SD Systems entered the S-100 computer boards business as a company called SD Sales in Dallas Texas which had been selling new and used electronic components.  Via a consulting firm called Micronix they designed a Z80 card for the S-100 bus. In 1977 they came out with this Z80 based CPU card for the S-100 bus. They called the card the "Z8800 CPU" card. Clearly this was well before the time anybody had heard about Zilog's 16 bit CPU's. The card came out around the same time TDL started selling their Z80 CPU card and for a short time these were the only two S-100 Z80 CPU cards available. Both cards claimed to be drop-in replacements for 8080 CPU cards in Altair and IMSAI systems. However the price of the SD Systems card was considerably cheaper than the TDL card. It sold well. They were to a large extent reliable but there were minor glitches -- particularly if the Z80 chip and crystal was switched from 2MHz to 4MHz. 
 
SD Systems were careful and diligent in updating customers with hardware "patches" and consequently built a loyal following. However in the end the changes became extensive.  Nevertheless the company persevered. They then designed a 16K dynamic RAM board that blew away almost all other early S-100 RAM boards. It was very well priced, was very reliable (not a trivial issue for dynamic RAM boards at that time) and became an instant success. They later extended this design to a 64K dynamic RAM board. This board probably was sold more in numbers than any other S-100 board. It essentially built the company. They extended their reach by designing a S-100 bus floppy disk controller board called the Versafloppy. Again a tremendous success. They extended it to the Versafloppy II which allowed double density disk recording. The company also had a SBC (Single Board Computer) S-100 board and wrote well documented software drivers etc for its products. While never as glamorous as its cousins Altair, IMSAI, TDL Victor Graphic Cromemco etc they mentioned a slow steady presence with little publicity well after many of the others had gone.
 
In the Summer of 1984 SD Systems did something new. They expanded their name to "Syntech Data Systems"  and introduced a number of great looking new boards.  They were then bought by Macrotech and everything was moved from Texas to Chateworth CA.  The origional SD Systems SD Systems went on to build lottery machines. They got radied by the Texas police for making illegal gambling devices and got stuck in a major legal battle.