Micropolis FDC board - "Overclocking" for 360 rpm drives

[MarsMan2020]

I picked up a couple of really nice Hitachi 1.2MB 5.25" drives at VCF this year - cast housings, black faceplates, originally used in IBM mainframes to load firmware updates.

I was hoping to use these to replace the Micropolis drives in my Vector MZ - but I can't seem to find any option to jumper the drives for 300 RPM operation. They appear to be 360 RPM only.

I am wondering if I fed my Micropolis FDC board a 360/300*2 MHz = 2.4 MHz clock, and adjusted the center frequency of the VCO in the PLL circuit to 360/300*1 MHz = 1.2 MHz - would the board work with a 360 RPM drive? Pretty much everything on the board is 7400 series logic, I would expect that it would handle a 20% speed difference OK. Might have to adjust the one-shot durations up/down to match as well.

The one big problem I would see would be some of the timing requirements with the driver software on the Z80, which needs to write preambles/data/postambles with the correct timing in response to flags from the FDC controller when it sees the sector pulses. Still, I'd be surprised if 20% caused a big difference there.

Any thoughts? Is this worth trying? Or should I try to trace out the controller boards on the floppies and find a way to get them to 300RPM instead?

(Note: I fully understand that using these drives will not be disk-compatible with my Micropolis drives due to being 96 TPI vs 100 TPI - but due to the Micropolis drives being such a pain, I only have a couple of disks anyway. I'd use my Vector MZ a lot more if I had more reliable drives.)

 

[Chuck(G)]

Don't know about the controller on the Micropolis board, but I used to use 300 RPM floppy drives to read disks written at 360. Then it was a matter of swapping a 24MHz crystal for a 20MHz one.

So, same basic idea.

 

[MikeS]

Have you actually tried one as the 'B' drive? ISTR that a 1.2HD drive worked fine as is, but for some reason I couldn't get the MZ to boot from it. Didn't follow it up very much though, whether it's a speed isssue, the settings on the FDC, or whatever.

Then again, my memory ain't what it used to be and maybe I did change the speed...

m

 

[Chuck(G)]

You also could see if you could locate a couple of Tandon TM-100-4M drives--they're compatible with the Micropolis ones.

 

[MikeS]

You also could see if you could locate a couple of Tandon TM-100-4M drives--they're compatible with the Micropolis ones.

I think he said that he's looking for "more reliable drives" ;-)

 

[Chuck(G)]

Well, MPI made some 100 tpi drives (92M?), but they're pretty scarce. Teac made a 100 TPI drive, but it was pretty awful.

That pretty much leaves Tandon.

Actually, well-maintained Micropolis drives are very reliable. Got a few of them here and they all work.

But why not use a standard 720K 5.25" drive, say a Teac FD 55F? 96 tpi, but apparently it's not an issue.

 

[deramp5113]

I've got 100 tpi Micropolis and Tandon -4M drives working in my Vector Graphic systems, but I've also used readily available Teac 55-GFR drives in the machines when originally restoring the machines because they're much more reliable. As noted, the 96 tpi diskettes won't interchange with 100 tpi drives, but interchange between the 96 tpi drives is fine.

Mike

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