ACT apricot - Page 47

SYSTEM BOARD
CRT Controller
The CRT control circuitry is, centred around the
Motorola 6845 CRT Controller. The hardware can be
software configured for
either standard 80 x 25 text mode, or
800 x 400 pixel graphics.
Floppy Disk Controller
The Floppy Disk Interface is the Western Digital
WD2797-02 FDC, a series of buffers, a decoding circuit and
the interface connector. Extensive use is made of the 8089
I/O Processor when reading and writing to disks.
Expansion Slots
The Apricot has two expansion slots which are
essentially
extensions of the standard system address, data
and control busses.
Interrupt controller
The Intel 8259A Programmable Interrupt Controller (PIC)
forms the interface between the devices capable of
generating
interrupt requests and the interrupt control line
of the 8086 processor.
Timer
The Intel 8253-5 Programmable Interval Timer (TMR)
utilizes
two clock inputs from a divider circuit to generate a
clock
output, which is connected to an interrupt request
line of the PIC. This provides the Clock Interrupt (see the
Software section for more information). Also, two outputs
are used to set the baud rates for the Z80 SIO.