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.

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.