I find myself wondering why when I use the serial cable on my Nextel phone for long trips why the battery dies so quickly. Well after some investigation and some previous knowledge I know why.
Well it starts with the fact that the GPS receiver is actively looking for and receiving signals. The next fact that I found through some investigation, soldering wires, and buzzing out circuit boards, was that the phone is not only powering its internal workings but also the external circuitry for the RS232 Driver Chip. Even though there seems to be circuitry in place to shut down the chip when it is not in use. It never is not in use when the phone is in NMEA Output mode. Well both of these add up to some major power draw for the phone battery.
Well now what is the solution? Well I thought that there is some power coming from one of the Handshaking signals so figured I would jumper the DTR from the computer to the power charge pin on the phone. Guess what? It works. The only problem with it is it only provides power when there is communication on the line.
In my application my laptop for GPS does not have a serial port so I have to use a USB to Serial Converter. Well here comes another mod. I used the 5 volt USB supply and jumpered it to the DTR pin on the converter side (USB Connector pin 1 to Serial connector Pin 4). Now whenever the serial cable is plugged through my converter to the computer the computer charges the phone.
Link for OEM Serial Cable Schematic (with Modifications)
As a few notes you can probably make your own serial cable with 4 capacitors, and 1 IC if you use your own power source and don’t use the Phone side Pin 7 to power the RS232 Driver chip. If you do this as a note connect Phone side Pin 7 to your power voltage as well as pin 3. I would not recommend using the Serial communication driven DTR in this case because since the RS232 Driver is not powered communication cannot start. Also don’t forget to power the chip with this source as well. The USB power I described earlier would work.
Nextel Connector Pinout
- Ground
- Usually a 33ohm Pulldown to ground
- Charge Voltage( >3v I believe)
- Tx
- Rx
- RTS
- Vbus (connect to pin 3 if not used as IC power)
- CTS
- DCD
- RE
- DTR (not used in my GPS config)
- DSR
- Serial Enable Not (if this pin is not pulled to ground your serial cable will not work)
- NC
- NC
- NC
- NC
I Believe this is valid for the following models of Nextel phones, Mine is an
i736:
- i860
- i830
- i733
- i730
- i736
- i710
- i530
- i305
- i205
- i836 - Thanks tim-1138 for your input
If you have any questions leave a comment.
Robogeek
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from BlogCadre on February 2, 2006 - 10:35pm
Ok you all may or may not remember my Nextel phone cable hack posts, Here and Here, but I have one more up my sleeve, "The USB Serial Cable" now you all might say what the heck how can it be USB and Serial at the same time. Well Click on the mor
