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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](4K DRAM.jpg)
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)](4K DRAM-1.jpg)
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
This page was last modified
on
01/08/2011