CompuPro - Disk 3
The CompuPro DISK 3 was an intelligent, high performance Winchester disk controller. It accepted up to four soft-sectored hard disks (Seagate 506 interface or equivalent), providing controller and DMA interface functions to the IEEE 696/S-100 bus.

All real time disk interactions are handled by the on board processor, eliminating the need for real time code in the host system. The only real time requirement of the host was its ability to handle the DMA transfer rate. The DISK 3 host interface consists of a single write-only port called an "attention" port. You could choose the address of the port by changing positions 1-7 on Switch 1, The port appeared as a pair of I/O addresses. CompuPro's software expected to see the Disk 3 set at I/O port 90H. To hold the bus CPU in a reset state: write a FFH to the attention port. The board communicated with the hard disk and S-100 bus master via a 16 byte common memory area. This memory had a very precise structure described in the manual. When set up you activated the board via the attention port by sending 0H to the attention port.  The board had its own onboard 8085 doing all the work.
 
The DISK 3 could be directed to generate an interrupt on completion of a command. All other communications between the DISK 3 and host system was handled via DMA. The priority with which the board will arbitrate for the bus to perform its DMA cycles is selected by jumper J11, positions P0 through P3. The DISK 3 was set to a priority of 9 at the factory.  The board was socketed for accepting either a 2716, 2732, or 27128 EPROM's.
  
The manual for this board can be obtained here.