I remembered there was such a thing as a serial mouse and I wondered how they worked. A bit of googling found me this link (which as of this posting has an SSL error). I'm going to reproduce it below but I wanted to write some notes about it first.

• The Microsoft Serial Mouse was the most popular mouse as of 1997, supported by and probably copied by all.
• The "resolution" was around 400-1000 CPI (counts per inch) compared to a gaming mouse today of around 16,000k. This had to do with internal buffering and the speed it updated the location of the movement to the computer.
• It could send 40 reports per second, where a report is a 3 byte update describing buttons and x and y changes.
• The original packet format is similar to midi. Each packet is a known length (3 bytes). For some unknown reason the first bit of each byte is ignored. The second bit is used for synchronizing packets. The first byte has the 2nd bit 1, and the following two have it 0.
• Because each byte only has 6 bits of usable data, they split the first two bits of each X and Y report off into the first byte. (See the awesome ascii art in the document below.) This gives 8 bits of data for each X and Y movement in each report (Represented as a signed integer.)
• Logitech expanded the format to 5 bytes (without wasting space) to support their 3 button mice.
• PS2 Also supported 3 buttons and didn't waste space. It was a bit faster as a standard too.
• I missed that baud rate was mentioned in the article. I guessed 40 reports a second at 3 bytes a report is 960 bps (bits per second) which might have been (at 4 bits per baud) a baud rate of 240. But it's clearly stated "1200bps, 7 databits, 1 stop-bit" for the MS and Logitech mouse and "1200bps, 8 databits, 1 stop-bit" for the Mouse Systems mouse.

This text (which is part of a "not so frequently asked questions" section of that site, which states "The information containted in these NSFAQ's have no assurance of accuracy other than I would not purposefully put up an inaccurate NSFAQ.") was curated by Paul Borman who copied this section from a Tomi Engdahl. URLs last only as long as the organizations do and unfortunately many of these links predate the wayback machine. None of the references currently exist however a few were thankfully archived their links have been changed to the archives.

Full text of Subject: What protocol do mice use? with formatting cleanup follows;

What protocol do mice use?

Subject: What protocol do mice use? Date: 09/16/97 Updated: 11/16/00

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This document was adapted from a web page produced by Tomi Engdahl <then@delta.hut.fi>.

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Microsoft serial mouse

Description

Pinout

9 pin  25 pin    Line    Comments
shell  1         GND
3      2         TD      Serial data from host to mouse (only for power)
2      3         RD      Serial data from mouse to host
7      4         RTS     Positive voltage to mouse
8      5         CTS
6      6         DSR
5      7         SGND
4      20        DTR     Positive voltage to mouse and reset/detection

RTS = Request to Send   CTS  = Clear to Send
DSR = Data Set Ready    DTR  = Data Terminal Ready
GND = Protective Ground SGND = Signal Ground

Serial data parameters:

1200bps, 7 databits, 1 stop-bit

Data packet format:

Data is sent in 3 byte packets for each event (a button is pressed or released or the mouse moves):

        D7      D6      D5      D4      D3      D2      D1      D0

Byte 1  X       1       LB      RB      Y7      Y6      X7      X6
Byte 2  X       0       X5      X4      X3      X2      X1      X0
Byte 3  X       0       Y5      Y4      Y3      Y2      Y1      Y0

LB is the state of the left button (1 means down)
RB is the state of the right button (1 means down)
X7-X0 movement in X direction since last packet (signed byte)
Y7-Y0 movement in Y direction since last packet (signed byte)

The high order bit of each byte (D7) is ignored. Bit D6 indicates the start of an event, which allows the software to synchronize with the mouse.

Graphical representation of a packet

              1st byte        2nd byte         3rd byte
          ================  ===============  ================
           - 1 ? ? Y Y X X  - 0 X X X X X X  - 0 Y Y Y Y Y Y
          ================  ===============  ================
               | | \ / \ /      \---------/      \---------/
               | |  |   |            |                |
               | |  |   \----\       |                |
               | |  \--------|-------|--------\       |
               | |          / \ /---------\  / \ /---------\
               | |         ================ =================
               | |          0 0 0 0 0 0 0 0  0 0 0 0 0 0 0 0
 Left Button --/ |         ================ =================
Right Button ----/            X increment      Y increment

3 Button Logitech extension

Mouse systems mouse

Serial data parameters:

1200bps, 8 databits, 1 stop-bit

5 byte Mouse Systems packet

        D7      D6      D5      D4      D3      D2      D1      D0

Byte 1  1       0       0       0       0       LB      CB      RB
Byte 2  X7      X6      X5      X4      X3      X2      X1      X0
Byte 3  Y7      Y6      Y5      Y4      Y3      Y4      Y1      Y0
Byte 4  X7'     X6'     X5'     X4'     X3'     X2'     X1'     X0'
Byte 5  Y7'     Y6'     Y5'     Y4'     Y3'     Y4'     Y1'     Y0'

LB is left button state (0=pressed, 1=released)
CB is center button state (0=pressed, 1=released)
RB is right button state (0=pressed, 1=released)
X7-X0 movement in X direction since last packet in signed byte
      format (-128..+127), positive direction right
Y7-Y0 movement in Y direction since last packet in signed byte
      format (-128..+127), positive direction up
X7'-X0' movement in X direction since sending of X7-X0 packet in signed byte
      format (-128..+127), positive direction right
Y7'-Y0' movement in Y direction since sending of Y7-Y0 in signed byte
      format (-128..+127), positive direction up

The last two bytes in the packet (bytes 4 and 5) contain information about movement data changes which have occurred after data bytes 2 and 3 have been sent.

PS/2 mouse

The standard PS/2 mouse (such as the Logitech mouse) defaults to 160 CPI and can be switched to 40, 80, 160 or 320 CPI via software. The Microsoft mouse driver for Windows 3.x and Windows 95 defaults to 160 counts per inch. The maximum tracking rate for PS/2 mouse is 40 reports/second * 255 counts per report, or 10200 counts per second. A 100 CPI mouse could discriminate motion up to 102 inches per second while a 400 CPI mouse could discriminate motion up to 25.2 inches per second.

Connector pinout

Pin     Wire Name
1       DATA
2       Reserved
3       Ground
4       +5V Supply
5       CLK
6       Reserved
Shield  Chassis

Packet Format

        D7      D6      D5      D4      D3      D2      D1      D0
Byte 1  XV      XV      YS      XS       1       M       R       L
Byte 2  X7      X6      X5      X4      X3      X2      X1      X0
Byte 3  Y7      Y6      Y5      Y4      Y3      Y2      Y1      Y0

L       Left button state (1 = pressed down)
M       Middle button state (1 = pressed down)
R       Right button state (1 = pressed down)
X0-X7   Movement in X direction
Y0-Y7   Movement in Y direction
XS,YS   Movement data sign bits (1 = negative)
XV,YV   Movement data overflow bits (1 = overflow has occurred)

Physical connector

The PS/2 mouse connector has the following pinout when looking at the connector on the back of the computer:

        4 u 6
       1  .  2
        3   5

1.      GND
2.      +5V
3.      DATA
4.      CLOCK
5.      Not used
6.      Not used

References