MDS-800 Floppy Boot Issue

[Robert Babanats]

Hi All,

I have just started restoring 2 MDS-800 systems each with dual floppy boxes.   I have both systems booting to the Monitor 2.0.   Each system will load the floppy head on Boot, but neither will seek track 0 and no data is written to the 3000 hex buffer.    I have a status of 0D hex at 78.   Is there a way to check the channel boards PROMs with taking them out of the system.  If you have to take them out what devices can read these PROMs?  I would appreciate any advice on how to troubleshoot these floppy controller boards.   

[Martyn Vale]

Hi Robert,

 

Make sure the felt pad is still on the arm that presses down on the floppy.

 

Check out my video here for some tips:

 

https://www.youtube.com/watch?v=fH3LTZzj-sc

 

Maybe create some fresh floppies or at least try to format one using IMG2MDS

 

https://www.youtube.com/watch?v=XonRevJNwek&t=4s

 

I’m assuming here of course you have a double density card set. There’s a full set of waveforms in the manual which are worth checking as well.

 

The PROMS are 3604’s I think and reading them might be a pain, however it’s unlikely they are damaged. I would recommend carefully removing all the socketed components and putting them back in to clean the contacts though.

 

Bye

Martyn.

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[Robert Babanats]

The part 10 video has been a great help in getting me as far as I have got.   I  have noticed that the response at 78H on my systems do not have the double density bit set as the one in the video.     Is the double density bit set by a setting or is the channel board or interface board physically a different board.   I also tried to create a disk using the IMG2MDS video  and the floppy seeks track 0 and then locks up.

[Martyn Vale]

 

Hi Robert,

 

The single density board set has a different floppy drive interface board often with “CDC” printed on the top of the PCB. The channel board as far as we know is identical, however the firmware is completely different.

 

Could you post a picture of your boards or quote the PWA numbers please.

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[Robert Babanats]

The channel board part number is 1000467 on both systems and the interface board part number is 1000603 on both systems.

[Martyn Vale]

 

That’s a single density system by the looks of it. Awful pic of my one here:

 

 

Some additional info from Herb here:

 

https://www.retrotechnology.com/herbs_stuff/intel_FDC_field_guide.txt

 

Unfortunately I’ve not done any work on the single density board yet as I’m using a modified Zendex Z200A for single density currently.

 

The single density format is a standard IBM format and you should be able to use IMD to create floppies or us something like the HxC Floppy Emulator or even possibly a Gotek. My favourite route would be to use a Greaseweazle with a D-Bit 8 inch adapter.

 

https://www.ebay.co.uk/itm/126182450561

http://www.dbit.com/fdadap.html

 

Probably worth noting that Single Density Board Sets are fairly rare although a few have surfaced over the last few years.

 

I guess you’ve already tried mix and matching the boards you have ? Also on the MDS-800 you must have them inserted in the correct order in the card cage I think the Channel goes to the left of the interface card from memory, but it’s in the manual anyway.

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[Herbert Johnson]

I sent Robert what amounted to a primer on how to evaluate and test
floppy drives in some detail. My read of the discussion is that it
focused on the MDS controller(s) and not the drives. But if the drives
don't work, the condition of the controllers is undeterminable, absent
test equipment. Drive details are many and tedious; so I sent my long
note privately. Robert can choose to share results and further
discussion if he finds my email useful or informative.

The reported fact (may not be a fact) that the drives seem to operate in
some way but not completely on BOTH systems, suggests the drives may be
at fault. Unused drives can fail from lack of cleaning and lubrication.
They can certainly damage diskettes during testing (or any time of
course) so that has to be addressed. Details of drive operation call for
resolution and repair if damaged or inoperable.

Put another way: he needs a known-good drive to proceed.

There's known issues and resolutions for most drives - I don't think our
friend has identified the brand and model of drives his systems happen
to have.many considerations and suggestions accordingly.

regards Herb JOhnson

--
Herb Johnson, New Jersey USA
http://www.retrotechnology.com or .net
preserve and restore 1970's personal computing
email: hjohnson @ retrotechnology dot com
or try later at herbjohnson @ comcast dot net

[Robert Babanats]

I have continued troubleshooting these systems and I have tried the other two CDC drives with the same results, the systems loads the head, does not seek track 00, and freezes.

The drives are ready with a 0D hex status and the signals from the drive look reasonable to me.  I have a couple of questions.

1.   I have noticed that the SEP CLK/ line from the interface board does not route to the Shugart drive, but the SEP DATA/ does.     Do these systems need the Shugart drives withe the optional clock. separator circuitry in the drive to operate?

2.   Is there a listing of the embedded firmware of the boot code for the floppy that would show the sequence of steps to boot from floppy.

[William Beech]

Robert,

The boot code is located in the Monitor ROM.  What version of the Monitor are you using?  I have recreated source for most that we don't hold a listing for.

Bill

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[Robert Babanats]

The monitor display reads Monitor 2.0

[William Beech]

Robert,

Here is the Monitor Docs, source and listing.  I also have the tools to build the source on a PC.

Bill

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[Robert Babanats]

Thanks so much for providing the monitor documentation.    I am looking forward to looking at these files and seeing if I can get a clue about what is going on with my MDS 800 systems.

[Sid Jones]

Sounds like it’s ‘scope time!

 

If you make a breakout lead for the 37 way drive connector to have a peek at the signals, you could check if the drive step and direction line get waggled.

 

It is possible to turn the drive lead screw (with the drive unpowered and without a disc inserted) to move the head away from the Track 0 position.

 

Then putting in a disc and doing a system boot should step the head back to track zero.

 

Martyn Vale has some excellent videos on intellec MDS restoration.

 

And pointed out that every single floppy disc pretty much uses the same interface lines! 8” disc drives can be emulated with certain 3.5” drives!

 

Regards

 

Sid Jones

 

PS Where are you located?

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[Robert Babanats]

I have the scope out and I need some confirmation of my findings.   

The cables that I have for my MDS-800 appear to be wired according to the SBC 201 documentation manual.   The cables do not connect the Shugart Drive SEP CLK  to the SEP CLK Drive signal (Pin 50 on the drive) and the signal SEP DATA does not connect to the SEP DATA Drive Signal (Pin 48 on the drive).

The wiring does connect the SEP DATA single on the interface board to the READ DATA signal on the drive ( Pin 46 on the drive).  

I used the scope to confirm that without the SEP CLK and SEP DATA signals from the drive, the SR CLK IN/  and the SR DATA IN/ signals going to the channel board are not operational.

Can anyone confirm that the single density floppy interface board 100603 requires that both the SEP CLK and SEP DATA signals from the drive be connected and operational?   If so I need I need to rewire my cables.    Secondly, should the data signal from the drive that is wired to the SEP DATA input to the interface board be.the SEP DATA signal from the drive or the READ DATA signal from the drive.

Does anyone have one of these signal density MDSl-800 systems that is operating.

Thanks for you input.

[Herbert Johnson]

I'm reluctantly responding, even though I do not have these controllers
or an MDS-800 system. I know something about floppy drives.

https://bitsavers.trailing-edge.com/pdf/shugart/SA8xx/39025-1_SA800_801_Service_Manual_Mar82.pdf

A Shugart 800 drive manual will describe the operation of the drive, how
"read data" pin 46 is not the same as "Sep Data" pin 48 and and "sep
clock" pin 50. The occasional use of sep clock/data is where the drive
separates the clock from the data instead of the floppy controller. Only
the earliest floppy controllers "asked" the drive to perform that.

Be sure the cables in question are for the Shugart drive and not the CDC
drive. See what I found in this document:

https://bitsavers.trailing-edge.com/pdf/intel/iSBC/98349B_iSBC_211_212_Diskette_Hardware_Reference_1977.pdf

(This copy doesn't support text search, it's not OCR'ed.)

the 100467 is the floppy "controller channel board" with the 3001's and
microcode. May be same board for each floppy interface CDC vs Shugart
but MIGHT be different microcode for each drive interface. Possibly it's
only one microcode ROM that's different - read the manual.

the 100603 board is the CDC drive "controller interface". Look at PDF
page 191, a drawing of the PC board, I'm reading it now, "FLoppy Disk
Controller Interface (CDC)" and legend box lower right "1000603" The
drawing shows a board number on-the-board, lefthand edge on component
side "PWA 100603".


The floppy drive signals in this document, do include "separated clock"
"Seperated data" PDF page 182. I do not know if they are in use.

The page numbered "7.3 electrical connections" PDF page 170 - READ IT.
It talks about the "interface board" and cabling. It says among other
things:, "If you ordered and SBC 211 or SBC 212, you received two
cables" followed by some connection description. "If you ordered an SBC
201, you must build your own cables or order SBC 951 (Shugart compatible
cables) or SBC 952 (CDC compatible cables) from INtel.)

This suggests different cables for the different drive brands/models.

I cannot determine with clarity, if the so-called "CDC" drive interface
board is also used with shugart drives. Maybe "CDC" is a straight-wired
cabling and "Shugart" involves some crossed wires? I am speculating!!

Also: see section 5.5.2 "SHUGART DRIVES" versus section 5.5.1 "CDC DRIVES".

also: see the 100605 schematics, to trace cable wiring by ohmmeter from
interface board through cables to drive.

And so on. Somewhere in this manual, may be further descriptions of
cabling and drive connections.

Reading your earlier posts, you don't appear to identify the CDC drives
by model. Searching this document for a model identification, I FIND
IT. Chapter 5 (PDF page 123) says quote ".. operate either a CDC 9404 or
Shugart 800/801 ... or other electrically comparable drives."

https://bitsavers.trailing-edge.com/pdf/cdc/discs/floppy/9404/


https://bitsavers.trailing-edge.com/pdf/cdc/discs/floppy/9404/75897465_9404B_floppyMaint.pdf

PDF page 29, document page 4-9 4-10 "Read Data Composite. This line
contains the unseperated data and clock information".

PDF page 37, document page 5-6 "J1 J10 and J7 pin assignments" ( drive
edge connector. "Composite read data" is pin 46. *No sepreated clock and
data*. Drive pins 48 and 50 are *unlabled* in the document. Check the
CDC schematics (and the drive itself) to see if those drive-pins go
anywhere or are open or grounded.

*Therefore* it's conceivable that CDC-style drive cables, don't support
a sep-data/sep-clock pair of signals from Shugart-style drives. And I
can tell you from my 8-inch drive experiences, the Shugart seperated
clock and data are *exceptional* among brands and models of 8-inch
floppy drives.

That's all I got. I'm pointing at documents I found. read the documents.

Regards Herb

On 9/12/2026 6:07 PM, Robert Babanats wrote:
> I have the scope out and I need some confirmation of my findings.
>
> The cables that I have for my MDS-800 appear to be wired according to
> the SBC 201 documentation manual.   The cables do not connect the
> Shugart Drive SEP CLK  to the SEP CLK Drive signal (Pin 50 on the drive)
> and the signal SEP DATA does not connect to the SEP DATA Drive Signal
> (Pin 48 on the drive).
> The wiring does connect the SEP DATA single on the interface board to
> the READ DATA signal on the drive ( Pin 46 on the drive).
>
> I used the scope to confirm that without the SEP CLK and SEP DATA
> signals from the drive, the SR CLK IN/  and the SR DATA IN/ signals
> going to the channel board are not operational.
>
> Can anyone confirm that the single density floppy interface board 100603
> requires that both the SEP CLK and SEP DATA signals from the drive be
> connected and operational?   If so I need I need to rewire my cables.
>  Secondly, should the data signal from the drive that is wired to the
> SEP DATA input to the interface board be.the SEP DATA signal from the
> drive or the READ DATA signal from the drive.
>
> Does anyone have one of these signal density MDSl-800 systems that is
> operating.
>
> Thanks for you input.

[Robert Babanats]

Is the anyone who has a bootable SSSD ISIS diskette that would be willing to make a copy for me.    I can seen diskettes to be used of this copy.

I might have my MDS 800 SSSD system that will boot if I can get a bootable diskette.

Thanks

[Martyn Vale]

 

Hi Robert,

 

The Single Density Disks are a standard format unlike the M2FM used on the Double Density Drives. You could probably make your own using a 5 ¼” to 8” Adapter:

 

https://www.ebay.co.uk/itm/115729590176

 

then all you need is a PC with IMD installed or even easier a Greaseweazle

 

https://www.ebay.com/itm/257091903336

 

you’ll need to slow down the step rate on the Greaseweazle, but I can look this up for you if needed. The advantage with this method is that even if your drives are out of alignment it’ll probably still work. Another option would of course be an HxC or Gotek Floppy Emulator along with the adapter above.

 

I could create a 8” Disk for you but I’m in the UK and I’m guessing you’re in the States so shipping would cost a fortune and there’s no guarantee it would be usable by the time it got to you.

 

I may be able to modify IMG2MDS to do Single Density, but this would take some time.

 

Bye

Martyn.  

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