Apple Desktop Bus Mouse

Apple Desktop Bus Mouse
Apple Desktop Bus Mouse (G5431)
Manufacturer	Apple Computer, Inc. (made by Logitech)
Type	Pointing device
Release date	September 1986 (with the Apple IIGS)
Interface	Apple Desktop Bus
Compatible	Apple IIGS and ADB-equipped Macintosh computers
Dimensions	96.5 × 53.3 × 27.9 mm
Weight	0.175 kg
Predecessor	Apple Mouse (DE-9)
Successor	Apple Desktop Bus Mouse II
Model	G5431
Resolution	100 ± 10 pulses per inch (200 in high-resolution mode)

The Apple Desktop Bus Mouse (part number G5431) was Apple's first mouse for the Apple Desktop Bus (ADB). It was introduced with the Apple IIGS and then with the Macintosh II, the first Macintosh with ADB. It is an opto-mechanical single-button mouse made for Apple by Logitech, with an Omron microswitch under the button; the FCC ID is BCG4G8G5431. Apple's hardware guide calls it the Apple Standard Mouse, used by all Apple computers with ADB except the Macintosh Portable, which has a low-power mouse and trackball. Apple's specifications give mechanical tracking with optical or contact shaft encoding at 100 ± 10 pulses per inch, and a size of 27.9 mm high, 53.3 mm wide and 96.5 mm deep, weighing 0.175 kg.

How it tracks

A rubber-coated steel ball turns two capstans, each connected to a slotted interrupter wheel, one for X motion and one for Y. Two beams of infrared light shine through the slots onto two light detectors, offset so that as the wheel turns they produce two square waves 90° out of phase, the interrupt signal and the quadrature signal; which one leads gives the direction of movement. A microprocessor in the mouse counts the pulses and also acts as the mouse's ADB transceiver, talking to the ADB transceiver in the computer, which passes the data to the VIA. The earlier Macintosh Plus mouse sends the square waves straight to the computer instead.

Resolution

With Device Handler ID $0001, which the mouse has at start-up and after a reset, it counts 100 ± 10 per inch of travel on each axis; with $0002 it counts 200 ± 10. A driver switches mode by writing a new handler ID to register 3. Deskthority gives 199 counts per inch.

ADB interface

Connector

ADB connector signal assignments

Pin	Signal	Description
1	ADB	Bidirectional data bus; open collector, pulled up to +5 V through 470 Ω on the computer's logic board
2	POWER.ON	On the Macintosh II family, a key on the ADB keyboard grounds this pin to pin 4 to switch on the power supply; not connected on other models
3	+5V	+5 volts
4	GND	Ground

All ADB devices together may draw no more than 500 mA. Cables should be no longer than 5 m, with no more than 100 pF per metre. The bus can address up to 16 devices, but Apple does not recommend daisy-chaining more than three on one port, because of connector resistance and signal degradation.

Timing

Each bit is a bit cell: the line goes low and then high, and a 0 and a 1 differ in how long the low part lasts.

ADB timing specifications (Apple Table 8-14)

Parameter	Nominal	Host tolerance	Device tolerance
Bit-cell time	100 µs	±3%	±30%
"0" low time	65 µs	65% of bit-cell time ±5%	65% of bit-cell time ±5%
"1" low time	35 µs	35% of bit-cell time ±5%	35% of bit-cell time ±5%
Attention low time	800 µs	±3%
Sync high time	65 µs	±3%
Stop bit low time	70 µs	±3%	±30%
Global Reset low time	3 ms	3 ms minimum	3 ms minimum
Service Request low time	300 µs		±30%
Stop-bit-to-start-bit time	200 µs	140 µs min, 260 µs max	140 µs min, 260 µs max

The 100 µs bit cell gives a raw rate of about 10 kbit/s.

Transactions

A transaction is an Attention pulse (long low), a Sync pulse (short high), an 8-bit command (four address bits, two command bits and two register bits), a stop bit, the stop-to-start interval, and then 2 to 8 bytes of data and a stop bit.

• Mice and other relative-position devices default to address $3; keyboards are at $2. Apple's support notes call the primary pointing device ADB device 3.
• A device must answer a Talk command with data within 260 µs.
• A device asks to be polled with a Service Request: it holds the bus low during the stop bit, lengthening it by at least 140 µs.
• Holding the bus low for at least 3 ms is a Global Reset: every device clears its pending service requests and returns to a state where it accepts commands.
• A device has up to four registers of 2 to 8 bytes. Registers 0 and 3 are defined by Apple; 1 and 2 are device-dependent. Register 3 holds the device address, Device Handler ID, a service-request enable bit and an exceptional-event bit.

Register 0

Register 0 in the Apple Standard Mouse (Apple Table 8-4)

Bit	Meaning
15	Button status; 0 = down
14-8	Y move counts, two's complement; negative = up, positive = down
7	Not used (always 1)
6-0	X move counts, two's complement; negative = left, positive = right

The Y count comes first, and each count is a 7-bit two's-complement value, so one report covers −64 to +63 on each axis.

Troubleshooting and repair

Erratic or jumpy cursor

A jumpy mouse is usually a dirty one.

1. Remove the ball and clean it.
2. Clean the capstans and interrupter wheels with a cotton swab and isopropyl alcohol and blow dust out of the wheel slots.
3. Clean the emitters and detectors on both axes.
4. Inspect the wheels for broken slots and the shafts for play; a damaged wheel means a donor mouse.

Button faults

The button works an Omron microswitch. Check that it reads closed when pressed and open when released, and check the button lever for obstruction. When replacing the switch, desolder it carefully to avoid lifting pads. For a button that seems to stay down, Apple's sequence is: restart; restart with extensions off; shut down, disconnect all ADB devices for at least 10 seconds and reconnect; zap the PRAM; then replace ADB cables and devices.

No response at all

1. Measure +5 V between pin 3 and pin 4 of the ADB connector.
2. Pin 1 should idle high through the 470 Ω pull-up on the computer's logic board. If it sits at 0 V with nothing plugged in, the fault is in the computer; a bus held low for 3 ms or more is a Global Reset.
3. Check all four conductors for continuity while flexing the cable at the strain relief.
4. With an oscilloscope, look for the 800 µs Attention pulse and 65 µs Sync pulse from the host, and for the mouse answering within 260 µs of a Talk command. If the host transmits and the mouse never answers, the fault is in the mouse.

Cable replacement

Record the existing wire-to-pin mapping before cutting anything, and keep the replacement close to the original length; Apple limits ADB cabling to 5 m in total.

Foot pads

Apple offered no replacement kit for the Teflon pads on the bottom of the ADB mouse.

Hot-plugging

Do not unplug and reconnect an ADB device while the computer is running. The device reverts to its default address while the computer keeps trying to reach it at the address it was assigned at startup.

See also

• Apple Desktop Bus
• Apple Desktop Bus Keyboard
• Apple Desktop Bus Mouse II
• Apple Mouse
• Apple IIGS
• Retrobrite