[time-nuts] Lucent KS-24361 GPSDO arrived today several questions

Sat Nov 8 13:44:11 EST 2014

Hi

There are a wide range of chips you can use for “proper” RS-422. There are also a range of USB RS-422 adapters for < $25. I would investigate the adapters, starting with cheap ones that claim FTDI chips inside. I’d also look for a claim of Windows 8 drivers. That’s  not so much because I *use* Windows 8, but because it implies they have checked in on a modern OS.

The adapters I’m using are 12V powered RS-422 to RS-232 converters that I’ve had forever and ever. I probably got them on eBay a few centuries ago. Mated up with generic 232 to USB converters they seem to work just fine for what I’m doing. 

One very real question - do you *need* to talk to the box? About 99.9% of the time in my setup, the answer is no. Check the LED’s, make sure all is fine, move on. The serial cable is just one more ground loop to worry about. Checking it when you fire it up for the first time - sure, you need to do that. Stewart’s approach works *fine* for doing that. It’s also ok for a check once every six months to a year to see if the beast is happy or not. 

If you are going with a home built converter, start with the FTDI cables from Mouser. Either:

http://www.mouser.com/ProductDetail/FTDI/TTL-232R-5V/?qs=sGAEpiMZZMuGxYVy11yKKrDbHO9RZyo6Pg7KqAgrCmE%3d

or it’s wire lead cousin. Both are $20. Both are legit FTDI gear. Yes you can get similar stuff on eBay for about half the price. Who knows which chip is in what in that case. 

Either a perf board and DIP parts or a $2 custom pc board (1” square is *plenty*) and SMT’s will give you a fine home-brew RS-422 converter. When you are not using it for RS-422, unplug the board and use the adapter for other stuff. Maybe make a 485 and a 232 adapter as well. All the plugin boards and parts probably won’t double the total project cost over just buying the cable. 

————

I’m not at all sure I should bring this up, but there *is* a pps on the output of the KS-24361. If one wished, they could do a board that did 422 to 232 and put the PPS on some pin that a NTP driver might be happy with. The whole NTP driver / GPS time offset / Leap second issue is one we have spent a lot of time on. I might  also could convert the pps to CMOS logic and hook that up to a coax connector. If you did NTP  then serial would be a full time sort of thing.  

My guess is that you would eventually go to some sort of pc board to get at the PPS, NTP or not. Do you pull it off the interface connector (and maybe grab the GPS data as well?). Do you get it from the PPS/RS-422 connector and keep things isolated? 

Lots to think about. Lots of choices. No one one size fits all solution. 

Bob

> On Nov 8, 2014, at 11:47 AM, Bob Stewart <bob at evoria.net> wrote:
> 
> 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 <kb8tq at n1k.org>
> To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
> 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 <alanh137 at gmail.com> 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
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> 
> 
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