Cromemco 4FDC   
The 4FDC was Cromemco's first Western Digital chip based floppy disk controller and was designed to work with both 5 & 8" drives.

The board also contained an RS232 serial port (which was the default CRT terminal port on boot up) with software-selectable baud rates from 110 to 76,800. It had a 1K resident 2708 ROM containing Cromemco's RDOS BIOS (@C000H). The 4FDC was really designed to drive a pair of Persci 277 8-inch single-density floppy drives that they had in their System 3 computer. These drives used a fast voice coil actuator and not a stepper motor to position the drive read write head. Fast but often went out of alignment.  More important the data separator electronics were on the drive itself - like today's hard disks!  Because of the location of the data separator, an unmodified 4FDC can not be used with 8-inch drives that don't have single-density data separators on the drive electronics. Later Cromemco replaced the unreliable Persci's. They had to also change the 4FDC. This lead to the 16FDC and 64FDC boards which contained both single and double density data separators on board.
 
BTW, the card soaks up some 15 IO ports out of the S-100's possible 256s.
 
The manual for the board can be obtained here.
The schematic for this board can be obtained here.