Service manual

Service Manual
Similarly, a key release is ignored while scanning is off.
Commands may be received at any time. Therefore,
commands can be interleaved with acknowledge replies
from the ARM, eg keyboard sends KDDA (first byte),
keyboard receives command, keyboard receives BACK,
keyboard sends KDDA (second byte), keyboard receives
command, keyboard receives SMACK. If the HRST
command is received the keyboard immediately enters
the restart sequence. The LEDS and PRST commands
may be acted on immediately. Commands which require
a response are held pending until the current data
protocol is complete. Repeated commands only require a
single response from the keyboard.
ARM commands
Mnemonic
Function
HRST
Reset keyboard.
LEDS
Turns key cap LEDs on/off. A three bit field
indicates which state the LEDs should be in.
Logic 1 is ON, logic 0 (zero) OFF
DO controls CAPS LOCK
D1 controls NUM LOCK
D2 controls SCROLL LOCK
RQM
Request mouse position (X,Y counts).
RQID
Request keyboard identification code. The
computer is manufactured with a 6-bit code to
identify the keyboard type to the ARM. Upon
receipt of RQID the keyboard controller
transmits KBID to the ARM.
PRST
Reserved for future use, the keyboard
controller currently ignores this command.
RQPD
For future use. The keyboard controller will
encode the four data bits into the PDAT code
data field and then send PDAT to the ARM.
Mouse interface
The mouse interface has three switch sense inputs and
two quadrature encoded movement signals for each of
the X axis and Y axis directions. Mouse key operations
are debounced and then reported to the ARM using the
Acorn key up / key down protocol. The mouse keys are
allocated unused row and column codes within the main
key matrix.
Switch 1 (left)
Row code - 7
Column code - 0
Switch 2 (middle)
Row code - 7
Column code - 1
Switch 3 (right)
Row code - 7
Column code - 2
For example, switch 1 release would give 11010111 (&D7)
as the complete row code, followed by 11010000 (&D0)
for the column code.
Note: Mouse keys are disabled by NACK and MACK
acknowledge codes, and are only enabled by SACK and
SMAK codes, ie they behave in the same way as the
keyboard keys.
The mouse is powered from the computer 5V supply and
may consume up to 100mA.
Movement signals
Each axis of movement is independently encoded in two
quadrature signals. The two signals are labelled
REFerence and DIRection (eg X REF and X DIR). The
table below defines the absolute direction of movement.
Circuitry in the keyboard decodes the quadrature signals
and maintains a signed 7-bit count for each axis of mouse
movement.
Initial
state
Next
state
REF
DIR
REF
DIR
1
1
1
0
1
0
0
0
Increase count by one
0
0
0
1
for each change of state.
0
1
1
1
1
1
0
1
0
1
0
0
Decrease count by one
0
0
1
0
for each change of state.
1
0
1
1
When count overflow or underflow occurs on either axis
both X and Y axis counts lock and ignore further mouse
movement until the current data has been sent to the
ARM.
Overflow occurs when a counter holds its maximum
positive count (0111111 binary). Underflow occurs when
a counter holds its maximum negative count (1000000
binary).
1-12 issue 2, June 1991 Part 1 - System description