Trouble with 8" Tandon TM848-02

[marmotking]

I have some 8" disks I'd like to image, and a Tandon TM848-02 connected to my PC via a dbit adapter. The dbit adapter is connected to the end of the pc's floppy cable (i.e. where A: would connect, after the twist). I can write disk images to it using my catweasel MK-IV. However, when I go to read them back, I start getting sector errors around tract 58 to 62. It seems to vary. Now it could just be a bad drive. The media is brand new. But there's another problem too. When I put the terminating resistor in the drive, it shorts the PC power supply or does something unholy to it such that it won't turn on. So, the drive is being used unterminated, when it actually should be terminated. Someone once told me through someone else that perhaps the shorting problem is a blown decoupling capacitor? I don't have the schematics for the drive, so I'm not sure if this is true and I also wouldn't know which capacitor it is. Anyone have any information about these issues? It would be greatly appreciated!

 

[Chuck(G)]

Well, the dbit adapter is a bit of a waste, as the 848 doesn't have a TG43/RWC input like older drives do--a simple wired connector would be adequate.

Terminating resistors are 150 ohms, snd are present on 7 signal lines, so at maximum (all lines pulled low) it amouns to (5/(150/7)) = about 200 ma., The drive itself is rated to draw no more than 700 ma, so you're still under 1 amp on the 5v lines.

Where I've seen trouble on these is with the decoupling cap at the +24 input--and the 7812 voltage regulator. You may want to check both.

The service manual is on bitsavers, so maybe that will give you some information.

Not my favorite 8" floppy drive.

 

[MikeS]

Unfortunately AFAIK the manual on bitsavers is for the plain TM848; the -02 (and revisions) is considerably different, using an 8748 instead of discrete logic.

On at least one of mine there's an 8th resistor in the pack which is *not* connected to Vcc; no idea what it's for.

Actually I kinda like the TM848, especially the -02s.

@OP: An odd problem indeed, the PC PS shutting down when you insert the terminator; any short would still be limited by a 150 ohm resistor. If you've got a few discrete resistors 150 ohm or even a little higher in value, why not try inserting one resistor at a time into the socket and see what happens?

As Chuck says, I'd just bypass the RWC compensation if it's not too difficult; have you got any known good disks written on another drive by any chance?

 

[Chuck(G)]

Thanks for catching that, Mike--the TM848-02 != TM848-2. Nothing like confusing the customers. Does the 848-02 still use the (vastly overloaded) 7812 to drop the +24 to +12?

But you know my opinion of Tandon drives in general...

 

[MikeS]

Thanks for catching that, Mike--the TM848-02 != TM848-2. Nothing like confusing the customers. Does the 848-02 still use the (vastly overloaded) 7812 to drop the +24 to +12?

Yup, although the later one does use an LM340.
And yes, their numbering system left a lot to be desired; mine's a TM848-02 mod II, guess after using -2 to indicate DS and -02 to indicate the MPU version they had to turn to a Roman numeral II ;-)

But you know my opinion of Tandon drives in general...

... and in particular his later HD products... ;-)

Although I had problems with the original 848s the later ones did seem to be considerably more reliable, but these days I prefer a nice 1.2MB 5.25" drive.

 

[modem7]

When I put the terminating resistor in the drive, it shorts the PC power supply or does something unholy to it such that it won't turn on.

Correct type of terminating resistor, and orientated correctly?

The dbit adapter is connected to the end of the pc's floppy cable (i.e. where A: would connect, after the twist).

The D Bit FDADAP web site contains, "It has 34- and 50-pin connectors which can be connected to the PC floppy controller and the 8" disk drive using simple straight-through ribbon cables". A friend of mine who has the adapter found that he had to use straight-through cables to get a working solution.

 

[MikeS]

Correct type of terminating resistor, and orientated correctly?

Yeah, I was going to ask that, at least the value and if it is indeed a terminating resistor; I don't think the orientation matters in this case since it's a DIP package with the resistors just going straight across AFAIK.

The D Bit FDADAP web site contains, "It has 34- and 50-pin connectors which can be connected to the PC floppy controller and the 8" disk drive using simple straight-through ribbon cables". A friend of mine who has the adapter found that he had to use straight-through cables to get a working solution.

Good point, but I'd think that if there were a problem with the drive select or related signals then it would not have read anything; still, something to check, especially in view of the problem with the terminator.

 

[Chuck(G)]

Even if the resistor pack had been other than that recommended, it would not have caused a short-circuit, unless there's something really messed up with the signal cable--and the terminator pack was made up of low (e.g. 20 ohm) resistances.

The bitsavers service manual should still be useful for the mechanicals, such as alignment, etc. as well as signal and power definitions.

 

[MikeS]

FWIW the *user* manual on bitsavers is for the -1E and -2E which is the -02 model AFAIK. No schematics but it does give some tech info and also covers the many jumper settings on both revisions of the -02 PCB.

http://www.bitsavers.org/pdf/tandon/TM848_userMan_1983.pdf

There are an XC and an IC jumper which control whether write current is controlled by the drive or the controller; the OP might want to check those.

 

[Chuck(G)]

FWIW the *user* manual on bitsavers is for the -1E and -2E which is the -02 model AFAIK. No schematics but it does give some tech info and also covers the many jumper settings on both revisions of the -02 PCB.

http://www.bitsavers.org/pdf/tandon/TM848_userMan_1983.pdf

There are an XC and an IC jumper which control whether write current is controlled by the drive or the controller; the OP might want to check those.

If the OP is using the DBIT adapter, should that make any difference?

 

[marmotking]

First, thanks for all the replies!

Ya, I know that I don't need the dbit for this drive, but I like it because it makes the cabling very easy. The drive is labeled as TM848-02, but it does not use a microprocessor, it's all discrete logic. So, it's apparently not the same as the -1E or -2E since it looks like they use a microprocessor. Here are photos of it.

http://download.marmotking.com/img_9480.jpg
http://download.marmotking.com/img_9481.jpg

So it doesn't appear to match the -1E or -2E. Looks more like the drive described here, which does show 150 Ohm pull ups.

http://maben.homeip.net/static/S100/tandon/diskette/Tandon TM848 Specification and Schematic.pdf

I measured the terminator that was with the drive when I got it. It's definitely not straight through. I tried a straight through one that i had laying around here and it gave worse results. None of the sectors read right with that one. So, I'm not sure.

It's also interesting to note that I have a second one of these drives and it does the same thing when terminated. Perhaps my PC power supply is bad?

 

[MikeS]

Right you are; my mistaken assumption that the -02 meant one of the CPU versions but it looks like -02 is indeed equivalent to -2 and it's the mod II that denotes the newer types.

In that case the service manual on bitsavers probably does apply to your drive although there were many revisions and mods; as it happens though, two of mine are the same PCB & unit rev as yours, rev E1 and M respectively.

What do you mean the terminator is not "straight through"? It should just be 150 ohms between opposite pins, i.e. 1-16,2-15,...8-9, and no connections between any of those pairs; effectively 8x150 ohm resistors side by side. A direct connection instead would not be good, effectively shorting +5V to ground through the related driver.

 

[marmotking]

The terminator appears to be bussed. I can measure 130 ohm or 260 ohm between any two pins. I seems like it's a "bussed" network rather than an "isolated" network.

For example, pin 1 to 16 measures 130 ohms. pin 1 to 15 measures 260 ohms.

 

[MikeS]

The terminator appears to be bussed. I can measure 130 ohm or 260 ohm between any two pins. I seems like it's a "bussed" network rather than an "isolated" network.

For example, pin 1 to 16 measures 130 ohms. pin 1 to 15 measures 260 ohms.

Assuming that's out of circuit, that sure doesn't sound right.

 

[Chuck(G)]

Okay, here's how you figure which kind of resistor that you need.

If it's a non-bussed terminator, all pins on one side of the DIP terminator socket will be connected together (e.g. 15,15, 14...9(. If it's bussed, then one pin will be connected to +5, the others will be connected to various input lines on the signal connector.

 

[MikeS]

Okay, here's how you figure which kind of resistor that you need.

If it's a non-bussed terminator, all pins on one side of the DIP terminator socket will be connected together (e.g. 15,15, 14...9(. If it's bussed, then one pin will be connected to +5, the others will be connected to various input lines on the signal connector.

I happen to have a PCB from the non-LSI modII version in front of me, and pins 9-16 are connected together and to +5V. Like I said, what he needs is 8x150 ohm resistors side by side.

 

[Dwight Elvey]

Hi
He needs to measure on the drive the resistance between the pins of the
terminator socket and the +5v and ground. Otherwise, we are just guessing
what he needs( unless we have an exact one working ).
The fact that it works at all without a terminator is also interesting.
Most drive that don't have at least one drive with termination on
the line will not work. Many will not work if there are too many terminators
( fixed a few with that myself ). If he has another newer drive on the line,
most of these have some type of active termiation.
Dwight

 

[Chuck(G)]

Why ground, Dwight? Floppy terminators are simple (usually 150 ohm) pullups to +5. That socket ought not to have any pins that exhibit continuity with ground.

The usual setup with a Catweasel is a single drive, though I have used 2 on occasion, so I doubt that there's another terminator in play.

The fact that an extra 200 ma of +5 draw causing his power supply to poop out is a bit disturbing. I'd recommend a separate power supply for the drive and the DBIT gizmo. It should need to supply less than 1500 ma to both, if spec sheets are to be believed.

 

[MikeS]

Hi
He needs to measure on the drive the resistance between the pins of the terminator socket and the +5v and ground. Otherwise, we are just guessing what he needs( unless we have an exact one working ).
Dwight

We're not guessing, Dwight; we have the service manual and I do in fact have four identical working drives with terminators.

This PS issue seems odd indeed, but we don't know what's in that terminator; a direct short instead of a resistor would certainly load the PS (and probably also take out the corresponding driver ).

Since it looks like the OP needs a correct terminator anyway, my suggestion is still to insert individual 150 (or somewhat higher) resistors into the socket one at a time and see what happens.

 

[Dwight Elvey]

Hi
If you have the right manual, then as you say, putting resistors
into the socket would work fine, if he can't find the correct
terminator.
As for Chuck. I've seen unused pins on terminator resistor sockets
tied to ground. If the terminator crosses power and ground, it
can do just what he is talking about. It doesn't hurt to check.
Dwight