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SD SYSTEMS - Z8800 CPU Board.
This was SD Systems first S-100 card. As mentioned in the history section it was a very early Z80 card for the S-100 bus. The card had a 2MHz crystal and a corresponding Z80 chip. A schematic of the final circuit is shown here. In spite of the atrocious schematic diagram the circuit design was fairly simple.

Z8800 - z80 CPU

The card was designed to be a drop in replacement for a 8080 CPU card in Altair and IMSAI computes. The card was sold as a kit and had one slightly unusual feature of running the ground lines across the front of the board with two vertical metal strips. This was a time when most boards had only 14 and 16 pin TTL chips so all bus drivers were 8T97's. Likewise discrete resistors were used to pull up lines to 5 volts. Nevertheless the two 7805 voltage regulators were more than adequate to handle the boards needs. There was no on board ROM or anything else for that matter it was meant to be a direct Altair/IMSAI replacement.   There were some minor differences however.
The "Stack" and "Interrupt Enable" panel lights were continually lit on front panels.  These signals were not provided by the Z-80 (or any Z-80 CPU card), therefore these lights were always lit.  Also the Address lights did not "flicker" in the same manner as with a 8080A card during a program "RUN". This was due to the slightly different timing that the Z-80 exhibits on the address and data lines.  This difference were visual only, and were of no practical concern. For users of the Processor Tech VDM video board with this board the VDM required a slight modification.  This mod is necessary because WR occurs one "T" state earlier in the Z-80 than on the 8080A.  The user had to disconnect PSYNC from the VDM board.  This causes the VDM to strobe the memory and port address comparator once every "T" state as opposed to once every "M" state.  This mod did not affect the normal operation of the VDM when used with the Z8800 or an 8080A CPU. The mod was as follows: Remove IC 18 (74LS132) from its socket. Bend Pin 5 up and replace IC 18 making sure Pin 5 DOES NOT make contact.
When DMA devices such as the Cromemco Dazzler were used with the Z8800 CPU, a modification was needed.  The trace going to Pin 1 of IC 19 should be cut and Jumpered to Pin 8 of IC 19,  This insures the CPU clocks will be on the buss during DMA cycles.  SD Systems supplied a brief but excellent comparison between the differences between the operations of an 8080 and Z80. They can be seen here.


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This page was last modified on 04/24/2014