
    Wiring required for Kiwi  - the Precision Timestamp Utility

                       *** NMEA Version  ***

Printer Port (trigger input, 1PPS from GPS and optional LED)
------------

A DB25 (Male) is required to plug into the LPT or Printer Port (female).

The following Pins need to be wired.

Pin 13
------

1PPS from the GPS, this must be an active low, i.e. the UTC
second occurs as the signal goes from HIGH to LOW.  Most GPS
receivers provide a high Z output that is the opposite, so a
common emitter transistor can act as a buffer and inverter as
follows.

-----------------------------------------------------------------

If the GPS has active high 1PPS (ie UTC occurs at transition low to high)

                           TX COLLECTOR-----Pin 13----[4.7K]----Pin 8
GPS 1PPS O/P----[47 K]-----TX BASE
GPS Ground-----------------TX EMITTER--------------------------Pin 25

TX = NPN Small signal Transistor (eg 2N2222)

The above cct also "isolates" the GPS if either unit is powered
or not, and allows use of 3.3 volt OEM units.

The above can be wired on the DB25 connector.
-----------------------------------------------------------------

PIN 10
------

This is the trigger or "manual Switch" input to the PC.  It is
active LO, so if using a TTL input, the timestamp is taken when
the logic goes from hi to lo!

I connected a manual switch via a 25 metre "single core
shielded" cable to pin 10 (note the 4.7K resistor to pin 9 to
provide pullup). The outer shield of the cable connects to Pin
25.

Switch ]-----------cable-------------Pin 10----[ 4.7K ]----Pin 9
Switch ]-----------cable-------------Pin 25

The Switch type is a momentary SPST i.e.
When "pressed" the switch closes the circuit.
When released internal spring action opens circuit

For precision timing, be careful selecting a switch, low cost
units can generate poor timing because of switch bounce. The
software has a built in 50mS debounce, but small switches can
often be quite "noisy". Use a switch that has a "click" feel to
it, this "snap action" makes for much cleaner switching.

PIN 2
-----

This is the "optional" LED output. Wire the "cathode" of the LED
to PIN 2, then the anode (via a 1K resistor) to one of the 
following pullup sources. In option 7, the LED is pulsed for 50
msec each time the "trigger" or manual switch activates (used in
astronomy for time tagging a video). In option 8 (time pips) the
LED flashes in synch with the audio pips, if not "muted".

PINS 6,7,8,9
------------

These are set HIGH for the duration of the program. These can be
used separately as pull up sources via resistors for the two
inputs and the LED output.

Pins 18-25
----------

These "should" all be earth, but if in doubt, use Pin
25 as the sole "earth connection" for the 1PPS and Switch.

Serial Connection:
------------------

N.B. Please make sure that the serial output from the GPS is
meant to be connected to a PC RS232 port. Many OEM units require
conversion from CMOS/TTL before connecting to a PC.

Here are some links to help with the wiring if you do need to
provide conversion from CMOS/TTL to RS232.

http://www.kmitl.ac.th/~kswichit/MAX232/MAX232.htm

http://www.seattlerobotics.org/encoder/aug97/cable.html

Here is a board that can do the conversion (for those who do not
like to breadboard).

http://www.tapr.org/tapr/html/Fvpib.html

So assuming you have RS232 from the GPS, here is the wiring to
connect to the PC.


PC Comport 25 Pin
-----------------

2-Tx--------------------------Rx-GPS rs232 I/P

7-------------GND----------------GPS GND

3-Rx--------------------------Tx-GPS rs232 O/P


PC Comport 9 Pin
----------------

2-Rx--------------------------Tx-GPS rs232 O/P

5-------------GND----------------GPS GND

3-Tx--------------------------Rx-GPS rs232 I/P

Once the above wiring has been done, run the software KIWI.EXE
and enjoy many experiments with accurate timing!
