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IMSAI - PIC-8 Board
The PIC-8 Priority
Interrupt-Programmable Clock board provided the IMSAI 8080 Microcomputer System
with an eight level Priority Interrupt capability and a software-controlled
interval clock.
![IMSAI PIC-8](PIC-8.jpg)
The PIC-8 had the capability to service
either single or multiple interrupt requests. When enabled and receiving an
interrupt request, the Pic-8 determined if the request priority is higher than
the software-controlled current priority, and if necessary issues a restart
instruction that directed the 8080 system to one of eight priority controlled
restart locations. For multiple interrupt requests, the 8214 determines the
highest priority request, and processes it normally. It should be noted that
the system did not store inactive requests, and that a peripheral device must
hold an interrupt request until it was serviced by the microprocessor. The
current priority status register could be software set to any value desired to
prevent low priority interrupts from being generated until the priority status
register is reset to a lower value. The status register may be set to 0 if it
is desired for all levels of interrupt to always occur. The PIC-8 board also
included a clock circuit which provides programmed control at intervals ranging
from .1 millisecond to 1 second. The program can select from among 3 jumper
selected interval rates, or it can turn all three off. The 3 rates are
jumper-selectable to any of the following values: .1 ms, .2 ms, 1 ms, 2 ms, 10
ms, 20 ms, 100 ms, 200 ms, or 1000 ms. Additionally, one bit of the DATA OUTPUT
port is connected to a transistor and jumper pads for a special-purpose
programmer-controlled output. Room is provided on the circuit board for a small
speaker or other, user-supplied circuitry.
Also provided (left hand side) were 5
16-pin IC hole patterns with power and ground decoupling for special purpose
user circuits. These hole patterns are drilled to accept wire wrap sockets.
Power on the board is regulated by an integrated circuit power regulator with
current limiting protection. Tantalum ceramic bypass capacitors are supplied
with the board. The board was G10-type double-sided laminate with plated
through holes and contact fingers are gold-plated over nickel for reliable
contact and long life.
The (updated) manual for this board can
be obtained here.
Other IMSAI S-100 Boards
4KRAM
8080CPU
EPROM-Programmer
MPU-B
PIC-8 PIO
PROM-4 SIO
VIO
MIO
FDC (FIB/FIM)
This page was last modified
on
01/08/2011