Hi Bob,
I've been using the "cheat" method that Stewart posted. It's mostly working,
but I do see occasional errors. So, I ordered the following USB to RS-422
adapter. Now your post makes me worry that there's still going to be a
problem. I'll post results back to the list. It is a FTDI device. Hopefully
it's a "real" FTDI device and not subjected to getting bricked.
USB 2.0 to RS-485/RS-422 RS485/RS422 DB9 Serial Adapter Cable FTDI FT232 FT232R
And I was also wondering if anyone has put together an adapter using LTC-1485
or similar chips?
Bob
From: Bob Camp <[email protected]>
To: Discussion of precise time and frequency measurement <[email protected]>
Sent: Saturday, November 8, 2014 10:10 AM
Subject: Re: [time-nuts] Lucent KS-24361 GPSDO arrived today several questions
Hi
You make another good point, that is worth repeating.
With RS-422, it’s not just the usual RX to TX and TX to RX confusion. You can
have all that right and still get it wrong. The + and - of each must be
properly identified and connected.
That sounds easy, but there is a catch. If you are used to RS-232, you are used
to a negative voltage being a “one” and a positive voltage being a “zero” data
wise. It’s upside down from the conventions on say, CMOS logic (one = more
positive voltage).
RS-422 uses a convention where the + output is higher than the - output when
you have a “one”. That’s the normal idle state for a serial output. Again, it’s
a simple DVM check on something it’s VERY easy to get wrong. Simply put - at
idle, the + output is the one that should be the higher voltage (3.5V in this
case).
Of course one should beware - It’s very easy to run though the archives and
find *lots* of places I get this kind of stuff mixed up. Like I said, it’s not
as easy as it seems.
(It’s even more confusing when you start talking about the control lines …
thank goodness we don’t seem to have CTS and RTS involved on these boxes).
One further disclaimer, this is all for RS-422. That’s what the HP / Lucent
boxes have on them. Mil-STD-188-114B signaling is very similar, but without
the 2.5V offset on the levels. They *should* talk to each other,
troubleshooting a mixed setup like that is a bit more complex. If your adapter
is actually a 114B adapter, things will be a bit different when you mate it all
up.
All of this differential signaling stuff is designed to be used in noisy
environments. You *can* have a 2V sine wave driving one end’s ground compared
to the other end and still get perfect data transmission. With some driver /
receiver combos you can have a lot more than 2V and still do 100Kb/s error
free. It’s also great for setups where you want to isolate the grounds of two
systems. It’s not as good as opto isolation, but it’s a lot better than a hard
ground wire.
Bob
> On Nov 7, 2014, at 9:09 PM, Alan Hochhalter <[email protected]> wrote:
>
> I got in a hurry and made mistake #1 listed by Bob a few replies back. I
> hooked one up to the RS422/1PPI port transmit pins (same as on J8) and was
> receiving data fine in the PC so I just hooked up the other pair of
> terminals to the receive pins for J8 and couldn't get any response to
> commands.
>
> I finally found that I had initially connected the Lucent box ouput pins to
> the wrong terminals on the converter. The Time code data on J6 was still
> getting into the PC even though it was connected wrong. So I just wired
> the Lucent box input pins to the other pins on the converter that are
> really inputs and couldn't get J8 sto work either.
>
> What I ended up with to make it work is: J8-9 to RX-, J8-5 to RX+, J8-8 to
> TX- and T8-4 to TX+.
>
> Alan
> _______________________________________________
> time-nuts mailing list -- [email protected]
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
