5.25" disk drive signal question

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5.25" disk drive signal question

Postby elianda » 2019-5-16 @ 09:38

Hello,

I recently got two TEAC YG-437F 5.25" disk drives that were used as Robotron KS5601.

From the documentation I could find at http://www.mpm-kc85.de/html/D004FDDrive.htm the drive is 80 tracks double sided.
Since I don't have a KC85 any more I wanted to test the drive on a regular PC.

So I jumpered both to DS1, but ran into another problem. The IBM/PC interface wants on Pin34 the Disk Change (DC) signal, whereas the Shugart Interface has Drive Ready (RY) there. The drive has no sensor for DC and does not support DC. The only other option beside RY is to jumper XT, which sets the signal on pin 34 permanently on HIGH.

Jumpered to RY I get in MS-DOS 'drive not ready' which is somehwat expected.
Jumpered to XT the drive seems to work as 360K drive.
Set to 1.2 MB type in BIOS formatting fails with 'Error on Track 0'. I suspect this has something to do with pin 2 which is disk change detect on shugart and density select on IBM/PC?!?

Someone with a bit more knowledge of drives can explain me what is going on?
e.g. why does the drive when jumpered to XT works as DD but not as HD? Can it be operated without DC signal on a IBM/PC or does it induce other issues?
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Re: 5.25" disk drive signal question

Postby Deunan » 2019-5-16 @ 09:53

The FDC on PC side has to output a density signal to let the drive know what frequency to use when reading/writing. Some drives ignore this signal and instead rely on their own media sensor. In such drives you can't format a 1.2MB floppy to 360k or 1.44MB one to 720k. Well, not without fooling the sensor and that's actually easy to do with a piece of masking tape. Do note that some drives have a mechanical detection (a microswitch) so the tape must be sturdy enough for that.

Does your drive have media sensor? If it's the same drive as on the KC85 photos then I can't see one. There's only the write protect LED barrier. Could be it just doesn't support HD mode at all?
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Re: 5.25" disk drive signal question

Postby retardware » 2019-5-16 @ 10:08

Did there exist any manufacturer for HD diskettes in the COMECON?

Iirc these Robotron-used 5.25" 80trk drives were all 720k (eg DSDD).
And, I always believed there is no density sensor on 5.25" drives.
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Re: 5.25" disk drive signal question

Postby Deunan » 2019-5-16 @ 10:51

retardware wrote:And, I always believed there is no density sensor on 5.25" drives.

You know, I think you are right and I just confused this with the C64 method of making a floppy usable from both sides on single-head drives. What I said above is true but for 3.5" drives.
However, if the drive has no density input signal then it's permanently tied to HD, or most likely in case of 5.25, DD. So the format command will fail because the drive is simply unable to cram all 15 sectors onto a track using 250k frequency.
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Re: 5.25" disk drive signal question

Postby elianda » 2019-5-16 @ 12:03

Ok so the default disk format when used with a KC85 is:

1024 bytes/sector
5 sectors/track
80 tracks/side
2 sides
equals 819.200 byte.

I further digged up a manual in german: http://www.robotron-net.de/Service/Dokus/TEAC_MFS16.pdf
It references that the drive is a variant of the TEAC FD-55F: http://v1050.classiccmp.org/chipset/fd55.pdf

According to page 9 the drive should be able to put 16 sectors a 256 byte on a track: With 16 * 256 * 80 * 2 = 655360 byte.
Now from the DOS supported disk layouts archived here: https://jeffpar.github.io/kbarchive/kb/075/Q75131/
I think the best fit would be the 5.25" drives 360K layout, just with 80 tracks instead.

I will try if the drives does that.
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Re: 5.25" disk drive signal question

Postby elianda » 2019-5-16 @ 15:18

So the trick is to set in BIOS for the 5.25" drive: 3.5" 720 kB.

Then format allows to format a disk with format b: /T:80 /N:9 to 720 kB.

If the BIOS is set to 5.25" 360 kB then format says parameters are not supported.
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