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Mice: Serial vs. PS/2


Many people own a computer that has a PS/2 style mouse rather than the more common serial mouse, and when it breaks down, they find that it's hard to replace. But as good fortune would have it, somebody came up with an adapter plug that will let you connect a serial mouse into a PS/2 jack! Good idea, right?

Wrong.

First of all, what is the difference between the mice? Well, the only obvious difference is the connector. PS/2 mice use a round 6-pin plug, and serial mice use a semi-rectangular 9-pin plug. What is not obvious is that PS/2 mice connect electronically to the keyboard controller chip inside your computer, whereas serial mice are designed to talk to the RS-232 controller in the computer. Both controllers are actually serial, but the PS/2 is a special case designed to run at a fixed speed (determined by the computer) and for input only.

For those who are interested, I found a description of the internal connections of a PS/2 to serial adapter plug:

PS/2 signal pin pin Serial signal
data 1 5 ground
reserved 2 5 ground
ground 3 2 data receive
+5V 4 3 data send
clock 5 7 ready-to-send
reserved 6 - not connected

It appears that the ground and data pins have been crossed because one device expects data 1 to be low and the other expects it to be high. This isn't important. What is important is first, that they tied the reserved pin #2 to ground; so if this pin is actually used for something on certain PS/2 devices, it's just been screwed up. Second, the keyboard controller runs with a fixed clock which all devices are expected to follow; where serial devices, on the other hand, set their clock rates internally and just use the RTS signal once for each byte transmitted.

Now, some mice exist that are designed to work with both PS/2 ports and serial ports, and it is for these mice that the serial-PS/2 adapter was made. But if you just have a plain serial mouse, there is no reason to expect this adapter to work, and in most cases it doesn't. If you aren't using the serial port on your computer, plug the mouse in there (9-pin to 25-pin adapters are fine, because both are RS-232 ports). Otherwise, use the mouse that was designed for that port.


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