Intel 8272 based floppy controller & Missing Address Mark

David Kuder dk at
Thu Nov 28 23:34:20 CST 2019

After digging in the manual for the controller card, it appears they have set it up to only support single density FM formatting.  Even still, formatting the disk 568KB FM with either clock speed 8MHz or 4MHz (and 500KHz / 250KHz WCLK respectively), same outcome with missing address mark.  I think I’m going to scrap the floppy card and design a fresh one using the FDC37C78. Less things to go wrong.




On Thu, Nov 28, 2019, 1:08 AM Fred Cisin via cctalk <cctalk at <mailto:cctalk at> > wrote:

I'll agree with Chuck, that FM/MFM seems a likely possibility.

index blindness can prevent seeing the first sector, but it should see 
other sectors.

What is on the disk that you are trying to read?

You are using an 8" drive, but you mention settings for 720K. 
720K (MFM) is normally 250K bts per second (300 K at 360 RPM in a 1.2M)
8" (MFM) is normally 500K bits per second.
8" FM is 250K bits per second. 
Is the data transfer rate consistent with what you are trying to read?

Do you have a scratch disk that you can spare?
Set up the code to FORMAT a track (eventually appropriate loops to format 
the whole disk).
After formatting a track, and without doing any stepping to another track, 
try to read it.  FM/MFM and data transfer rates and even sector 
header (CHRN) should therefore be compatible.
If you have another machine, try reading THAT track on the other machine 
and check what it sees.

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