Micropolis Floppy Disk Drive Micropolis 100085-01-8A, Specifications, Support, Specs, Manual, Images

Micropolis 100085-01-8A

Controller Board


Model Capacity Size Height Media Type
Micropolis 100085-01-8A - - - -

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.

The 100085 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 It was later superseded by controller PCB model 100163.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.

Sync Byte
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. More exactly, the data field per sector is 256 Bytes of actual payload plus 10 Bytes of dummy padding.

The checksum is a one byte error detection code that provides error detection in read operations. The checksum is computed by the CPU as follows:
a) The accumulator and carry are initially cleared;
b) Each byte of the header and data fields is added to the accumulator with carry.

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.


Related information

A detailed chronicle of getting a Micropolis 1053-II floppy subsystem working on a rare Micro Expander Inc. S-100 bus machine via 100085-01-8A controller board (dash number 100086-52-9B) can be found on German vintage computer tinkerer funkenzupfer's website.

For S-100 bus based computers, hardware manufacturer NorthStar implemented a very similar controller with its MDC and MDS 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.