5 is the offset, not interleave. "g=c800:5" simply means execute the program from memory address C8005.

But where can i find the interleave number? I also found out that this controller wasnt ment for the bigger MFM harddisk drive.. It should that the DTC 5150X MFM controller is a ST506 interface only controller ( for the first generation MFM drives 5- 20MB.

I believe the best controller I found for these ST 251s was the WD-1002A WX1. This one will support the full 40MB capacity. The model without the "A" in the name only does 20MB drives. I also discovered that you could run an ST 251 on an RLL controller (like the WD-1002A-27X) and format the drives to almost 60MB. Not sure how reliable that is though.

You can choose any interleave, and only thing you will loose is some performance. Interleave is a factor used to optimize the performance of HDD, based on its native head movement speed and plate rotation speed in relation to each other. Basically, you tell the formatting program not putting the sectors adjacently but in a different order. This is quite good a reading:

en.wikipedia.org/wiki/Interleaving_(disk_storage)

and as you can see from the above article, there are programs to determine best interleave.

Robin4 wrote:

But where can i find the interleave number? I also found out that this controller wasnt ment for the bigger MFM harddisk drive.. It should that the DTC 5150X MFM controller is a ST506 interface only controller ( for the first generation MFM drives 5- 20MB.

Here is a helpful article:

http://www.minuszerodegrees.net/manuals/DTC%2 … ion%20Guide.pdf

The drive table on the last page has ST-251 listed so I wonder the controller would support this drive.

Interleave also relies on the system you are running the drive in. PC XT Interleave was 6:1, PC AT was 3:1. I believe Norton utilities for dos has a utility called Norton's Calibrate which can find the fastest interleave. I remember running it on a PC XT with a V20 and it changed the interleave from 6:1 to 4:1.

Also, interleave will only impact performance. You can safely use the wrong interleave setting and the hard disk will work without problems (but slower).

A lot slower - if the interleave was set too tight, it caused the "hard disk not much faster than floppy", as the controller read on sector, then missed the next and had to wait a whole revolution.

On a controller with a track buffer (some of the later era ones), the interleave was 1:1 - the controller buffered the track.

On AT systems, 3:1 or maybe 2:1 - by staggering the sector numbering 2 or 3 apart, it gave time for the controller to be ready for the next.
The very slowest XT needed 6:1 or even 7:1 - meaning it could access something over 2 sectors per revolution - that setting will work on faster systems as well, but limits the disk speed compared to an optimize setting.

Yes, Norton Calibrate was the program, remember using it in the days of MFM & RLL drives - probably not included in the modern utilities, though. It uses the test cylinder of the drive to check different interleaves, and then converts the drive interleave.