Service manual

I/O programming details
External latch A
External latch A is a write only latch used to control
parts of the floppy disc sub-system:
Bit
Name
Function
0-3
Floppy disc sel.
These bits select the floppy disc
drive 0 through 3 when written
LOW. Only one bit should be LOW
at any one time.
4
Side select
This controls the side select line of
the floppy disc interface
0 = Side 1 (upper)
1 = Side 0 (lower)
5
Floppy motor
This bit controls the floppy disc
on/off control motor line. Its exact
use depends on the type of drive.
6
In Use
This bit controls the IN USE line of
the floppy disc. Its exact use
depends on the type of drive.
7
Not used.
Service Manual
External latch B
External Latch B is a write only register shared
between several users who must maintain a
consistent RAM copy. Updates must be made with
IRQ disabled.
Bit
Name
Function
0-2
CD[0:2] should be programmed
CD[0.2] LOW for future compatibility.
CD[1] controls the floppy disc data
separator format.
CD[1] = 0 Double Density
CD[1] =1 Single Density
3
FDCR
This controls the floppy disc
controller reset line. When
programmed LOW, the controller is
RESET.
4
Printer Strobe
This is used to indicate valid data on
the printer outputs. It should be set
HIGH when valid data has been
written to the printer port and LOW
after about 5
[5:6]
AUX [1:2]
Not used.
7
HS3
Not used.
External latch C
External latch C is a write only register that is used to
control video sync polarity and clock speed.
1-6 Issue 2, June 1991 Part 1 - System description