[LinuxPPS] Oncore Refclock interface & thermostat

[Hal V. Engel]

On Monday 05 January 2009 22:46:19 Remco dB wrote:
> Again, but now a new subject ;-)
> Hal,
>
> I will answer the thermostat questions later.
>
> I have the same setup as you concerning the interface and use USB as power
> supply.
>
> See my weblog http://remco.org and scroll down, or look at
> http://rembl.org/wordpress/wp-content/uploads/2008/04/oncore_iface.jpg
>
> Remco

I do this a little different since I use a TTL to RS232 converter kit that I 
buy from an ebay vendor and my PPS interface also looks different.  The TTL to 
RS232 kit is more costly than using a hex inverter like Remco does.  But I am 
more of a software guy and so don't mind spending a little more to make the 
hardware side as simple as possible.  This only adds perhaps $6 or $7 to the 
cost of the build but makes the build very simple and almost fool proof.   I 
quickly put together a schematic for my version of the interface circuit which 
I have attached to this email.

I have used this TTL to RS232 kit for two Oncore projects:

http://cgi.ebay.com/RS232-to-TTL-converter-3V-5-5V-KIT-AVR-PIC-MCU-
WinDAS_W0QQitemZ380089550171QQihZ025QQcategoryZ4663QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Other similar kits or pre-built interfaces will work just make sure that pin 1 
of the port is not tied to ground.

This kit takes less than 10 minutes to build and other than wire, the 10 pin 
connector for the oncore, a USB cable, shrink wrap tubing and a 100 ohm 
resister has everything you need for the interface.  

I take the PPS line (IE. Oncore pin 6) through a 100 ohm resister and solder 
this directly to pin 1 of the DB9 port on the back side of the TTL to RS232 
board.  This is how the "raw" PPS signal is setup in the McKinney Technology 
Oncore interface circuit which is no longer available (see the TPAR web site 
for more details).   I then run wires from the TX and RX  connectors of the 
RS232 board to the connector for the Oncore (pin 9 and pin 8 respectively).  
At that point all that is needed are the power connections to the Oncore 
connector (pin 3 and 7 to ground and pin 2 to +5V) and to the TTL to RS232 
module and you are good to go.   Everything on the TTL to RS232 board that you 
connect to is clearly marked except for the DCD pin of the serial port.  On 
the Oncore side use the Oncore Engineering Notes document for the pin outs of 
the connector:

http://www.tapr.org/pphlogger/dlcount.php?id=tapr&url=http://www.tapr.org/pdf/UT_Eng_Notes.pdf

One other difference from Remco's circuit is that I don't bother with wiring 
up a backup battery.  This is the part of the circuit between Oncore pins 1 
and 2 in the lower right of Remco's circuit diagram. If you setup SHMEM in the 
oncore driver this is not needed since the Oncore driver will use the SHMEM 
file to initialize the Oncore to the state is was in the last time it was 
running.  This will reload the almanac and other things that would normally be 
available in memory if there is a backup battery.  One of the things I have 
learned is that using the ntp oncore driver along with a recent SHMEM file is 
a fast way to initialize and test a new unit since you can watch the 
clockstats file to see if it is working and you will know in about 1 minute if 
it is OK or not.

To create a flashing LED that indicates an active PPS signal I use two hex 
inverter stages to isolate the LED from the PPS line.  But using an LED is not 
needed (IE. it is geek bling) and it is marked as optional in the schematic.  
I based this part of my circuit on the part of the McKinney Technology Oncore 
interface circuit that does the same thing.  All that is needed is an LED (if 
you have an old computer case laying around just salvage some LEDs from it), a 
220 ohm resister, a bypass cap and a 74HC14 hex inverter.  If you buy all of 
this at your local electronics supply store it will cost about $5 or $6.  I 
build this dead bug style and I just glue the 74HC14 dead bug to the bottom of 
the case this is mounted in.

As far as tools all you need are a soldering iron (needs a small tip), a 
crimping tool for assembling the wires to the connector that attaches to the 
Oncore (I paid $8 for one from the local electronics store).  You will also 
need solder, some shrink wrap tubing, wire in various colors and hardware to 
mount the components. 

If I leave off the blinking LED I can have one of these working in about 1/2 
hour if I have all the parts on hand even working at a careful unrusted pace. 
Adding the blinking LED adds about 20 minutes to the build time and about $6 
to the cost (if you are using a salvaged LED about $3).  If you are doing your 
first one expect it to take longer since you will want to go slow and double 
check your work as you do things.  Almost anyone should be able to build one 
of these since this is a very simple electronics project.  Having detailed 
schematics should also help since you will not be guessing about how things 
need to go together. 

The schematic shows a fuse in the power line.  I have not used a fuse in any 
of my builds but using one is the correct thing to do.  This will help protect 
the various components in case there is ever a short in the circuit. 

Hal
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