GPS - Making Your Own Data Cables

Now that you are familiar with the different types of cables that you can combine with your
GPS,it’s time to take a look at how to make some of them.I’ll begin by taking you through the
process of making a data cable.Specifically,this data cable is one for a Garmin eTrex GPS unit.
Materials You Will Need
To successfully build a data cable,you will need the following supplies and tools:


- A suitable connector for your GPS (these are available from www.pfranc.com or
www.lynks.co.uk).
- A cable with a 9-pin D connector on the end—any old serial port cable will do,such as an old
mouse or modem cable.Failing that,you will have to buy a cable).
Soldering iron
- Solder
- Pliers/wire cutters
- Screwdriver (Crosspoint or Phillips)
- Electronic multimeter or circuit tester (if you have a multimeter,it will have an “ohms”
setting—use this for circuit testing).

Cable Assembly You assemble the cable as follows:

1. Take the wire with the 9-pin D connector at one end and cut off any connector at the opposite
end. Cut the outer sheathing off the trimmed end and expose the inner core of wires,as shown in Figure :




2. Strip the ends of the wires,as shown in Figure



3. Using a multimeter,you need to determine which wires corresponds to the data in and
data out and ground pin on the 9-pin connector,as shown in Figure 2-7.To do this,set
the multimeter to “ohms”(the 20 ohms setting will do just fine).Attach one probe to the
bare wire;wrap a length of paper clip around the other probe,and probe the holes in the
connector.Try them in turn;you are looking for a reading of zero,indicating that you
have the right wire for the pin.The pins are as follows:
■ Pin 2:Data out
■ Pin 3:Data in
■ Pin 5:Ground
Carefully note which wires correspond to each pin.If each is colored,make a note ofthis.



4. Trim back all the unneeded wires and cut the wires from pins 2,3,and 5 to a length of
approximately 1 inch (25 mm).Trim the sheath from these wires back about 1 ⁄4 of an
inch (6 mm),as shown in Figure.



5. Now take a look at the pFranc eTrex connector shown in Figure .Notice that it has
four pins.The pins are numbered 1,2,3,and 4,with pin number 1 being the pin furthest
away from the cable.The pins are as follows:
■ Pin 1:Power supply (+)
■ Pin 2:RXD (data in)
■ Pin 3:TXD (data out)
■ Pin 4:GND (-)



6. Time to begin assembly of the pFranc connector.The connector includes five parts:
■ Three plastic parts that make up the connector assembly
■ One metal screw
■ One strip of metal that has the four or eight pins attached to it (depending on the kit you receive),as shown in Figure.



7. Carefully break off the pins (you only need three if you are making a data cable) from the
sheet of metal by bending them off the metal strip.Take care and you will end up with
four pins.

8. After breaking the pins free from the metal strip,bend them to 90 degrees as shown in
below Figure .



9. Now it’s time to solder the wires to the pins.At this stage,it doesn’t matter which pins
the wires are soldered to because you can rearrange them as necessary later.
After soldering,gently push each pin,one at a time,through the hole in the plastic piece
with the large rectangle opening,as shown in Figure .

10. Now you are ready to place the pins in the proper position in the plastic holder (the base
of the connector).The proper assembly is as follows:
■ The wire corresponding to pin 2 (data out on the 9-pin D connector) connects to
pin 3 (data out) on the GPS connector.
■ The wire corresponding to pin 3 (data in on the 9-pin D connector) connects to
pin 2 (data in) on the GPS connector.
■ The wire corresponding to pin 5 (ground on the 9-pin D connector) connects to
pin 4 (ground) on the GPS connector.



The fourth pin is only required if you are making a power cable or a combo cable. If you’re connecting to a 25-pin DB-25 connector, swap over pins 2 and 3 at the PC connector.

A clever way to hold the pins in place while assembling the connector is to use a piece ofwire (or a needle) through the loops of the pins to hold them in place,as shown in this Figure .This is the best hands-free way I’ve found of doing this. Remember to remove the wire after assembly.



Bring the two parts of the connector together and then place the plastic hood on theconnector. Make sure that all the cables seat properly and there is no chance ofshort-circuiting.Then add the screw and close the connector,as shown in Figure. Remove the pin,needle,or wire used to hold the pins in place and the connector is finished




Testing

You should always test your cables before first use to ensure that there are no short circuits or
bad connections.Use a multimeter to do this,as shown in Figure. Make sure that each
pin on the GPS connector corresponds to the appropriate pin on the 9-pin connector.Also
ensure there are no short circuits to any other pins. Refer to your multimeter’s user manual for details on how to operate it.