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IMSAI - PROM 4 Board
This was an early S-100 EPROM card taking only 1702's.  I cannot locate a good color image. If anybody has one I will place it here.


The PROM-4 board provides up to 4K of addressable Read-Only-Memory, utilizing the Intel 3702-1702 PROM devices. The board contained 256 bytes of memory for each 8702-1702 chip installed. Address lines A0 through A7 are run directly to all PROM positions to select one of the 256 internal byte positions, while address lines A8 through A11 were used to select and enable one particular PROM position through 8205 decoders. Address lines A12 through A15 were jumper-selected to determine the board's enabling address. The board was enabled when the 74LS30 NAND gate (C1 in schematic in manual) inputs are all high, namely when the selected address appears on the address bus, and the status line SMEMR is high.  The Processor Ready line is controlled by a 74195 shift register via an 8T97.  The 74195 provided a user-selected memory read delay, selectable with jumpers in the delay select socket.  The 74195 shift register is reset on the rising edge of the inverted Board Enable (BDENA) signal. When addressed and enabled, an 8702-170Z PROM puts out its data on the DO through D7 lines. The data output lines of all PROMS are tied to these lines, and these lines are buffered via 8T97 sections to the DI0 through DI7 back plane bus lines. Power for the card logic is provided by a +5 volt regulator and a -5 volt regulator and a -4 volt Zener diode combination to yield +5 and -9 volts. 
The 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



This page was last modified on 10/25/2013