[Elecraft] [KX3] RS232 xcvr control when serial ports are occupied

Mon Jul 9 12:04:10 EDT 2018

As a ‘field engineer ‘ for a global company in the 80s, I’d occasionally
find one of those RS232 breakout boxes tucked behind some production
system, jumpered to make the connection work, blinking away, because some
technician finally hit upon the magic combination and then left it in place
rather than building a proper cable.
I’ve seen lots of devices whose designers took great liberty with the RS232
standard...a breakout box and some gender changers was indispensable in
working with a variety of serial devices. Sometimes even that plus a
healthy amount of experience wasn’t sufficient, as when marginal voltage
swings, extra long cables, and different ground potential at each end
conspired against you.
Lots of fun!
On Mon, Jul 9, 2018 at 10:10 AM Cady, Fred <fcady at montana.edu> wrote:

> Thanks Don.  It must have been interesting working all that out.  I used
> to enjoy giving my students a history lesson explaining the control and
> handshaking signals in the RS232 connector from basic principles, starting
> with the Bell 103 modem.  That didn't help  explain the terminology and
> signal naming problem that Andy had. Manufacturers were terrible at keeping
> to the standard.  And really, in isolation, it doesn't make sense that a
> signal named TxD would be a input and not an output.  Documentation was
> scanty and so the scope, or my favorite test tool -- the RS232 Blinky Box
> -- had to be gotten out to tell what was what.
>
> The now  used for other purposes signals are:
> DTR -- from the terminal to the modem to tell the modem the Data Terminal
> was ready.  Now used for CW or PTT.
>
> DSR -- From the modem to the terminal to indicate the modem was ready
> (generally power on).
>
> The Bell 103 could send data only in one direction at a time so RTS and
> CTS were included to control the flow of the data.
>
> RTS -- Request to Send from the terminal to the modem to request the line.
>
> CTS -- Clear to send from the modem to the terminal to say the line was
> not busy and to go ahead with sending the data.
>
> RTS and CTS were also used to control the data flow from a fast device to
> a slow device.  I think there were some kenwood radios that used this
> scheme.
>
> DCD -- Data Carrier Detected was used for the modem to tell the terminal
> that the carrier from the other data set was being generated (the phone
> line was working).
>
> RI -- Ring Indicator asserted by the modem when it heard a ringing signal
> at the other end of the telephone line.
>
> As I remember it.
>
> Cheers,
> Fred KE7X
>
>
>
>
> ________________________________
> From: elecraft-bounces at mailman.qth.net <elecraft-bounces at mailman.qth.net>
> on behalf of Don Wilhelm <donwilh at embarqmail.com>
> Sent: Sunday, July 8, 2018 10:04 PM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] [KX3] RS232 xcvr control when serial ports are
> occupied
>
> The computer is the DTE device, and those devices designed to connect to
> the computer via the serial port are normally wired as DCE devices.
>
> The original thought when the IBM PC serial port was designed was that
> the PC would be used as a terminal connected to a modem, thus the
> decision was that the PC should be a DTE - and for the first several
> years the PC was used only as a terminal device.  Yes, I was involved
> during those early days, but not as a part of that decision making
> process - I was involved with the IBM PC modems.  My manager said I had
> "M" engraved on my forehead.
>
> Now we have the situation where the computer is the controller.  It
> would be very confusing to change the wiring of the computer, so it
> remains as a DTE, and connecting devices (while not modems) are
> configured as a DCE,
>
> 73,
> Don W3FPR
>
> On 7/8/2018 10:15 PM, Cady, Fred wrote:
> > Hi Andy,
> >
> > Don is correct.
> >
> > Here is a little blurb that might help.
> >
> >
> http://www.ke7x.com/home/miscellaneous-k3-information/unexpected-agc-behavior/rs232-interfaces-1
> >
> >
> > ________________________________
> > From: elecraft-bounces at mailman.qth.net <elecraft-bounces at mailman.qth.net>
> on behalf of Don Wilhelm <donwilh at embarqmail.com>
> > Sent: Sunday, July 8, 2018 7:21 PM
> > To: ANDY DURBIN; elecraft at mailman.qth.net
> > Subject: Re: [Elecraft] [KX3] RS232 xcvr control when serial ports are
> occupied
> >
> > Andy,
> >
> > No TXD is TXD all the way through the path.  Yes, the DTE TXD will have
> > the drivers, and the DCE will have receivers, but it is TXD from end to
> > end.  RXD is the other way around because the DCE drives that line.
> > Trsnsmit and receive are named with respect to the DTE.
> >
> > 73,
> > Don W3FPR
> >
> > On 7/8/2018 8:39 PM, ANDY DURBIN wrote:
> >> "TXD is really pin 3."
> >>
> >>
> >> One end's TXD is the other end's RXD. That's obvious to those who have
> used RS-233 for a while but if someone needs to ask then it may not be
> obvious. Find the TXD pin on the source and connect it the RXD pin on the
> destination. Sometimes it's easier to look with a scope than to try to
> understand the documentation.
> > ______________________________________________________________
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