[ntp:questions] almost decided which new PPS GPS to buy

David Lord snews at lordynet.org
Mon Mar 5 01:26:51 UTC 2012


Ron Frazier (NTP) wrote:

......

> 
> I may try pulling the signal directly from the PPS directly to DCD as 
> you describe.  If it works, the procedure might be a bit simpler.  By 
> the way, are the data lines on the serial port of the Sure board running 
> at standard RS-232 levels?

The TxD and RxD data lines are via an RS232 chip

> 
>>> D) Good timing performance.
>>>
>>> E) High sensitivity for potential indoor operation.
>>>
>>> F) David Taylor's excellent tutorial on setup:
>>> http://www.satsignal.eu/ntp/Sure-GPS.htm
>>>
>>>   From my reading, it appears that I need to add the following to the
>>> Sure Electronics package of parts:
>>>
>>> 1) DB-9 Male - DB-9 Female serial cable
>>>
>>> 2) Motherboard header - DB-9 port adapter
>>>      
>> Sorry, what is this for?
>>
>>    
> 
> Only one of my PC's has a serial port.  There is no external connector 
> available on the rear panel.  There's only a row of header pins on the 
> motherboard.  This adapter will plug into the motherboard header pins on 
> one end and provide a traditional DB-9 male connector on the other end.  
> It has a metal plate which screws on to one of the card slots, so the 
> DB-9 connector is exposed at the rear of the computer.
> 
>>> 3) Plastic box for mounting.  Misc hardware like screws, standoffs, etc.
>>>
>>> (David T. or others, I don't suppose you can suggest any specific part
>>> numbers for those hardware items from Radio Shack [do you have those in
>>> the UK], Mouser, or DigiKey.  Just thought I'd ask.)
>>>      
>> A serial line you can buy at your local dollar/euro/pound store.
>>
>>   
>>> My plan of attack is as follows, although the sequence may vary:
>>>
>>> A) Get the unit working "at all", using USB NMEA data only.  I would
>>> hope to see performance equal to my BU-353.
>>>
>>> B) Get the unit working with NMEA data from the RS-232 port, going
>>> through a Trendnet TU-S9 USB - Serial converter, based on the Prolific
>>> chipset.  Performance should, again, be similar to the BU-353 (which
>>> also uses the Prolific chipset internally).
>>>      
>> ?? not sure what you mean by this. Note that you are going to get of the
>> order of 10ms ( after correcting for the 500 or so ms offset of the nmea
>> output). Part of the problem is that the nmea sentences are of variable
>> length. Each character takes about 1ms (at 9600 bd) to come out.
>>
>> You need the pps to get good timing (sub ms)
>>
>>    
> 
> Internally, the USB GPS I have, the BU-353, is using a Prolific serial - 
> USB chipset.  So, when I plug it in, it creates a virtual com port.  
> However, it doesn't pass any handshaking signals as far as I know.  Or, 
> perhaps they're just not connected.  The TU-S9 USB - serial adapter I 
> have uses the same chipset, but it DOES pass all the handshake signals 
> coming from the DB-9 port.  So, I want to connect a serial cable to the 
> new GPS, and the other end of that to the TU-S9, and the other end of 
> that to a USB port on the PC.  If I have PPS active on DCD, this SHOULD 
> bring the PPS into the PC through the TU-S9 adapter.  Yes, it will be a 
> USB message, not a true hardware interrupt.  But it should at least work.
> 
> I'm already getting better than 10 ms just using NMEA data only on a USB 
> virtual com port.  I've got the GPS outputting data at 57,600 baud to 
> minimize latency and jitter.  I'm also outputting the GPZDA sentence (as 
> recommended in one of the Trimble manuals).  I'm pretty sure this 
> sentence is fixed length.  It only has time in it, and a couple of other 
> minor things.  I'm getting + / - 6 ms accuracy polling the device, on a 
> USB port, every 8 - 64 seconds.  Jitter is averaging 2.4 ms or so.  So, 
> I'm already less than 10 ms, at least just a bit.  I'm hoping to get + / 
> - 1 ms performance once I get USB going through an adapter with 
> handshaking output.

The serial data doesn't use handshake which is why using
a high baud rate might give problems.

You need to use PPS to get lower offsets. I've no idea if
PPS will work via a serial to usb adapter but I'd guess
that it will and you will get an improvement.

David


>>> C) Get the unit working with NMEA data from the RS-232 port, going to a
>>> real serial port.  I have to tinker with the motherboard on another
>>> computer to do that.
>>>
>>> D) Get the unit working with NMEA + PPS, with data from the RS-232 port,
>>> going through the TU-S9, which claims to pass all the handshaking
>>> signals.  I'll have to do the mods that David recommends on the board
>>>      
>> See above for this.
>>
>>   
>>> for this.  Hopefully, the same procedure will still work on the recent
>>> boards.  This is where I'm hoping to get + / - 1 ms performance.
>>>
>>> E) Get the unit working with NMEA + PPS, with data from the RS-232 port,
>>> going to a real serial port on the other computer.
>>>
>>> This should be an interesting adventure, hopefully ending with a time
>>> server running at 100 us accuracy or better.
>>>      
>> You can get 2-3us if you use a real serial port with dcd interrupt.
>>
>>
>>    
> 
> 2 - 3 us would be great.  Even 100 us would be great, since my needs are 
> not critical.  On a USB port, I will consider 1 ms to be great.
> 
>>> Sincerely,
>>>
>>> Ron
>>>
>>>      
> 
> 



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