Industrial Micro Systems - Winchester Disk Controller
This is a controller for Winchester Hard disks -- not ST506 hard disks.

The controller can interface with up to 4 Winchester hard disk drives. However each drives requires a special "interface" board shown here.   

The interface card provides sector signals to the controller for read/write/format operations by using a phase lock loop reference synced with the drives index pulse.
 
Commands to each interface card from the controller are placed via the 8255 on the board.  The manual explains in detail the process. The controller has a 1K RAM buffer on board with a 4 byte CRC error detection system.   Sectors are 512 bytes in length. The board software is set to I/O ports A0H - A7H.
  
The limited manual I have (no schematic or ROM software) can be obtained here.

Other IMS S-100 Boards
16K RAM   32K RAM    64K DRAM    80186 CPU    FDC 400    FDC 930    HDC    IO Board    Z80 CPU  
8K RAM   4 Line Serial     CPZ 4800   LANS100    CPS-Q6A   Other Boards