Mice: Serial vs. PS-2
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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