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The Memotech MTX Series

MEMOTECH  DISK UPGRADE

This is a work in progress - to be continued . . . .

 

Background

In the heyday of home computers in the 1980s, floppy disk drives were the only affordable random access storage devices available. In the early part of the decade, 5.25" disk drives were dominant but were gradually superseded by 3.5" drives.

The media used in floppy drives consists of a flexible plastic "disk" with a magnetic coating, encased in a flexible (5.25") or rigid (3.5") carrier. Due to the nature of their construction, floppy disk media is very susceptible to damage, it is sensitive to foreign bodies including dust and condensation, 3.5" diskettes offer better protection than 5.25", but are not immune from the problems. Storage conditions such as temperature and humidity, greatly influence the longevity of the media, but all diskettes will degrade with time, the magnetic coating can become brittle and separate from the substrate and/or mould can grow on the surface of the media. As well as rendering the affected diskette unreadable, putting a badly degraded disk into an otherwise good floppy disk drive can also deposit foreign bodies on the drive heads, even worse, these deposits can also damage other media subsequently put into the drive.

These days, floppy drives in PCs have been replaced by more reliable, higher capacity, media such as CDROM, DVD, USB flash drives etc. Unfortunately, there are currently no USB interfaces available for Memotech computers, other solutions, including the HxC SD card floppy disk emulator, are available, and while they work well, they are not cheap and obviously cannot read legacy media.

As described on my FDX disk page, the first floppy disk drives available for use with the Memotech range of computers were 5.25" drives manufactured by Qume and Teac. The QumeTrak 142 and Epson SD-521 were compatible with Memotech Type 03 disks, i.e., DS/DD 40 tracks, with a formatted capacity of 320kbytes. These drives were supplied with the FDX and early SDX disk upgrades, later SDX controllers had integrated 3.5" disk drives, including the NEC FD1036A.

As Memotech systems are now close to 30 years old, problems with original diskette media is are only to be expected and problems with the drives themselves are highly likely. You may be considering replacing the ageing drive(s) on your Memotech disk system, if so, you might find the information on this page useful.

 

Potential Upgrade Issues

The original design of the floppy disk drive used jumpers on the drive to set its address which would then be selected by the disk controller using pins 6, 10, 12 & 14 of the Shugart interface. The IBM PC muddied the waters somewhat by not adhering to the Shugart "standard" for drive selection and cable termination.

To simplify the assembly process for manufacturers, later PCs have all drives factory configured as ID 1 and the floppy interface cable does the device selection by incorporating a twist in the cable - the device at the end of the cables has pins 10 to 16 reversed, resulting in the drive at the far end of the cable becoming ID0 and the PC interface only supporting two floppy drives.

Floppy drives are usually connected to the host controller using 34 way ribbon cables, in most cases, the cables are internal to the computer and so are relatively short, say, 12-24 inches long, if external drives are used, the cables will almost certainly be longer. To optimise the signal quality, like other data busses, floppy disk cables should have termination resistors at the end of the cable farthest from the controller. With a few exceptions, in older 5.25" drives, such as the Qume, termination was done with a 150 ohm resistor package plugged into a DIL socket on the last drive on the bus which connected resistors between the signal lines and +5V. The terminator is connected to only the output lines from the controller, i.e., DS0, DS1, DS2, DS3, Motor On and Head Load. 

The PC also modified the way that bus termination is done - newer drives, 3.5" in particular, do not have removable terminator packs, all drives have a terminator package permanently installed, usually with values of 360 ohms. When two drives are connected to the bus, the terminators are effectively in parallel, resulting in a combined resistance of 180 ohms - close enough to the "standard" 150 ohms.

These points need to be borne in mind when considering replacing drives in a Memotech disk system with newer drives from the PC era. Drive selection using a twisted floppy cable will not work - the interface cable should not incorporate a twist and drive selection must be done on the individual drive(s). This can be problematic on newer 3.5" drives when many do not have jumper selectable IDs.

 

My FDX Disk Upgrade

Original Disk Configuration

I bought a twin disk FDX system in 1984 which had two QumeTrak 142 drives installed, in addition, I purchased a second FDX off ebay.uk in 2009, which had the same model drives fitted. My original media was starting to degrade and the drives were becoming increasingly unreliable, no doubt due to the degraded quality of the media being used. Although I could have tried to source better quality, newer media, I decided that the best option was to replace the 5.25" drives with 3.5" drives.

The highest capacity supported by the FDX disk controller is with Memotech Type 07 drives - DS/DD, 80 tracks, giving an FDX formatted capacity of 640kbytes. This type is compatible with 1MB capacity DS/DD drives which gave a formatted capacity of 720kbytes on an IBM PC. New DS/DD drives are hard, if not impossible to come by, but HD (1.44Mb) drives are still readily available, these drives have the same number of tracks as 720k disks, but have a higher recording density - 18x512 byte sectors per track, rather than the 9 found on 720k disks. A HD drive can also read and write 720k disks but needs to know what disk type is in the drive - the drive has a media detection function -  another hole on the opposite side of the diskette from the write protect tab is present on 1.44Mb media. Although the drive can be tricked into treating HD media as DD by taping over the density hole, this is not a good idea - see this page for why.

 

Replacement Drives

I had previously had good results from using a Sony MPF920 drive with my SDX, so I decided to install two of these drives in my FDX. The version of the drive that I was able to source off ebay.uk does not have a jumper selectable ID, but with care, it is possible to make a small modification to the PCB as described on my SDX page.

When I was trying to resurrect my failed FDXs, I had purchased a pair of external 5.25" 40/80 track disks originally designed for use with a BBC micro. Since I had found that these drives worked well with my FDX, I decided that it would give me extra flexibility if I could connect these drives alongside my 3.5" ones. The FDX controller can support up to 4 floppy drives and has a cut-out on the rear of the case intended for an external drive cable connection, but unfortunately, Memotech did not install connectors for external drives.

What I have in mind, is to extend the internal floppy drive data bus to a bulkhead connector on the rear panel, this would allow me to attach the external drives as required.

   
The internal floppy cable, having three female IDC connectors for the disk controller and two internal drives, with a connection to a bulkhead connector on the rear panel.
The external floppy cable, with a straight through connector to the external drive enclosure.

 

   
   

The most obvious problem with this is the differing requirements for bus termination when external drives are and are not connected. If a standard terminator pack were fitted on the last drive in the chain, it would be in parallel with the "hard wired" 3.5" drive terminators and would bring the combined resistance down to around 80 ohms :-

1 / R = 1 / 360  + 1 / 360  + 1 / 150

R ≈ 81 ohms

This is potentially going to be a worse situation than if no external terminator was fitted, in that case, the bus would still have termination resistors in the expected range, albeit that they were not installed on the last drive in the chain. As the total cable length will be relatively short, I am optimistic that there will not be a problem when I connect external drives - but this will only be proven once once I have been able to try it.

Of course, this is contrary to what the FDXC1 disk controller manual says: "The combined length of both daisy chains [5" and 8"] must be less than 10 feet and each must be properly terminated. Proper termination requires that if double sided drives are installed, then the drive at the physical end of the chain should be double sided, this ensures that the side select line is properly terminated." Whilst all drives will be double sided, the termination will not be as intended by the Shugart standard, i.e., not at the last drive in the chain.

 

Mounting Arrangements

My page describing disassembly and assembly of the FDX provides more detail on how the original 5.25" drives were installed; the FDX used a "unique" mounting arrangement, rather than the standard fittings used by IBM PCs and clones. Although the drives themselves had the standard 5.25" half-height form factor with the usual fixing positions, they were installed the "Memotech way" - i.e., customised for the FDX.

   
The original floppy drives sit on an aluminium plinth and are secured from underneath, there is no real way of removing a 5.25" drive from the plinth while it is still in the FDX chassis.

Four hex headed bolts, like the two that you can see at the bottom of this photo, are used to fix small, right angled, brackets to the sides of the drives which are then used to fix the drives to the plinth.

As you can see in the next photo, other hex headed bolts are fitted from underneath and mate with the threaded portion of the right angled brackets.

A profile view of the plinth. The two "feet" shown at the left hand side of the photo straddle the PSU towards the rear of the case, each is fixed with a bolt through the case bottom and secured with a nylock nut.

The front of the plinth is supported by a ledge in the front panel and two smaller diameter bolts are held captive in a channel in the front panel, these bolts are also secured with nylock nuts.

This is a view of the underside of the plinth.

As described earlier, each drive has four "L" shaped brackets bolted to its sides, the bottom of the brackets are tapped to mate with the bolts that you can see on the underside of the plinth.

I was going to need 3.5" to 5.25" adapters to mount the new drives but the ones that I found on ebay.uk were all plastic. I preferred to use metal ones like this one which I sourced on ebay.com.

The bezel is a clip in plastic panel, but the mounting tray is steel.

Unfortunately, the existing holes in the plinth were not suitable for the new drive mounting trays and additional holes had to be drilled as shown in this photo..

To make sure that the new drive fascias were properly aligned in the front panel, the location of the new holes were marked with the plinth and tray in position in the FDX.

The new drive adapters, minus the front bezels, fitted to the plinth.

Using a right angled screwdriver, it is possible to install and remove 3.5" drives from the trays without removing the plinth from the FDX.

Drive trays mounted on the plinth, refitted in the FDC, with front bezels attached.

(The PSU below the drive plinth is the replacement ATX PSU that I fitted in place of the original Astec PSU)

   
   
   
   

Drive Configuration

The floppy disk controller must be configured to reflect the type of drives that are installed in the FDX.

The design was for four DIP switch packs to be fitted in locations 9E, 9D, 9C and 8B on the board. There should be 8 corresponding diode networks in locations 8E, 8D, 8C and 7B. On this board from Jan Seyfarth's FDX, only one switch pack and one full set of diodes are present - the others have hard wired links and missing diodes.

A close up of the two "switch" arrangements that you might find on an FDX disk controller board. The configuration options are described in the FDX disk controller manual, an extract from the relevant page can be found here.

In this case, it appears that drive 1 ("B") was configured for a 5.25" Double Sided 40 track drive and drive 2 ("C") was configured for an 8" Single Sided 40 Track drive with write pre-compensation required for all tracks greater than 43.

It looks like Jan's board had the switch pack installed "after market", you can see what appears to be the original diodes in positions 5 and 8 and the switch pack is also different from the ones installed on my two FDX disk controllers.

The "switch" position on the right (board position 9E) has link & diodes corresponding to the same positions on the switch packs on my original FDX. Fortunately, my board has a full set of switch packs and diodes fitted. This makes changing drive types much easier than if I had to move wire links and install missing diodes. If I had had to make modifications to my board, I would have installed a switch pack and diodes to make changing drives easier in future.

The relevant "switch" positions for 5.25" (or 3.5") drives are :-

  • SW5 - ON for a Double Sided drive
  • SW6 - ON for a 96 Tracks per inch (i.e. 80 Track) drive
  • SW7 and
  • SW8 - set the track-to-track stepping rate, both OFF for the fastest (6 ms) rate

So, changing from my original QumeTrack 142 (DS/DD, 40 Track, Memotech Type 03) drives to Sony MPF920 (DS/DD, 80 Track in Memotech Type 07 mode) meant setting SW6 ON and setting SW8 OFF to change from a 12ms step rate to 6ms - the fastest available. The 3.5" drive is capable of higher step rates, probably 3ms, but the FDX controller treats the drive the same as a 5.25" one.

   

System, Applications Software and Data Files

In normal circumstances, an upgrade from 5.25" to 3.5" disks would need to be done in a two stage process, one drive would be changed and then used to create 3.5" System disks and to transfer any legacy software and data from 5.25" to 3.5" disk. In my case, at one stage, I was unable to boot from 5.25" disk and created a 3.5" CP/M System disk on a DOS PC from a Teledisk image. At a later stage, I was able to recover most of the programs and data from my 30 year old 5.25" disks.

   
   
   

Disk System Enhancement

Whilst the 3.5" floppy drives were much more reliable than the original 5.25" drives, I wanted to enhance the storage facilities on my FDX.

Many vintage computers have had CF (Compact Flash) or SD (Secure Digital) memory card devices developed to emulate floppy and hard disk drives. Unfortunately, there are no integrated storage card options, like Andy Key's REMEMOrizer for the MTX, available for the FDX (yet ?), so even with the largest floppy disk supported by the FDX (Type 07, 640k), a lot of disks are required to store all of my old programs and data. As well as that, the long term reliability of floppy disk media is not guaranteed and backing up a collection of even a moderate quantity of floppy disks is a chore that I can do without, There had to be a better solution . . . . . . .

I chose to go for the HxC Universal Floppy Disk Emulator, developed by Jean-François Del Nero (Jeff).

You can see the status of my HxC upgrade on this page.

 

 
   

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