Connecting a PDA to a GPS Receiver
Important Note!  Use this information only as a guide and verify this information elsewhere until you are sure you know what you are doing and have the correct information for your situation.  If you don't put a cable together correctly, you may destroy the cable, the PDA, and/or the GPSR.  You must establish how the hot sync serial cable that you have works.  The information given below may not apply to the cable you have.

I have made a cable that works but the information below may contain typographical errors, might be misinterpreted, and may not apply to your particular cable or PDA.  I will not be responsible for damaged equipment.

My experience has been with a Sony CLIÉ SL10 and a Garmin Venture, GPS 76, and 12XL.  The information below may help someone understand what to do with other PDAs and GPSRs.

Some cables that are sold are not correctly designed to connect any particular PDA to any particular GPS receiver.  You can end up with a cable that will work in only one direction.  By that I mean data sent from the GPSR arrives at the PDA without any problems.  However data sent from the PDA does not make it to the GPSR.  If you run a program on the PDA that collects NMEA data and you put the GPSR in the mode to output NMEA data, you will find that it works because NMEA data flow is in only one direction.

Functions requiring GARMIN-GARMIN mode require bi-directional data flow and hence will not work.  These functions are what the PDA needs to get waypoints, routes, and tracks, etc. from the GPSR.  The PDA sends commands to the GPSR to request that the GPSR send the data to the PDA.  The GPSR never receives the commands.

The connector on the CLIÉ (not the cable) seems to be setup up (named) as DTE equipment.  The computer that you connect a PDA to is also DTE equipment so a "NULL" modem is put in between.  A "NULL" modem crosses TD and RD and also various combinations of the handshaking lines.  For instance CTS is often connected to RTS in a NULL modem.  RTS/CTS is usually used for hardware data flow control.  This is done for you in the hotsync cable - it serves as a null modem cable too.

You would probably need to supply a high voltage (probably somewhere in the range of 5V to 15V) to the RTS line, pin 7 of the DB9 to get the CLIÉ to transmit.  This is to supply a "TRUE" logic level to RTS for hardware data flow handshaking.  This line should be going to the CLIÉ CTS input in the commercial serial hot sync cable. (This is not intended to supply power to the cable circuitry but it might also be used that way depending on how the cable was designed.)

If you purchased a serial hot sync cable that was correctly designed to work from any CLIÉ's battery power and supply proper RS232 level signals, then it might be possible for a serial cable to work without doing the following.!!!  However the serial hot sync cable may have been designed to work by using the DTR line from the computer as a power supply (and/or the RTS line). In this case you may need to supply a high voltage (5V to 15V) to the DTR (and/or RTS) line to supply the cable circuitry with power.

Note for serial cables in general: using RTS as a power supply is a bad idea.  RTS could go high/low for hardware handshaking during data exchange.  DTR would likely be high during the complete data exchange so the cable designer probably should use DTR as the power supply not RTS.

I made an "adapter" cable that plugs into the Garmin GPS with a male DB9 on the other end.  I connected a 9V battery to RTS and DTR to supply power to the serial hotsync cable and supply a "TRUE" value to the CTS pin of the Clie.  It wasn't really necessary with the serial hotsync cable I have to connect to DTR, since the cable seemed to get power from RTS, but I did it in case I get a serial hot sync cable some day that needs to get power from DTR.

I strap the 9v battery to this male DB9 connector with two wires running into the connector housing from the battery connector (a little flimsy plastic thing with the typical 9V battery connectors on it) plus the cable from the GPS.

There is a common kind of DB9 connector housing that is somewhat rectangular, about as wide as a 9V battery, and a bit shorter in length.  The 9V "piggybacks" on it fairly well.....  Sort of.....  Yuk!.

Diagram of PDA to GPSR Cable

The DB9 pinouts for a PC (and the serial hotsync cable) are:

1 - DCD, 2 -  RD, 3 - TD,  4 - DTR, 5 - grd,  6 - DSR, 7 - RTS, 8 - CTS, 9 - RI

DCD = data carrier detect, RD = receive data, DTR = data terminal ready, DSR = data set ready, RTS = request to send,CTS = clear to send, RI = ring in

For "Data Terminal Equipment" (DTE) the following are outputs:
TD, DTR, and RTS
For "Data Terminal Equipment" (DTE) the following are inputs:
RS232 was specified to connect "Data Terminal Equipment" (DTE) to "Data Communication Equipment" (DCE).

An IBM PC type computer is wired as DTE equipment and a modem is wired as "Data Communication Equipment" (DCE) so a modem outputs on RD, DCD, DSR, CTS, and RI.  When DTE equipment is connected to DCE equipment, this means that RD is connected to RD, TD is connected to TD, CTS is connected to CTS, etc.  If you want to connect DTE to DTE then you need a "null modem" where various wires are "crossed".  For instance: RD to TD, CTS to RTS, etc.

There are many different Sony CLIÉ PDAs.  Some are of the T and NR series. Others are of the S or N series (older ones). These have a different connector and interface than the T and NR series.  Make sure you know which ones you are dealing with.

Return to the Main Tips and Hints Page
Return to the Index (Home) Page
Copyright © 2002, 2003 Dan Anderson. All rights reserved.