I am thrilled to report that after some juggling which will be described below my faithful 1979 Cromemco Z2 is now interfacing with a 5.25" HD (High Density) diskette drive.
Why is this important? Well ...
The original Cromemco Z2 was able to connect to both 360K 5.25 and 1.2MB 8 inch floppy drives using (at best) the Cromemco 64FDC diskette controller. Today however 8" drives are no longer produced and 8" diskette media is rare, expensive and somewhat unreliable.
If you look at the electrical specifications of a 5.25 inch 1.2MB diskette you will notice that it is remarkably close to that of an 8" diskette drive. So the cunning plan is to connect the 5.25" drive instead of the 8" one.
But there is one problem. When IBM introduced the PC in the early 80's they changed the signal definition for pin 34 of the diskette drive. Initially pin 34 was to represent READY (RDY) but IBM changed it to Disk Change (DC). After a few years manufacturers optimised most diskette drive designs for RDY only and often did not even have jumper option to select RDY or DC. So without further ado here is the end to end procedure that worked for me
Pre-requisites
- Teac FD-55GFR 1.2MB 5.25" diskette drive . This drive has jumperable Ready
- Some fine wire. I bought some from eBay UK. You are going to use this to connect the 64FDC 5.25 inch disk Ready to the 64FDC 8 inch disk controller READY
- The Cromemco 64FDC disk controller. Well in principle anything newer than a 4FDC will work i.e. a controller which has a non Persci 8" disk electronics and IDC connector. My current 8" drives are YE Data 174.
(Click on the photo to enlarge) and identify the arrow showing pin 22. This is a pin closest to the card edge. The inner row of pins are the odd numbered ground lines. You dont want those! In fact the IDC numbering looks like this for the 8 inch drive connector 
and like this for the IDC34 5.25 inch drive connector
You will need to repeat this procedure to locate pin 34 of the 5.25 inch floppy IDC 34 interface. Pin 34 is the pin closest to the edge connector on the left.
Procedure
- Switch off the Cromemco computer and remove the 64FDC controller. Using my excellent diagram locate pin 22 and using the fine wire wrap around pin 22.
The put back the original 8" drive IDC 50 pin connector. That will keep your wire in place.
Now connect the other end of the wire to pin 34 of the 5.25 inch IDC interface as shown above (click on image to enlarge).You are now ready to reinstall the 64FDC and of course don't forget to connect the 2 wire Interrupt daisy chain cable if you are running Cromix.
- Install the Teac FD-55GFR diskette into the front panel of the Cromemco Z2. Make sure RDY is selected. I also selected the diskette as DS1 (counting from DS0 this would become a B: drive)
And now we are done!
And here is the beautiful result. Cromemco Z2 is on top with the new working HD diskette drive. On the bottom the 3 drive YE data 174 diskette chassis I built in 1980. Only 2 of those drives are now connected (C: and D:), A: and B: now being supplied from the Z2.
