'V-Max secondary checks'
Author:Lord Crass (guest: search)
Date: Sun, Apr 03rd, 2011 @ 02:00 ( . )

I only had a bit of time with the C64 today, but I was able to try writing one of these long tracks with a 1541 (with Maverick's track editor though, not nibtools).

My assumption about 308RPM/291RPM was based on a regular track being 7800 bytes. This is incorrect. At maximum density setting, the drive spec says it writes 307692 bits/sec.

307692 / 5 / 8 = 7692 ($1E0C) bytes per track

This makes the problem even worse though. The Track Editor read in track 16 as being $1F76 bytes. I was able to adjust the drive speed to write a track this long, and it did write that track back, but when re-reading it with the track editor, it seemed to take longer to read the copy than the original. The resulting data looked correct, but perhaps this borders on unreliable depending on your diskette quality.

So, $1F76 (8054) bytes on a track is an increase in capacity of 4.71%

300RPM * 0.953 = 286RPM for tracks 16/17

Tracks 12/13 are 7806 bytes in length. In fact, tracks 1-13 are typically written at the same density. To achieve this density on a copy:

7692 / 7806 * 300RPM = 295.6RPM

Tracks 14/15 are 7598 bytes in length:

7692 / 7598 * 300RPM = 303.7RPM

And track 18 is handled by V-Max as standard density for that speed zone (285714 bits/sec = 7143 bytes per track). V-Max is showing 7156 bytes on this track, which is close enough to standard, so this track is written at 300RPM)

You can reduce syncs and track gap bytes for 16/17 as the protection doesn't check sync length on them. This should reduce the track to about 7908 bytes which could help you from overwriting the start of track. You still need to slow the drive down to about 286RPM though to get the proper density.

If nibtools won't write to a drive that slow, perhaps you can modify the source to either adjust the limits, or remove them entirely. It's in the write.c file. The line is

if( (motor_speed > 310) || (motor_speed < 290))

Or, if you don't have the dev tools to compile nibtools, but you're handy with a hex editor, you can try modifying the binary nibwrite.exe.

Search for: 00 00 91 43
Replace with: 00 80 8E 43

This will change the lower RPM limit to 285, which should be sufficient.

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