Nice old mouse. No offense but your pin numbers are wrong on the IC's. See pic and also attached the LA6339 datasheet. On the 5 pin cable connector the middle pin is definately ground. Am in mid dinner fixing but will check back asap and post anything else I see... am a bit distracted. sorry

Oh, thanks for correcting that, I even was looking at the pinout earlier but neverheless messed it up 😀 there is no rush with fixing that mouse, I just want to make sure it won't blow anything up while it's connected.
I found this site on getting power off RS232 http://www.abcelectronique.com/annuaire/monta … erie-rs232.html

so I guess that DTR and RTS lines are mostly used to generate power and two of these pins might go there. Pin 5 seems to be connected to VCC with a diode, and there is also a capacitor next to it.

I also found a datasheet for the controller (4-bit microcomputer) https://www.digchip.com/datasheets/download_d … -number=MB88201
R10/START pin is connected (pin number 5) is connected to the transistor base, collector is connected directly to cable 1 and there is a resistor between cable 1 and 2

Pin 5 is connected to a negative lead of a capacitor, and pin 4 is connected to positive lead of the same capacitor... neither of them are connected to ground

All right, so...
the wiring was fine all the time! I mean, it worked with another computer.
But it was causing something which seemed to be a massive voltage drop on T3200SXC.
That laptop worked only with Logitech Mouse Man. I tested two additional mice, and the result was the same (fan and HDD slowdown, no boot after RAM check).

I found that voltage drop disappeared after disconnecting pin 5 (GND) in DB9 connector in another mouse (A4 tech) which caused the same problem. The system booted and mouse driver was installed, but after starting EDIT or simfarm mouse cursor didn't show up and driver got unloaded.

So I looked at the internals of Logitech Mouse Man, which worked fine:

First I tried to mod A4tech mouse. I disconnected pin 5 (GND) in DB9 connector and connected it to pin 6. (DSR). I also bridged pin 4 (DTR) with pin 6, just like with Mouse Man. And it started working in T3200SXC... but there is nothing connected to GND now.

With that old NCE mouse i did this:

MOUSE | DB9
green | pin 6
yellow | pin 4
orange | pin 3
brown | pin 2
red | pin 7

So I only relocated green cable to pin 6 and that's it. Again, there is no GND now. In Mouse Man GND was connected, but it didn't cause any problems. Other mice don't work without this modification. I also tried connecting GND through 150ohm resistor, but the voltage drop was still there. Are there any serial mouse standards which don't use GND connection?

I am using non-standard power supply, I wonder if that might cause it. But I only used Mouse Man before replacing PSU and it didn't cause any problems. I made new PSU with buck converters for positive and negative rails, here is schematic: DIY AT DC-DC power supply (replacement for Toshiba T3200SXC and others)