Ways and problems of connecting the palmtop to differend kinds of devices via Infrared

Contents of this page:

• Preface
• Introduction
• Connecting to cellular phones
• Connecting to other PCs for file transfer
• What about a general IrDA driver for the palmtop?

Preface

I removed my instructions to build an IR transceiver device for Desktop PCs from this site, because there are now many other instructions available on the web which are way better than mine was. Another reason is that I used an Infrared transceiver component which is not available anymore, the HP HSDL 1001. The newer Vishay TDFS 4500 and similar products are a better choice nowadays.

You can find instructions to make infrared transceivers for your desktop PC here:

• http://www.infraredport.com by Dschen Reinecke, in German and in English
• http://stebo-online.purespace.de only in German.

Please note that these transceivers connect directly to the "Infrared" connector on your motherboard. No way to connect them to a serial port or USB. For that purpose there are special devices called "IrDA dongles" available. You can buy these devices in several variations, the most common one nowadays may be the USB version.

Introduction: About IR communication with the HP DOS palmtops

The main issue with the palmtops HP 95LX, 100LX, 200LX, 1000CX and 700LX is, that these devices are not fully IrDA compliant.
IrDA defines several network layers. The lowest layer, the physical one, is SIR, also known as HPSIR, because HP invented it. This layer is supported by the HP LX palmtops, i.e. all IrDA devices arephysically able to establish an Infrared communication path to the palmtop.

But the logical way of communications causes problems: Almost all IrDA compliant devices which are available these days (e.g. cell phones, PDAs, Laptops, LAN adapters, digital cameras...) make use of some of the other layers defined by IrDA. These other layers are all implemented in software, so theycan be switched off if the software is modifyable, e.g. on laptops which run Linux. Maybe even IrDA drivers in Windows 9x can be made work in plain SIR mode by modifying the Windows registry (see PC part of this page below).

SIR (serial Infrared) is basically the RS232 protocol, but with other signal levels and impulse lenghts. All other IrDA protocol layers (IrLAP, IrLMP, IrCOMM, IrLAN, IrOBEX) are connection management and handling stuff, such as adjusting the baud rate, recognizing the peer device, setting up "virtual" ports which can be used by software which wants to talk to "normal" ports like LPT or COM or network devices and so on. As soon as software relies on these upper IrDA layers, a communication with the HP palmtop as is is impossible.

Why do I say "as is"? Read on!

Connecting to cellular phones

Well, there are exactly two programs by now which are able to drive the palmtop´s IR port in a more IrDA-compliant mode. Only a few parts of the full IrDA protocol are implemented there, so it is not possible to do all IrDA things with that software.

Both programs are written by Andreas Garzotto, one of them is commercial software and sold by D&A Software. It is called "WWW/LX". This is an Internet suite with TCP/IP stack, mail client, web browser, and clients for various purposes, such as FTP, finger, get, ping etc. This one supports talking to IrDA cell phones via IR, so you can use a cell phone as a modem or you can use AT commands in terminal mode or even automated to administrate your cell phone (phone book up/download, ring tones, logos etc.) WWW/LX implements parts of the IrCOMM, IrLAP and IrLMP protocols.

The other one is free software called "IR.EXE". This one implements the connection free part of the IrOBEX protocol in addition to IrLAP. IR.EXE is also available over the D&A software FTP server. You can send and receive OBEX objects with this software, which is also supported by cell phones, PDAs and other devices.

So if you want to connect to cell phones via IrDA, there is generally no big problem. Dependent on your needs you have up to two choices.

Regarding the specifications of different tested cell phones, please read my "mobile HPLX" page!

Connecting to other PCs for file transfer

If your computer and OS (or IR dongle device drivers) are capable to talk the plain SIR (serial Infrared) protocol via its IR port, everything is fine. Then you can use every application which you also would use with a serial cable connection to transfer files (Transfile Win200, Laplink Remote Access, zip, InterLink/InterServer, X-Finder....).

If your computer can "only" talk the full-featured IrDA protocol, you may have a problem. If you connect an IrDA transmitter device to your computer, Windows 98 recognizes it and sets up "virtual" COM and LPT ports. These port types are defined by the IrDA standard (IrCOMM), but are not suitable to communicate to the palmtop, because they use these advanced IrDA features (keywords IrCOMM, IrLTP, IrLMP, IrLAP etc. - I don´t want to go into detail here, please refer to http://www.irda.org). The problem is that these "virtual" ports don´t use the plain (HP)SIR protocol, but all these add-on connection handling protocols, which the palmtop isn´t capable to handle. The above mentioned programs WWW/LX and IR.EXE are limited for use with cell phones and IrDA modems.

On the other side, you may need these IrDA-specific features on your PC if you want to use it to talk to a mobile phone, a newer PDA or some other real IrDA device.

What about a general IrDA driver for the palmtop?

This is an idea which has been mentioned a lot of times in the HPLX mailing list. But unfortunately, it is simply not possible to create a driver which sits between the application and the IR port, because the IR port is directly addressed by all DOS programs with their read / write routines. They speak directly to the hardware. So no other program or driver would have a chance to modify the data stream in any way before it is written to the serial port.

Internet connection sharing for HP Palmtops

Or: How to connect a handheld or other PC via serial line to a PC to use its Internet connection
Especially: Sharing an HP Omnibook 800CT wireless LAN access over IrDA with an HP 200LX palmtop

last updated:

Contents of this page:

• Introduction
• Windows Server setup (MochaPPP)
• Linux server setup (pppd and IP masquerading / forwarding)
• Client setup

Introduction:

If you have a PC at your home or at work which is connected to the Internet via a fast and reliable connection, such as DSL, Ethernet, ATM, X25 or even via modem or ISDN, and if you have a handheld computer which you also want to connect to the Internet from time to time, for up- and downloading emails, for example, you may not want to establish an extra expensive Internet connection via modem or GSM, but rather want to make use of the existing connection.

This is possible. I will describe here how to set up a PC running Linux or Windows so that you can connect your handheld via serial port or IrDA to the PC and use the Internet connection of the PC also with your handheld.

Since I use the HP 200LX palmtop and only Linux on my PCs, I will focus mostly on these points of view, but I will also describe how to set up Windows on the "server" side and any communication software on the "client" side.

Windows server: MochaPPP:

First download MochaPPP from http://www.mochasoft.dk/download.html (look for the "Mocha W32 PPP" in the "Palm and Visor" section) and install it on your Windows machine.

Then

1. run the program MPPP.EXE.
2. Set speed to 38400 (once this works you can go higher and explore the limits, make sure you have always set the client machine to the same speed!)
3. Tell MochaPPP what device you use (if you use the HP 200LX palmtop, tell MochaPPP it was a WinCE device)
4. Since I have never worked with MochaPPP I cannot say exactly how to set it up for an IrDA connection, but since Windows usually assigns a virtual COM port to an IrDA interface, you should probably simply shoose this virtual COM port as the port to be used by MochaPPP.
5. Then connect your already configured client device to the PC. Use a nullmodem cable, not a straight-through serial cable! For more information about these different kinds of cables, also related to the palmtop HP200LX, please see my RS232 page!

Unfortunately, Windows 2000 and Windows XP use another driver concept as the older Windows versions, so there is no "virtual COM port" assigned to an IrDA device anymore. There are thiird party drivers to solve this problem:

• A free open-source solution: http://www.ircomm2k.de
• The commercial IrComm2000 driver: http://www.extendedsystems.com

Click here to jump to the client setup guide.

Linux server: pppd and IP masquerading / forwarding:

Assuming you have already a working internet connection, I will now guide you through the steps necessary to share that Internet connection with a device attached to the serial port or IrDA port of your linux machine.

The short story

A simple shell script can do the job under certain circumstances. I suggest you to try that way first, and if it doesn't work, use the detailled instructions below.

The shell script looks like this (I have named it "/sbin/palmtop_gateway"; you have to run it as root):

#!/bin/sh

# Setup IP masquerading on your Internet connection network device
# (change "wlan0" to the appropriate device, e.g. ppp0, eth0, ippp0...):

iptables -t nat -A POSTROUTING -o "wlan0" -j MASQUERADE

# This is for IrDA connection on an HP Omnibook 800 notebook only
# comment it out if you don´t use an Omnibook 800:

modprobe vlsi_ir

# This is for IrDA connections only (not only Omnibook 800)
# comment it out if you use a serial connection instead of IrDA:

ifconfig irda0 up

# This is the PPP server (PPPD) call within an infinite loop:

while [ 1 ]
do
/usr/sbin/pppd /dev/ircomm0 38400 172.16.96.10:172.16.96.11 proxyarp local noauth silent nodetach ktune ms-dns 217.5.100.1
done

Adapt the pppd line to your needs:

• /usr/bin/pppd is the location of the pppd binary,
• /dev/ircomm0 is the name of the port where the device is connected which shall use the Linux computer's Internet connection. (/dev/ttyS1 for "COM2", /dev/ttyS0 for "COM1", /dev/ircomm0 for an IrDA port...),
• 38400 is the baud rate, you can probably go higher, but try this one first,
• the following two IP addresses are the one pppd uses on the Linux machine and the one the client should get. You can probably use the ones used here, except you already have a private subnet which uses these IP addresses. They must be unique in your subnet.
• proxyarp tells pppd to handle ARP requests on behalf of the connected device
• local Tells pppd not to use modem control features.
• noauth Bypasses all authentication. You can also set it up to use e.g. CHAP or PAP. But noauth should be enough for private use it you do not use a dialup connection between the Linux PC and the device which you connect
• silent Tells pppd to wait for the connecting device to send LCP packets rather than trying (and failing) when no device is connected. Replaces the 'passive' option in older versions.
• nodetach Tells pppd to stay attached to the controlling terminal rather than forking off as a background process. We want ONE pppd in charge of this vty!
• ktune tells the kernel to enable IP forwarding
• ms-dns xxx.xxx.xxx.xxx Tells pppd to send our DNS server (217.5.100.1 in my case) parameters to the attached device. This avoids having to hard-code values on the client, in case things change.

This shell script needs the executable "iptables", a newer version of pppd which supports the ktune parameter and the NAT (network address translation) kernel modules.

Usage: Simply adapt this script to your needs and start it. If it leads to error messages, try to find the reason. If it doesn´t, connect the device which should participate in the internet connection of the Linux computer and let it connect. When you don´t need the Internet connection sharing anymore, stop the script by pressing (and maybe holding for some seconds) Ctrl-C.

If this setup doesn´t work for some reason, or if you want a more sophisticated setup where you don´t need to manually start a script, read on!

Click here to jump to the client setup guide.

The long story

Attention: The following things have to be done as root, so be careful!

The first thing you need is the ability of Linux to share its Internet connection at all. This is done by a mechanism called IP forwarding and IP masquerading. If you want to learn more about these techniques, please read the IP masquerading howto. There are more than one way to setup IP masquerading and forwarding, please consult the manuals and docs of your Linux distribution to learn how to do it in a distribution-specific way. I describe it here in a generic way which should work for all distributions using a recent kernel version (i.e. 2.4.0 or above). If you have another kernel version, you should read the IP masquerading howto.

First, switch on everything which is necessary for IP forwarding and masqerading in the kernel configuration and compile a new kernel. Maybe this is not necessary in some cases. If you don´t feel comfortable compiling a new kernel, don't do it first and try if it works with your current kernel. You need to go into the "networking options" and switch the netfiler on. Then in the netfiler section everything related to NAT and IP forwarding has to be enabled.

My relevant sections in /usr/src/linux/.config look like this:

#
# Networking options
#
CONFIG_PACKET=y
# CONFIG_PACKET_MMAP is not set
CONFIG_NETLINK=y
CONFIG_RTNETLINK=y
CONFIG_NETLINK_DEV=m
CONFIG_NETFILTER=y
# CONFIG_NETFILTER_DEBUG is not set
# CONFIG_FILTER is not set
CONFIG_UNIX=y
CONFIG_INET=y
CONFIG_IP_MULTICAST=y
# CONFIG_IP_ADVANCED_ROUTER is not set
# CONFIG_IP_PNP is not set
# CONFIG_NET_IPIP is not set
# CONFIG_NET_IPGRE is not set
# CONFIG_IP_MROUTE is not set
# CONFIG_ARPD is not set
# CONFIG_INET_ECN is not set
# CONFIG_SYN_COOKIES is not set

#
#   IP: Netfilter Configuration
#
CONFIG_IP_NF_CONNTRACK=m
CONFIG_IP_NF_FTP=m
CONFIG_IP_NF_QUEUE=m
CONFIG_IP_NF_IPTABLES=m
CONFIG_IP_NF_MATCH_LIMIT=m
CONFIG_IP_NF_MATCH_MAC=m
CONFIG_IP_NF_MATCH_MARK=m
CONFIG_IP_NF_MATCH_MULTIPORT=m
CONFIG_IP_NF_MATCH_TOS=m
CONFIG_IP_NF_MATCH_TCPMSS=m
CONFIG_IP_NF_MATCH_STATE=m
CONFIG_IP_NF_MATCH_UNCLEAN=m
CONFIG_IP_NF_MATCH_OWNER=m
CONFIG_IP_NF_FILTER=m
CONFIG_IP_NF_TARGET_REJECT=m
CONFIG_IP_NF_TARGET_MIRROR=m
CONFIG_IP_NF_NAT=m
CONFIG_IP_NF_NAT_NEEDED=y
CONFIG_IP_NF_TARGET_MASQUERADE=m
CONFIG_IP_NF_TARGET_REDIRECT=m
CONFIG_IP_NF_NAT_FTP=m
CONFIG_IP_NF_MANGLE=m
CONFIG_IP_NF_TARGET_TOS=m
CONFIG_IP_NF_TARGET_MARK=m
CONFIG_IP_NF_TARGET_LOG=m
CONFIG_IP_NF_TARGET_TCPMSS=m
CONFIG_IP_NF_COMPAT_IPCHAINS=m
CONFIG_IP_NF_NAT_NEEDED=y
CONFIG_IP_NF_COMPAT_IPFWADM=m
CONFIG_IP_NF_NAT_NEEDED=y
CONFIG_IPV6=m

Also make sure the driver for the serial port and / or IrDA interface is included into the kernel or compiled as module. I assume it works already. If not, please make it work before you go on. Especially an IrDA driver may be a bit tricky to set up. I use the HP Omnibook 800CT with its IrDA driver vlsi_ir by Martin Diehl to share the wireless LAN access with my HP 200LX palmtop. Please note that the vlsi_ir driver is included in the Linux kernel since version 2.4.9. In order to make IrDA work on my Omnibook I use this script:

#!/bin/sh
modprobe vlsi_ir
modprobe ircomm
ifconfig irda0 up
echo "1" > /proc/sys/net/irda/discovery
echo Discovery says:
cat /proc/net/irda/discovery

(The last three lines are optional. They show you if another device is in the range of the IrDA interface. You will see the name it identifies itself with.)

In order to set up the netfilter, you need the iptables package (if you do not have it already), which you can download at http://netfilter.samba.org/. I use version 1.2.4.

Also you need the pppd (PPP daemon), which is certainly part of your Linux distribution.

Now we can start setting the whole thing up. In order to enable IP forwarding and masquerading, execute

echo 1 > /proc/sys/net/ipv4/ip_forward
/usr/local/sbin/iptables -t nat -A POSTROUTING -o "eth0" -j MASQUERADE

The first line enables forwarding (not necessary when using a recent version of pppd! The pppd parameter ktune can do that job for you!), the second line enables masquerading. Replace eth0 by your network device which is connected to the Internet. (Probably ppp0 if you use a dial-up connection, ippp0 for ISDN, wlan0 for wireless LAN...., find out with "ifconfig" while you have an active Internet connection). the iptables binary may be placed on another location than /usr/local/sbin - change that if necessary.

Palmtop user Ed Padin reported success using ipchains instead of iptables using this command:

ipchains -l forward -j MASQ

If you have troubles starting iptables, try to unload the ipchains module, which can disturb iptables (rmmod ipchains).

You can put the IrDA module calls and the iptables / ipchains setups into /etc/init.d/boot.local, so it is executed automatically at boot time.

Now you must set up the Linux computer as a PPP server for the attached device, using the pppd. This can either be done using a simple pppd call from a shell script as described above, or you can use init to start the pppd as a deamon in background. Here is an example how to call it for runlevels 2,3,4 and 5:
Append a line to /etc/inittab:

palm:2345:respawn:/usr/sbin/pppd /dev/ircomm0 38400 172.16.96.10:172.16.96.11 proxyarp local noauth silent nodetach ktune ms-dns 217.5.100.1

(Adapt the parameters as described above) When you have appended this line to the inittab, run a telinit q to let the system reread the file and use the new settings. maybe a killall pppd is necessary to kill an old pppd and to let the new one start automatically after a change of parameters. You can check easily if the correct pppd runs by typing ps -x | grep pppd.

That´s it. After configuring your client device you should be able to use the PC´s Internet connection over the serial port. Click here to jump to the client setup guide.

Client configuration:

In general you can use any software on the client machine which is able to talk the PPP protocol over the serial port or the IrDA port. For setting up a Palm, please go to the top of this page and follow the Palm link under "Related pages". Here I will describe the process of setting up the WWW/LX software on the HP 200LX palmtop, but this should also show how to set up every other piece of software.

Here are the relevant parts from my WWW/LX config file www.cfg:

[Setup]
...
PPPD_linux_IR=pppd on linux Omnibook IrDA

[pppd_script]
s=\r
@=3

[PPPD_linux_IR]
Port=-1
OmniGo=0
PPP=1
Modem=0
Baud=38400
My_IP=0.0.0.0
DNS_IP=0.0.0.0
DNS2_IP=0.0.0.0
Script=pppd_script
ModemInit=
Dial=
Login=
Password=

Explanation:

• The Setup entry is WWW/LX specific and only adds a short description to the setup name.
• [pppd_script] is the script WWW/LX uses when connecting over the serial line. It is a very simple script which does the following:
  • s=\r -- send a carriage return (= press the Enter key)
  • @=3 -- pauses for 3 seconds before PPP connection begins.
• The remaining lines are the settings WWW/LX uses for the connection:
  • Port=-1: Use port COM1 and set it to IrDA mode. Attention: On the HP 200LX this is only possible with WWW/LX, not with pther software, because the IrDA protocol is totally implemented in software, there is no generic IrDA support by that device! See also my IrDA page! Use Port=1 for a wired serial port connection (i.e. on the Server side /dev/ttySx or COMx).
  • PPP=1: Use the PPP protocol
  • Modem=0: Use a direct serial line
  • Baud=38400: Must match the speed setting on the Server side!
  • MY_IP, DNS_IP, DNS2_IP=0.0.0.0 tells WWW/LX to determine the correct values during the login process. You can also give it statically here if you have troubles with the dynamical setup. The dynamical setup has the advantage that you have to change it only on the server side if something changes.
  • Script=pppd_script: The script to use for the login process
  • ModemInit=, Dial=, Login=, Password=: Since no authentication and no modem is used, these parameters do not need any value.

Attention WWW/LX users who use setcom.bat in conjunction with modem.com to let the palmtop determine automatically the necessary WWW/LX settings: Since a wired connection to a Linux PC running pppd (and eventually even MochaPPP on Windows machines) causes modem.com to think there is a modem and let setcom.bat then execute wwwset.exe with your modem settings, make sure these settings do not interfere with your remaining setup for this kind of connection!

See also: Tutorial: Disassembly of a 200LX 

(EMI = Electromagnetic Interference: Communication problems when the HP Palmtop cannot communicate via InfraRed anymore when a mobile phone is near, e.g. in setups where the Palmtop should use a mobile phone via IrDA for Internet access.
Contents of this page:

• Introduction
• Diagrams (schematic and board layout)
• My Measurements

NEWS:

I have bought a used 700LX to study how HP solved the EMI problem there (since the 700LX is designed to work with a GSM phone on top, it must be somehow shielded against the EMI, and tests with ring.com and a seperate GSM phone so the dehaviour of the 200LX and 700LX regarding EMI coulr directly be compared, showed that indeed the 700LX is only disturbed marginally by a mobile phone). The results can be viewed on the separate 700LX page: Click here!

Introduction:

The problem appeared, when the first people wanted to use their HP 100/200 LX to connect to the Internet using the IrDA interface of a mobile phone in the summer of 1999. It is nice to have the ability to simply put the palmtop upon a table, put the cellular phone besides it, and surf the web, download email etc.
Actually it should be possible without problems, because the palmtop has got an IR port, and more and more cellular phones also have an IrDA port which looks to a PC just as a standard modem. The ´internet suite´ WWW/LX by D&A Software is the only software which is able to speak the IrDA protocol over the palmtop´s IR port. Actually the IR port of the palmtop speaks the "HPSIR" protocol which cannot be handled by modern mobile phones. So you have to use WWW/LX if you want to go online via IR.
But you will run into the following problem: The connection is either terribly slow, or it isn´t possible at all to get a working connection.

The people who ran into that problem first were Andreas Garzotto (programmer of WWW/LX), Stefan Peichl and I, Daniel Hertrich. Later Mark Willis, Joeseph Buford and Jerome V. Healy joined us to help. With time I got many requests for help regarding that subject, but a real solution isn´t found yet!

The problem is caused by the following:

Everytime we want to establish a connection to the internet with the mobile phone via infra red (IrDA-protocol by WWW/LX), the palmtop receives strange disturbance signals. These signals are caused by voltages induced by the electromagnetic field surrounding the antenna of the mobile phone.

Actually these signals must have a frequency of 900MHz, 1800MHz or 1900MHz (GSM, PCS), depending on the frequency of the cellular network. But these modulation frequencies are pulsed with a frequency of about 1,6 kHz (maybe that´s only true in my case - I don´t know if the pulsing frequency differs between cellular networks!).
And these pulsing is probably the frequency which causes the problems.
I don´t think that the very high frequency of 1-2GHz causes problems. because this frequency is so high that it cannot be decoded by the decoder hardware of the palmtop (and even is not shown by a normal oscilloscope) - see the pictures below!
But the 1,6 kHz signal is shown and decoded! And it is decoded into many, many characters that prevent the IrDA protocol from working properly.
The signal that actually comes through the photo diode has a pulse frequency of around 10 kHz (depending on the character that was sent by the sending device).

If you would like to test these interference by yourself, you can use Stefan´s software ´RING.COM´. It displays every character received by the IR receiver circuit (also the induced signals) and every character causes a click in the speaker. And the average of ingoing characters per second is measured. So you can see how bad these interferences are in your case. Maybe send as a reference a vcard over IR from your telephone. That will not disturb the IR port, but only the actual IrDA data will be arrive in RING.COM (because the antenna pf the phone doesn´t send anything). Then establish some kind of radio connection (i.e. do a call) and place the phone near the palmtop. You´ll see and hear the disturbances arriving in RING.COM!
You can download RING.COM with documentation  here.

Another experience is that with Siemens phones (S25, S35i), the disturbances are so bad, that most of the times a connection isn´t even possible, but with Nokia phones (8810, 7110, 6210...) you get a connection in most cases. You have to find the ideal distance between palmtop and phone in order to suppress most of the disturbances but keep the IR link working. For Siemens phones, this distance is about 15-20cm, for Nokia phones you can place the phone with a much bigger distance from the palmtop. About 50cm should still be possible. Several people worked out little tricks: For example with the Siemens phones (probably also with others) it helps if you take the phone in your right hand during the internet connection: touch the display with your palm, lay your fingers arund the upper edge of the phone and touch the cak of the phone (where the antenna connection is) with your third and fourth finger. That seems to bundle the emitted electromagnetic disturbances in a way that it doesn´t reach the palmtop anymore.

But of course we want to find a durable working solution!
First I thought, the right solution would be to build a high pass filter into the circuit of the IR Receiver to filter out all high GSM frequencies.

I thought we should replace the component no 7 (see picture below) by a properly dimensioned high pass filter. I tried this. But without any luck! :-(
I tried to replace component no 7 with different capacitors of values 150 pF up to 330 pF (My calculations showed that  220pF  is the right value to drop 1,6kHz signals by approximately 15dB and leave the 10kHz sinals untouched. With a filter of 1st grade, a higher attenuation of the 1,6 kHz signal is not possible -- the filter of 1st grade is built by the inserted capacitor and the resistor to ground (no 12 and 13) with a value of 100kOhm + 6.81kOhm = 106,81 kOhm).

Before I did these tests I thought that the GHz-frequencies directly caused the problems and inserted several low pass filters into the receiver circuit. None of them worked)

One solution that already works in some palmtops is to connect pin 2 of the photo diode (the right one, if you watch the diagram below) to ground. It´s not such an elegant solution, because it drains extremely much power and we don´t know if it can hurt the circuit. And it doesn´t work in every machine. About every third machine we tested that ´low cost solution´ in wasn´t affected by the EMI anymore. The other machines couldn´t at all use the IR port anymore. We don´t know why. But Stefan used this solution for about half a year successfully and his palmtop survived it.

Drawings (schematics and board layout)

The circuit board layout with component numbers (description of the components below):

The components of the IR receiver circuit:

PD: the photo diode
1:    R = 21,5 k
2:    R = 21,5 k
3:    Transistor KST 5179 or 2N 5179 by Samsung (pins see below) (Data sheet (PDF, 50kB) downloadable here!)
4:    Transistor KST 5179 or 2N 5179 by Samsung (pins see below) (Data sheet downloadable above (=part #3)!)
5:    C = xx xF (value unknown)
6:    R = 215 R
7:    C = 0,01uF (marking "KA4")
8:    C = ´685´
9:    C = ´105´
10:  R = 10 k
11:  R = 21,5 k
12:  R = 6,81 k
13:  R = 100 k
14:  IC ´TLC372C´ by TI  description here (HTML with llink to PDF data sheet)

The pins B,C and E of the Transistors 3 and 4 are placed this way:

  ------
B O    |
  |    |
  |    O C
  |    |
E O    |
  ------
 

The circuit diagram of the IR receiver (self-explored by Joeseph Buford and me):

My measurements

I made the measurements under the following conditions: An opened 100LX lying on the table, the Siemens S25´s IR port and the 100LX´s IR port are facing each other with a distance of about 3 cm.

You can see the arrangement here (on this picture the S25 is a little bit more away from the 100LX than 3 cm):

The following picture shows my measurements of the signal voltage of an IR-received character (measured at pin 3 of the 372 chip no. 14) without any interference.

Horizontally, one division is 0.2ms, so the frequency of the pulses is approx. 10 kHz
Vertically, one division is 2 V, so the peaks are approx. +/- 1V
 

The following picture shows my measurements of the voltage induced by the S25´s electromagnetic field:

with the same scalings of both axes as the picture above.
The signal has got a frequency of about 1.6 kHz (period cycle is abt. 0.6 ms).

If you would like and/or feel able to help, ideas are greatly appreciated by: Andreas Garzotto, Stefan Peichl, Mark Willis, Joseph Buford, Jerome V. Healy, me and many, many other users of the HP 200LX!
Please email me! I´ll forward your idea to the others.
Thanks to all who help(ed) us and to all who will help!