Archived 2013-12-08
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There are two ways to connect the HP DOS palmtop (100LX, 200LX, 700LX, 1000CX) to a mobile phone:
via a data cable or via infrared. Regarding the infrared connection please read my EMI problem page and my phone features page!

If you want to connect your phone via a data cable to the HP LX, please read on.

The cable and circuit described here is suitable for most mobile phones which have a serial port. Originally it has been designed for the Siemens S25 phone (the pictures show that phone), but it works for many other models, too. Only the plug has to be adapted.

In most cases you can also use the original data cable which is available for your mobile phone. But there are several potential problems:

  • You'll have to carry a bulky combination of cables aroud: The phone's data cable, the HP connectivity cable and a null modem adapter in between the two cables
  • The original data cable may not work at all. This is because many cables (especially cheap ones available on ebay etc.) are powered by the PC's serial port. This works for standard PCs, but not with the HPLX, because the palmtop's serial port is not able to supply enough power to the cable. The cable described here is powered by the phone via its accessory power line.

Compare the bulky Siemens-HP cable combination
(on the left) with my self-made cable!

It may be necessary for you to reques data/fax services from your mobile network provider before you are able to use the mobile phone as a modem. If it is enabled, the phone / cable combination lets you use the phone as a regular modem (plus many GSM-specific features).

Making a data cable

What you need:

  • 1 MAX3232 (I recommend the SMD package)
  • 4 capacitors 0,1uF
  • 1 capacitor 10uF
  • 2 Z-diodes 2,7V
  • a phone-side plug (I took mine from a cheap head set)
  • the material for making a HPLX plug (see my HPLX serial port plug page)
  • epoxy glue for covering the HPLX-plug that holds the electronics

I won´t describe the process in detail, I´ll only show you pictures of my cable. Please be creative!

First, the circuit diagram. I recommend that you download the picture and print it out because it may not be readable well enough in your browser. Note that the pin assignments on the right are only valid for a plug for Siemens S25/S35-family phones! For other pin assignments, please see the table below!

Circuit diagram

the parts for the HPLX plug and the circuit board

circuit board and MAX3232

MAX3232 soldered onto the circuit board

top and bottom of the circuit board with HPLX plug module attached and all wires except the cable to the phone.

working arrangment.

data cable covered with epoxy and plugged into the palmtop´s serial port

Getting a plug for your mobile phone / pin assignments

To get a plug which fits the port of your phone you have the following options:

  • buy an accessory which comes with a full-wired plug and use that plug (cheap head sets for example)
  • buy a single plug, a source in germany is for example

The circuit diagram above shows pin numbers for only the Siemens S25/S35 plug. Use the following table to get the correct pin assignments for your plug:

pin nameSiemens S25 etc.Ericsson T39m etc.
Rx pin 6 pin 4
Tx pin 5 pin 5
GND pin 1 pin 10
Vcc pin 4 pin 11

Following phones are pin compatible:

  • Siemens S25, C25, S35, C35, S45... (probably more)
  • Ericsson T39m, T68i, T68m, R520... (probably more)

Tips and Tricks

  • The cable shown here does not have handshaking lines (RTS/CTS). Some mobile phones have these lines and expect them to work. With such a phone, you may have connection problems using the cable (buffer overflows, missing fax lines, lost data, timeouts...). Some phones have configurable handshaking behaviour. For example, the Ericsson phone T39m (and probably similar models, too) need the command AT+IFC=1,1 to disable their RTS/CTS lines and switch to software handshaking (XON/XOFF).
  • Some phones have a fixed baud rate on their serial port. For example the Siemens S25, S35 and similar models are hard-wired to 19200 baud. Other phones, such as the Ericsson T39m, have an automatic speed detection. So take care about that when configuring your communication programs.
  • An easy way to test if the cable basically works is to fire up the DataComm application of the palmtop, configure it to use COM1, 19200 baud (or 9600, 38400...) and send an AT command, for example ATZ. After pressing Enter, the phone should respond with OK. If it doesn't, something is wrong.

Using a special printed circuit board (recommended!)

This email address is being protected from spambots. You need JavaScript enabled to view it. made a circuit board layout available that lets you make a the HP plug slightly smaller than my version which is described above. Another advantage is that you don't have to saw around to get these two small baords and that you don't have to care about the wiring between the components.
Günther released two PostScript files which contain the layout. You can download them here: for the top layer of the board for the bottom layer of the board

Günther even offered to make such boards, so if you are interested, please This email address is being protected from spambots. You need JavaScript enabled to view it. and he will answer you how much he asks you to pay and when he will make it ready for you.

Data cable for a standard PC serial port

The data cable described above should also work for use with a standard PC. Simply attach a female DB9 connector to the electronics instead of the special HP connector. The pin numbers are identical on both connectors.

Please note that the circuit described above is powered by the battery of the phone. Most data cables which are available in stores are powered by the PC´s serial port. Since the palmtop doesn´t provide so much power, I decided to let the phone power the cable. This should of course also work with a standard PC. I just mention that to avoid confusion.

Do you like to support me?

Dear fellow Palmtopper!

If you like, you may reward me for the work I put into this website with a donation.


For a donation in US$:

For a donation in €:

Your donation, even if it's just a single USD, will help me to:

  • finance the server that hosts the site
  • justify time I spend with maintaining the site and do not spend with earning money otherwise (wife is watching! ;D)
  • just continue to be motivated :)

Since my company "hermocom" does not exist for many years anymore, I am now spending my spare time and private money for all of this. Thank you very much! 🙂

Yours truly,
Daniel Hertrich