit Grover:
> Hi,
>
> Thanks for the response. I stumbled upon the solution quite by chance.
>
> My USB->serial adaptor has a NULL modem built into it, so attaching an
> external NULL modem was un-necessary and incorrect. It is a wonder that the
> devices survived this treatment.
>
> thanks again.
>
> regards,
>
> > > I am having trouble getting my BAFO 810 USB->serial adaptor to
> > > work. I have built the uplcom and ucom drivers into the kernel,
> > > and my USB->serial adaptor is recognized at boot, as can be
> > > inferred from the following lines from /var/run/dmesg.boot:
> > >
> > > ucom0: <Prolific Technology Inc. USB-Serial Controller, class
> > > 0/0, rev 1.10/3.\ 00, addr 2> on uhub3
> > >
> > > I am using minicom to drive /dev/cuaU0 at 115200 8N1.
> > >
> > > If I connect the Tx and Rx lines (pins 2 and 3) of the DB9
> > > connector, I can get echoes of my keystrokes on the minicom
> > > terminal.
> > >
> > > Unfortunately, I am unable to connect to an embedded device which
> > > supports serial connections at 115200 8N1. When I plug the serial
> > > output connector of my adaptor to a NULL modem and then to the
> > > target embedded system, I don't see any serial output from the
> > > device on my minicom terminal. This same device communicates
> > > correctly with the serial port of my linux desktop with minicom
> > > and 115200 8N1.
> > >
> > > As a further experiment, I manually connected pins 2,3, and 5 of
> > > the serial end of my adaptor to pins 3,2, and 5 respectively of
> > > the embedded device, but still no luck.
> > >
> > > What am I missing?
> > >
> > > regards,
> >
> > Rohit,
> >
> > hard to tell on the distance but if you're able to receive (using a
> > loopback adapter), it sounds like either side is most likely requiring
> > hardware handshake (DSR/DTR, RTS/CTS).
> >
> > Sometimes it helps to short cut the DCD line (to RTS/DSR), as some
> > device refuse to send if DCD is not set. If you're owning a break out
> > box, it should be snap to figure that out. At least you should check
> > using a serial line tester (those little tools signaling line status
> > using LEDs or use a Voltmeter to check for correct line status).
> >
> > Volker

Rohit,

correct terminology is: Instead of the usual DTE (provided by built-in PC 
serial ports), your adapter is a DCE. Connecting that to a DTE (modem, serial 
terminal) does not require a null-modem (crossed Rx/Tx, handshake) cabling.

Connecting DCE to DCE or DTE to DTE always requires a (crossed) null modem 
connection. DTE to DCE requires a straight 1:1 cabling.

Against everybody's assumption, a PC's serial port is mostly always a DTE. I 
think this may go back to the time (some 20 or 30 years) while PCs were seen 
just as smart (dumb?) terminals connected to a mainframe.

HTH

Volker

-- 
Volker Werth
system engineering

Bally Wulff Entertainment GmbH
Maybachufer 48-51
12045 Berlin, Germany

ph: +49(30)62002-109

Bally Wulff Entertainment GmbH, Maybachufer 48-51, 12045 Berlin, Postanschrift: Postfach 44 01 57, 12001 Berlin Tel.: 030-620 02-0 FAX: 030-620 02-200, Geschäftsführer: Sascha Blodau, Wolfram J. Seiffert, Tim Wittenbecher, Amtsgericht Berlin-Charlottenburg, HRB 91532 B, UST-ID DE 234 517 998
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Versand am 01.07.2008 13:16 von Werth Volker