The Dell Axim is available with an optional serial cable, this cable generally works fine with PC's to use ActiveSync.
However, when the cable is attempted to be used in an environment not supporting RS-232 control signals, such as with a GPS, the cable has some serious issues. The standard cable should work fine with a null modem adapter and a device such as a serial modem without any of the modifications since such devices generally support RS-232 control signals.
The problem is that the RS-232 level converter chip inside the bulge in the cable does not get power from the Axim unless the SEL line on the Axim side is pulled to ground.
Usually this is done when the device connected to the serial cable asserts DTR(pin 4) a transistor in the blob pulls the SEL line low and the converter chip activates. However, GPS devices only use Transmit and Receive lines(generally) so the Axim never realizes the cable needs power. Sometimes the cable will work to receive NMEA, but generally it won't work bidirectionally. With the transistor modification(and disabling activesync) you should be able to plug your GPS and start GPS software in any order without problems or lockups.(At present it does appear if the Axim powers off while the GPS is sending data and the GPS software is running it loses the GPS, I have found no fix for this yet.)
If you don't mind possibly breaking your Axim and Serial cable, you can open the bulge and solder a wire between SEL and GND on the Axim side of the adapter board.
On the other hand, if you don't want to hack your cable, there are several ways to make things work right.
The first way to bypass this is to use a parasitic power system where the Transmitted data from the GPS also is used to trigger the DTR line so the Axim knows something is attached. However, this only works when there is a stream of data from the GPS. NMEA sentences keep this triggered fine, but it probably won't work with a protocol the Axim initiates, such as a Garmin protocol transfer.
In testing, 100 ohms, and 1000uF works fine for a Garmin in NMEA mode at 4800bps. The Dell chip stays triggered and all the lines stay active. However, in Garmin mode, there is no transmission from the GPS so there is no way to trigger that circuit.
A battery adapter connects batteries to pin 4 which makes the Axim think there is a device attached.(The minimum voltage that triggers the Axim is about 1.6V) The problem with this is that the batteries will eventually go dead if they continue to be attached to the Axim part of the cable. The draw appears to be about 1mA, so about 30 days out of a set of AAA batteries is the estimate if continually attached. To deal with this, the cable can be detached from the Axim, and remain attached to to the GPS cable. This DOES allow 2-way communications between the Axim and GPS, Garmin mode works fine in testing with G7ToCE.
The use of the diodes allows the DSR signal to power DTR when the Dell port is activated to save a bit of power. The batteries connect to a Ge diode to reduce the voltage drop a bit. Both diodes can be eliminated by removing the connection to pin 6, And directly connect only the batteries to pin 4.
Connection Summary: Axim side, loop 7-8, Connect the bands of the 2 diodes to pin 4. Connect the other end of the Ge diode(or a 1N4148) to the positive on the batteries. Connect the other end of the other diode to pin 6 on the Axim side. Connect pin 5 axim to pin 5 on the other connector and to the negative side of the batteries.Connect pin 2 on the Axim side to 3 on the GPS side, and pin 3 on the Axim side to pin 2 on the GPS side.
Internal connections on Axim side
Assembled connector
This is a simple modification of the above circuit to add a transistor to control data coming from the GPS. When the Axim asserts DSR(pin 6) the transistor turns on and allows data to flow from the GPS, when DSR is off, no data flows from the GPS. Once you have disabled ActiveSync(ActiveSync->Tools->Options and uncheck Enable Synchronization when Cradled.) this change makes the Axim not go crazy when the cable is attached without a GPS program running. The Axim leaves DSR off by default, thus no data flows and it doesn't get confused. When the GPS program is started and opens the serial port, DSR goes active and data flows. This change can also be made to the first Parasitic power circuit to make it solve the same problem there for NMEA only uses. The transistor is probably not critical any small NPN should work reasonably well, for example a commonly available 2N2222A. The 1K resistor limits the base current to the transistor. It can be wired point to point inside the box like the above picture with just two more parts.
Thanks to Steven D. Wormley for this information
