The reason that we want to do this is that my old Cromemco computers primarily use 8 inch double sided, double density floppies with a formatted capacity of 1.2MB. Whilst in the last tutorial we saw that we could use 5.25 inch HD diskette drives, it would be even better to be able to use a 3.5 inch HD diskette drive and media because:
- This media is freely available (well just about)
- It is shielded and robust
- It should last another 25 years with ease
How is this going to work?
The Japanese have a standard format not used in Europe of using a 2MB, 3.5 inch diskette in a 1.6MB format. Selected 3.5 inch drives can be jumpered to operate in 1.6MB mode. Note that this format almost exactly mimics the 8 inch drive format and was clearly developed in Japan so that Japanese could move from 8 inch to 3.5 inch design with minimal software changes.
This chart shows the tech specs of the Samsung SFD-321B diskette drive
Notice that in 1.6MB mode the rotational speed is set to 360 rpm. That is important because Cromemco Cromix and later versions of CDOS check that the drive is revolving at around 360rpm else refusing to believe that it is an 8 inch diskette drive.
Here is the initflop Cromix source to show this:
if (!print) printf(STDOUT,"\nTesting:\n\t");
printf(STDOUT,"Rotational speed:\t%ld RPM\n",speed);
if (!speed) printf(STDERR, "\tNo index pulses or counter not functioning.\n");
if ((oflags & SMALL && (speed <> 306)) || (!(oflags & SMALL) && (speed <> 367))) printf(STDERR,"\tSpeed out of tolerance\n");
You will need the following
- A Samsung SFD-321B 3.5 inch diskette drive
Chosen because it is still freely available as a New drive from eBay or similar.
- A watchmakers eyeglass magnifier or something similar so you can clearly see the diskette PCB
- A soldering iron and some fine wire
- A PC-AT style twist floppy cable
- A Leatherman
First disassemble the floppy drive. This means pull off the plastic front, one screw for one side panel, 4 screws for the other side panel to expose the electronics.
This is how it looked before I changed anything
- Take your Leatherman and rip out pin 2 of the floppy drive on the male IDC connector
Pin 2 is the density select pin of the drive. We will tie this low but we don't want it talking to anything outbound of the floppy
Now identify the 3 areas that need to be modified
- Unsolder the 0 ohm bridge on the DC disk change connector and put it back on the adjacent RDY ReaDY pad set
- Take some fine wire and connect between pin1 (ground) and pin2 (density select) to force it permanently low.
- Locate OPA pads and connect a wire between them
At this point I'll go for the sympathy vote. Since both my hands are still partially paralysed from my fall on January 10th these three steps took literally hours. Eyeglass installed, holding the soldering iron, and wire and drive, well it just wasn't easy. I'm sorry to complain but there it is.
Here we see the PCB after my 3 modifications
Now re-assemble the drive, and connect to the Cromemco 64FDC disk controller thus: You must use the 2 non twisted connectors. First drive is selected as A: (i.e. DS0) and this 3.5 inch drive is already preselected as DS1 (B:)
And it all works
Formatting the 3.5 inch diskette for Cromix, then making a filesystem on it, then checking that the diskette is 100% readable. Finally asking the Cromix Driver to show its reported diskette characteristics.
Copy Cromix 162 files to the formatted diskette, then writing the 162 boot program to the boot sectors of the diskette.
Booting Cromix 162 from a single 1.2MB, 3.5 inch floppy diskette