I line with unusual names for their other boards Jade call this board the
"Double D".
The Double D was an intelligent S-100 based floppy disk controller. It was
capable of handling up to four full size (8") or mini (5") disk drives.
Provisions had been made for double sided drives. Single and double sided
drives could be mixed. The controller was capable of single density or
double density disks.
The board contained its own on-board Z80A microprocessor with 2K of static
memory. The on-board processor runs simultaneously with and transparent to
the S-100 bus. All critical timing was handled on board; data transfers were
fully buffered by sector in the on-board memory, two levels of interrupt
were implemented on the Z80A, and a wait state generator was used to
synchronize the on-board processor to the disk transfer rate. The host
system (8080, 8085, Z80, or anything else) need only transfer commands and
data through a block of static memory, which could be accessed from the bus.
This architecture provided a high degree of timing independence from the
host system. Also, since the disk controller program was contained on-board
in RAM, the board's operational characteristics were redfinable at any time
during system operation.
A WD1791-01 Formatter/Controller was used to encode and decode all data
transfers to and from the disk drives. It also provided for the generation
and checking of address marks, data marks, and the cyclic redundancy
characters. Write pre-compensation could be selected under software control
at three levels of intensity, providing flexible data recording. Data
separation was achieved by the use of a phase-locked loop to insure maximum
immunity to disk speed variation and to enhance data recovery margins in
both single and double density.
The manual (one of the best FDC manuals I have seen), for this board can be obtained
here.
The support software manual with BIOS etc can be found
here.
To read about the WD 1791 chip, click
here.
