Problems getting data out of an RS422 port

Problems getting data out of an RS422 port
Posted by aroby on 07 Nov, 2014 16:51
I picked up a pair of Lucent GPS / oscillator devices off eBay (see http://www.ebay.com/itm/LUCENT-SYMMETRICOM-Z3810AS-KS24361-L101-L102-HP-KIO-OEM-GPSDO-TIMING-SYSTEM-/321560316836?), along with a number of others from the time-nuts mailing list.

I cannot for the life of me get any data out of the RS422 diagnostic ports. I have tried a hack connecting the RX- and TX- ports to the RS232 TX and RX ports (which seems to have worked for everyone else). I have tried an RS422 to USB cable. I've tried an RS422 to RS232 plug convertor. Nothing works.

Yet, if I connect the box to the PC using the cable hack and put a scope on the TX-, I can send commands from a terminal program and decode RS232 data coming out (see attached) - just nothing gets back to the PC.

Has anyone encountered similar situations and have been able to solve them? I'm at a loss as to what to try next.

Anthony

#2 Reply
Posted by aroby on 07 Nov, 2014 17:58
Quote from: sunnyhighway on 07 Nov, 2014 17:22
Have you tried crossing the TX and RX lines?

Yes, tried that. I have other scope shots where I sent the command from the PC, it hits the device and then I see the data on the scope coming back from the device. I just can never get the PC serial or USB ports to pick up the data. My guess is it's to do with voltage levels or noise.

#3 Reply
Posted by texaspyro on 12 Jul, 2016 07:42
I also got in one of the Z2812A "ref0" units and added back in the Motorola UT+ GPS receiver per Peter Grade's instructions. I have it working now as a stand-alone GPSDO (and have Lady Heather working with it).

I also experienced RS-422 communications issues when feeding the RS-422 data into a RS-232 serial port. One a desktop box with a real serial port it drops most of the characters. On a laptop with a real serial port it occasionally drops characters. It works fine with a cheap Chinese USB to serial connector. It seems the levels that the RS-422 port puts out are on the edge of what a lot of RS-232 ports will accept.

#5 Reply
Posted by blacksheeplogic on 02 Aug, 2016 02:37
Quote from: texaspyro on 12 Jul, 2016 07:42
I also experienced RS-422 communications issues when feeding the RS-422 data into a RS-232 serial port. One a desktop box with a real serial port it drops most of the characters. On a laptop with a real serial port it occasionally drops characters. It works fine with a cheap Chinese USB to serial connector. It seems the levels that the RS-422 port puts out are on the edge of what a lot of RS-232 ports will accept.

That's more likely a flow control issue, use HW flow control, signal level shouldn't be an issue on a short cable, the specifications very broad. You can also pick up powered line extenders very cheaply which convert RS232 <-> RS422 if your using long cables, for example WTI LD-4 Line Drivers. I've had some issues in the past with the cheap USB RS232 serial interfaces so try a different brand. If you do a lot of serial it's worth picking up a serial protocol analyzer, they cost next to nothing these days for an old HP unit.

#6 Reply
Posted by texaspyro on 02 Aug, 2016 04:31
Quote from: blacksheeplogic on 02 Aug, 2016 02:37

That's more likely a flow control issue, use HW flow control, signal level shouldn't be an issue on a short cable, the specifications very broad. You can also pick up powered line extenders very cheaply which convert RS232 <-> RS422 if your using long cables, for example WTI LD-4 Line Drivers. I've had some issues in the past with the cheap USB RS232 serial interfaces so try a different brand. If you do a lot of serial it's worth picking up a serial protocol analyzer, they cost next to nothing these days for an old HP unit.

Nope, i't a signal level issue on the RS-232 line receivers in the computers (both have genuine hardware serial ports, USB to serial adapters work fine). The GPSDO sees commands sent to it just fine, but the returned data is garbaged. Not a flow control or buffering issue either. And I have protocol analyzers most people only dream about...

#7 Reply
Posted by blacksheeplogic on 02 Aug, 2016 05:09
Quote from: texaspyro on 02 Aug, 2016 04:31
Nope, i't a signal level issue on the RS-232 line receivers in the computers (both have genuine hardware serial ports, USB to serial adapters work fine). The GPSDO sees commands sent to it just fine, but the returned data is garbaged. Not a flow control or buffering issue either. And I have protocol analyzers most people only dream about...

You need a signal ground.