View Full Version : The 4FDC and 5.25" Diskette Drives

April 6th, 2010, 11:06 AM
There seems to be a lot of confusion about the compatibility of 5.25" drives with the Cromemco 4FDC. I'm starting this thread to provide a definitive answer.

It's well-documented that the 4FDC doesn't do data separation for 8" floppy drives; however, there seems to be question about whether that's true for the 5.25" option. Many web resources state that the 4FDC does not do data separation because the WD 1771 Floppy Controller does not have an internal data separator. This is untrue: there is a data separator, but Western Digital has this to say:

NOTE: Internal data separation may work for some applications. However, for applications requiring high data recover reliability, WDC recommends external data separation be used.

This explains why, as several other people have noted, the WD1771 almost always requires external data separation. Pin 25 of the WD1771, /XTDS, allows the internal data separator to be disabled in favor of an external separator (/XTDS is active low...tying to 0 V disables internal data separation).

The /XTDS line is selected, along with FCLK and READY, by the 4FDC's 8"/5" select line. Thus, when an 8" disk is in use, the data separator is turned off, and FCLK and READY are fed into the 1771. When a 5" drive is in use, the data separator is turned on, and FCLK and READY are disabled.

As such, any 5.25" DSDD floppy drive should be useable with the 4FDC, not just the Wangtek drive specified in the 4FDC manual. I'll confirm this and post as an update. Depending on the 8"/5" select line's logic, it may also be possible to use 5.25" HD drives as 8" drives with the 4FDC, providing that the 4FDC either detects that a 5" drive is in use, or the 8"/5" line is forced into a constant 5" state. I plan to test this possibility, along with the use of an external data separator circuit, at a later time.

April 6th, 2010, 06:49 PM
I used Tandon 100-2a's with my 4FDC and it seemed to work fine, at least the A drive worked.