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The 16FDC was Cromemco's second generation floppy disk controller board. The 16FDC provided a complete system for floppy disk operation including serial I/O for an RS-232 terminal, a pre-programmed ROM with system bootstrap and diagnostic routines, and of course full read/write/format capability for any combination of single or double sided, single or double density, 5" (mini) or 8" (maxi) floppy disks. The 16FDC normally handled up to 4 drives in a daisy chain configuration, although 16 drives could be chained if the drives decode the 4 drive select lines. The 16FDC included an 8 channel interrupt system which was connected to the flag bits of the disk controller, serial I/O, and a set of interval timers. One of the interrupt inputs could optionally be connected to a 512 millisecond crystal controlled clock for real time interrupt capability. All timing on the 16FDC was referenced to an onboard crystal clock. This included the head load delay (which was switched according to the size of the diskette), the head stepping rate, the motor turnoff timer, and the watchdog timer in an auto-wait circuit. The data recovery circuit uses a Cromemco unique phase locked loop to optimize performance for each size and density diskette. Single density data was recorded in the FM format while double density data uses the MFM format. Density on a given diskette could be vary from track to track - though this would have been very non-standard. It could however be used with dual density diskettes that require the first track to contain a label, written in single density, which informed the operating system of the density and number of sides of the remainder of the diskette. The phase locked loop circuitry incorporated an adjustable trim capacitor on the circuit board to optimize the system using a scope and special disk.A writeup describing how to hook up this board with Shugart and Persci floppy disk drives can be seen here.
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This page was last modified on 04/23/2014