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Serial to PS/2 Mouse Converter

 
 

Problem: Using a plug converter to plug a serial mouse into a PS/2 port causes the keyboard to fail and the mouse not to work. Removing the converter shows that the keyboard is still not working.

Hardware: Serial to PS/2 mouse converter.

Software: Windows 95.

Resolution Steps: This was a strange one. I tried the same mouse and converter on another PC and exactly the same thing happened! It didn't appear to be a software problem as the keyboards still didn't work when the PC's were started up in DOS. I then plugged the inoperative keyboards into a third PC and they worked ok. Conclusion: something was wrong with the keyboard controller on the motherboard.

I rang up the supplier of the converter and they assured me that it should work with any mouse and they'd never had any problems with them in the past.

Off to the web to see what I could discover about these serial to PS/2 mouse converters. It turns out that a PS/2 mouse port has a 5 volt power pin whereas serial mice steal their power from the RS-232 signal pins. For a mouse to handle both types of ports the mouse must be specifically designed to handle this (i.e. it must include additional circuitry to do this).

I also discovered that when it comes to PS/2 connections a single controller chip, the 8042, handles both the keyboard and the mouse port.

Since I don't do component level work on motherboards, and one of the PC's was still under warranty, I decided to call the manufacturer of the PC that was still under warranty, Compucon, and ask for some advice. Compucon's helpful support staff confirmed everything that I'd already discovered. Even though the warranty didn't cover silly people plugging in incorrect devices they repaired the motherboard at no charge. They also repaired the other PC that wasn't even theirs!

The supplier of the mouse converter finally admitted that it would only work with mice that were specifically designed to be multi-port capable and they gave me a refund.

Recommended Reading: Upgrading and Repairing PCs: The best PC hardware resource anywhere!

 

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