Micropolis 100085-01-8A is a floppy disk controller board, implemented on a 5-by-10-inch PCBA
that plugs into the S-100 bus. It relies on hard-sectored media
disks with 16 sector marking holes arranged around the center of the disk which an optical
sensor can pick up to detect the beginning of one disk sector. An additional 17th index hole
is located halfway between the holes for sector 15 and sector zero.
This board is one of Micropolis' first products. It was marketed closely with Vector Graphic Inc.'s
computers. Documentation was sometimes interchanged and this Micropolis board was sold as part of
Vector Graphic's early systems.
was bundled floppy controller for many drives from the Micropolis 1015/1016-Series
especially earlier drive models. It was later superseded by controller PCB model 100163
Although the model/part number 100085-01-8A is normally the main number identifying the PCB, there
are sub-variants of this floppy controller assembly, mostly differing in factory-set jumpers,
commonly identified by numbers similarly formatted like the main part number.
Here's a list of these "dash numbers". Each number is usually suffixed by a letter denominating a
hardware revision. For the below list, we suffixed the letter "B".
100086-01-6B for a PCB with Address F4, MFM encoding and external Oscillator
100086-02-4B for address F4, MFM encoding, internal Oscillator
100086-03-2B Address F4, DF, external Osc.
100086-04-0B Address F4, DF, internal Osc.
100086-52-9B Address F4, MFM, internal Oscillator, relocated PROMS
Disk Data Format
Data is recorded on the disk in concentric tracks. The outermost track
is track zero. Each track is divided into 16 sectors. The beginning of
each sector is indicated by a sector hole punched in the disk. An index
hole is located halfway between the holes for sector 15 and sector zero.
The sector and the index hole are sensed by a photo transistor in the disk
drive. Each disk sector consists of a preamble, sync byte, header, data, 4
checksum, and a postamble. Each of these fields are described below.
The preamble is composed of approximately 40 bytes of zero (0) data bits.
The preamble is automatically generated by the disk controller and is
necessary to provide tolerance for the mechanical alignment and electrical
characteristics of the sector/index sensor. It also provides a field of
known data pattern for synchronization of the read data decoder.
The sync byte is a byte of data (FFH) which is used in the disk
controller to define the beginning of useful data.
The header is a 2-byte block consisting of the binary track address
of the track on which the sector resides (0-76 (34)) and the address of
the sector (0-15). The header is used to verify that the proper sector is
being accessed in a disk I/O operation.
The data field consists of 266 bytes of user data.
The checksum is a one byte error detection code that provides error
detection in read operations. The checksum is computed by the CPU as
a) The accumulator and carry are initially cleared;
b) Each byte of the header and data fields is added to the accumulator
In write operations, the computed checksum is written immediately
following the data field.. In read cperations, the checksum is recomputed
fram. the read data and is compared with the checksum byte which is read. If
they do not compare, a read error has occurred.
The rest of the sector from the checksum to the next sector hole is
filled with zero data bits. The length of the postamble allows for the
mechanical tolerance in the placement of sector holes on the disk and
tolerance for disk speed and write clock variations.
For S-100 bus based computers, hardware manufacturer NorthStar implemented
a very similar controller with its MDC
cards which used hard-sectored disks like the 100085. The move away from
hard-sectored to soft-sector media on S-100 systems was sped up by the Western
Digital 1771 LSI floppy disk controller chip used on competing boards
from manufacturers like SD Systems