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MITS - 88-S4K   4k Dynamic RAM Board
Immediately after producing their 1K Static RAM board MITS proceeded to make a 4K Dynamic RAM board (see below). Unfortunately the electronic design rendered the board unreliable leading to loss of sales, returned boards and opened the window to competition from multiple other vendors who supplied more reliable (and cheaper) static 4K RAM boards.
The Altair board obtained its essential dynamic RAM refresh pulse from the 8080 microprocessor chip by a process called "cycle stealing." The problem with this technique is that sometimes the 8080 CPU is off doing something at a time when the memory needs to be refreshed. If this happens, the memory "forgets," and data is completely lost! To make "cycle stealing" work at all, precise timing is required. The company later came out with its own 4k Static RAM board.

MITS 4K Dynamic RAM

Board memory access time was 200-300ns.    They called the board a Synchronous RAM board meaning it relied on the CPU for refresh timing signals -- "no single shots" the brochure quoted. The latter presumably referring to their earlier problematic dynamic RAM  board.

The schematic for this board can be obtained here

The first attempt MITS took at a dynamic RAM board was in fact the 88-4KD shown here.   As I said above, it never really worked. People tried all kinds of jumpers and patches.  On top of that the board was badly designed. All of the wires on this board would have been unnecessary with a professional layout.  It's included here just for completeness or if you are a collector.  Don't even think of using it in your S-100 system!

4kD RAM (First Attempt)

The manual for this board can be obtained here
Other MITS S-100 Boards

1K RAM   4K RAM   4K Dynamic RAM  16K RAM   16K Dynamic RAM   4PIO  8080 CPU   VI/RTC   PCI   ADC

2K PROM    8K PROM    88-2 SIO    SIO-B   MITS FDC Boards   TurnKey Board


This page was last modified on 10/25/2013