NorthStar - MDC
This was the board that essentially launched NorthStar. It provided the hobbyist a simple reasonable way to access the then new 5" floppy disk drives.

NorthStar sold this board as a package along with a Shugart SA-400 mini-floppy disk drive. The disks were hard-sectored 256-byte sectors, each diskette could store 89.6K bytes of information, formatted as 35 tracks with 10 sectors per track in single density. Track-to-track access was 40ms and latency was 100ms. The data transfer rate was 125K bits per second. The drive of course comes assembled and tested.
 
The controller was a single PC board that plugs directly into the S-100 bus. Commands and data were transferred under software control by the technique of memory-mapped I/O (no I/O ports or DMA were used). Up to three drives could be controlled, with or without interrupts. The controller allows transfer of between one and ten 256-byte blocks of data between the diskette and RAM in a single revolution. CRC error checking was done for each block read from disk. The controller automatically turned the drives on and off to minimize head and diskette wear. The controller consists of LS and S 7400 chips and included its own crystal-controlled clock.
  
The NorthStar Disk Operating System and extended disk BASIC were included on diskette with the MICRO-DISK SYSTEM. A comprehensive monitor program for hardware and software maintenance was also provided on diskette.
 
The controller included an on-board PROM, pre-programmed to permit power-on start-up of the computer. The PROM program loaded the DOS from drive number 1 into memory and then branches to the loaded DOS.  Much of the low-level software for the DOS was contained on he PROM. The on-board PROM and the memory-mapped I/O together used 1K of the computer address space, starting at E800H in the standard NorthStar version.   There appears to have been 3 version of this board. MDC-A1, MDC-A1 and MDC-A4. I'm not sure what the difference is. Also the manual refers to the "system" as the MDS of Micro-Disk System. The later boards were actually called the MDS-AD2 or MDS-AD3.  I'm therefore not sure which manuals actually correspond to this board but try this one here.
  
NorthStar's Basic interpreter was quite good and for a small hardware house they did a good job with software support.

An overview of the NorthStar DOS and Basic software can be seen here.