ACT apricot - Page 185

PARALLEL INTERFACE
PB3 Display on (DON) to video interface. A logic
low on this control line allows the generated
video signal through to the Display Unit.
Conversely, a logic high prevents the video
signal
from reaching the Display Unit.
PB4 A/G select to CRT control. A logic high
(alphanumeric) on this control line sets the
CRT control circuitry to operate in the
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character display mode. A logic low (graphics)
sets the CRT control circuitry to operate in the
graphics mode.
PBS, PB6 Disk select lines to the floppy disk drives. The
selection of disk drive is determined as follows:
PB6
PBS
0 0
Selects
drive configured as drive o.
0
1 No selection.
1
0
Selects
drive configured as drive 1.
1 1
No selection.
PB7 Direction control for the transceiver of the
parallel interface. A logic low on this control
line enables data to be routed from Port A to
the output. A logic high reverses the direction
of data flow.
PC3 Display enable (DE) from CRT control
circuitry. A logic high on this control line
indicates the active line period of the Display
Unit. A logic low indicates the flyback periods.