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ExpandoRAM Dynamic Memory Cards.
SD Systems really built the business with their dynamic RAM cards which they called ExpandoRAM cards.  The initial  boards utilized 16K dynamic memory chips (4108's) and could be configured to work with 1,2 3 or 4 banks.  This was quickly increased to 64K RAM chips (4116's). Thereby allowing one S-100 board to take care of the complete 64K address space of the 8 bit Z80 CPU's used at the time. The boards were quite configurable and would fit in well with other memory cards within the 64K memory space.  They responded to the PHANTOM S-100 line so ROM's on CPU cards worked fine.

SD Systems ExpandoRAM

The boards were sold initially as kits but later were also sold fully assembled and tested. There were three major version of the board called the ExpandoRAM, ExpandoRAM II and ExpandoRAM III. Each had numerous revisions.  The ExpandoRAM III was IEEE-696 compatible. Memory access was about 367ns - fast at that time. The ExpandoRAM-III which came out in 1983 could be configured with 4164 chips allowing for a RAM card capable of addressing 256K of memory. It also ran with a 4MHZ Z80 with no wait states.

A typical schematic of these boards is shown here.

The manuals can be seen by clicking on the appropriate pdf file below.
SD Systems ExpandoRAM-I manual
SD Systems ExpandoRAM-II manual
SD Systems ExpandoRAM-II Update manual
SD Systems ExpandoRAM-III manual
SD Systems ExpandoRAM-IV manual

Howard Adams wrote an article in the Sept 1980 issue of S-100 Microsystems (Vol 1,#5, p44) describing how to modify the original ExpandoRAM series (which were designed to run at 2MHZ), to run at 4MHZ. The article can be obtained here.


Other SD Systems  S-100 Boards
8024 VDB    ExpandoRAM    ExpandoROM     SBC    VersaFloppy I & II   Z8800   PROM-100  
I/O8 Serial Board  4KRAM   Other Boards


This page was last modified on 01/16/2020