Inspection of Tandon TM100 designs for 96TPI vs 100TPI operation

(C) Copyright Herb Johnson 2007. Most recent revision dated Nov 16 2007. My S-100 Web pages are are at this Web link.

I have a copy of the Tandon TM100 manual as Vector Graphic (an S-100 company) copied and produced it. I read the manual, to look for any distinctions regarding 100TPI vs 96TPI operation. This manual includes schematics, alignment and repair information, so it's pretty complete.

The short story is that I saw NO STATEMENT describing specifically what made a drive "100 TPI" or "96TPI". I conclude it's the head/stepper mechanism, not any electronics. I don't believe a jumper or a PC board change will convert between these two.

As I read it and other Tandon docs, the TM-100 models are;

TM100-1 is single sided 40 track
TM100-2 is double sided 40 track
TM100-3 is single sided 80 track, 96TPI; TM100-3M is the 100TPI version
TM100-4 is double sided 80 track, 96TPI; TM100-4M is the 100TPI version.

(Historically, Micropolis produced a 100TPI drive early on, perhaps that's why Tandon used "M".). 35 and 40 track drives 5.25" floppy drives are all, to my knowledge, 48 TPI.

The VG/Tandon manual I have covers the -3 and -4 and -3M and -4M models. All 80-track drives have heads which write thinner tracks than 40-track drives. The heads alone do not make a drive 100tpi or 96tpi. The manual says the head is positioned over the track by a stepper motor/band and associated electronics. "This positioner employs a one-step rotation to cause a one-track linear displacment". Other statements in the manual describe one step equals one track operation.

I see no adjustment procedure to set TPI, only to align to specific tracks on an alignment disk, different tracks depending on 96TPI or 100TPI by model. I suspect it's the same physical track on the diskette, it just takes differnent track counts to get "there", but I don't know that for a fact.

Finally, the PC schematic drawings are listed as either "96/100TPI...unipolar" or "96/100TPI...bipolar". Unipolar and bipolar refers to differences in the head stepper electronics, which are not cross-compatible. The unipolar model has FIVE wires, four coils and a common ground. The bipolar stepper has FOUR wires, two pairs of coils each in parallel, without a common connection.There are apparenly no differences in the electronics for 96 TPI or 100 TPIoperation. (Thanks to Andrew Lynch for pointing this out to me.)

Here are two photos of Tandon disk circuit boards, from TM-100-1 and -2 drives I have on hand. here's one and here's the other The circuits near the head cables (center right in these photos) show different layouts, different numbers of transistors. (These may not be identical to the schematics I have for the -3, -4, -3M, -4M models, as these are older drives.)

All this strongly suggests that it's not the ELECTRONICS which determine track spacing, but simply the structure of the head and stepper mechanism . I saw no documentation on repairs to the stepping motor and head assembly; there were docs on other assemblies. So I don't think, based on my reading, that one can readily change a 96TPI drive to 100 TPI or vice versa.

Next, I looked at a Tandon TM-100 drive. I had the TM-100-1 and TM-100-2 drives at hand. These are 40-track drives, single or double sided. The head and stepper mechanism is typical. Here's a photo of that mechanism.

The head runs along a track of some sort. The stepper motor rotates a roller, around which is a metal band. Either end of the band is connected to the slide which holds the head. As the stepper rotates, the band converts rotation to linear motion and steps the head. The "step" size is determined by the angular rotation per step; and the diameter (or circumference, "pi" times the diameter) of the roller.

I presume the steppers are identical for the 80-track drives and for the 40-track drives. A typical stepper steps 200 times per revolution or 400 steps/rev. (I've not checked these.) I speculate, therefore, that the 96 TPI drive and the 100TPI drives have rollers of different sizes. A larger roller would move the head further as it would have a larger circumference. I'd have to count steps to determine the step count per revolution.

This confirms to my satisfaction, that one can't electronically change a 96TPI drive to 100 TPI or vice versa. Precise stepping has to be based on mechanical geometry, and that is based on the repeatability of a stepper motor from step to step; plus good mechanical hardware. I would not mess with ANY drive's steppers and bands if I could avoid it at all. Who needs a drive that produces disks that are SLIGHTLY out of alignment?

Herb Johnson

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Herb Johnson
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Copyright © 2007 Herb Johnson