Computers Overview
Commodore PET
Sinclair ZX80
Sinclair ZX81
BBC Micro
Sinclair Spectrum
Memotech MTX
    User Groups
    Video Wall
Memotech CP/M
Atari ST
Commodore Amiga
DEC 3000 AXP
Raspberry Pi



The Memotech MTX Series


SDX Disk Upgrade

(SDX Photos)

SDX Disk Upgrades

As described on the FDX disk option page, Memotech released a single 5.25" drive upgrade in an FDX case, which is sometimes referred to as an SDX system, however, the 5.25" upgrade was not labeled as an SDX. The Memotech Single Disc Operator's Manual appears to be a compilation of various other documents and does not present a very clear picture of the SDX options, but does describe both the Single Disc FDX type and the two SDX badged products that were released :-

External Drive SDX

This upgrade allowed the user to install one or two floppy disk drives alongside the MTX, but with a much reduced footprint from the FDX option. The disk controller was in an aluminium case, profiled to match the MTX, which connected to the expansion connector on the left hand side of the MTX. The product datasheet for the original SDX disk option notes that the disk controller is an FDC03, although some external disk SDX systems used an FDC05 controller (no FDC04 controller boards have been seen and probably do not exist).

The only obvious difference between the two controller types is that the FDC03 controller does not use an external power supply, it gets 5VDC from the MTX edge connector. The FDC05 has a 3.5mm jack socket on the rear panel which is used to supply additional 5VDC power. The floppy disk PSU provided an additional 5V supply to the disk controller via a small power cable between the disk drive unit and the SDX controller, terminated with a 3.5mm jack plug at both ends. I assume that Memotech found that the current draw over the MTX edge connector was too great and added external power to later controllers. 

The disk controller was capable of controlling two floppy drives which could be a mixture of 5.25" and 3.5", although Memotech only supplied 5.25" disks in this configuration. The floppy drive was housed in its own case which included the drive's power supply; a ribbon cable connected the floppy drive to the disk controller.

The external SDX was available with Type 03 (320kB) or Type 07 (640kB) disk drives but were too expensive for the majority of MTX owners to consider. Late in the machine's life, Memotech made lower capacity drives available with Type 00 (80kB) and Type 02 (160kB) disks but they were introduced too late to make much of an impact on sales.

Supported Disk Formats

The SDX FDC03/05 disc controller supported a subset of the config codes contained in the PROM of the FDXC1 controller :-

These config codes allowed the use of 100kb, 250kb, 500kb and 1MB (unformatted sizes) floppy disks, the FORMAT program was capable of formatting up to 8 different configurations, i.e., the four sizes on either 5.25" or 3.5" disks.

The first style of "new" SDX controller.

The case had a wedge profile that matched the MTX.

Additional photos can be found on the SDX Photos page
The PCB has 2 packs of 4 dip switches that were configured for the attached drive(s). The set at the rear of the case configure the first drive. The ROM often, but not always, carries a two digit number that signifies the drive type - in this case, "Type 02" (SS/SD), Type 03 (DS/SD) are more common.
SW1 (ON) Head Load solenoid present on drive
SW2 (ON) Double Sided Drive
SW3 (ON) 96 TPI Drive (80 Tracks)
SW4 (ON) Drive Step Rate 6ms (OFF = 12ms)

There is a copy of the Memotech "Flyer" for the "Memotech 5.25" Disc System" on the Articles page which includes some technical data on this version of the SDX,


Internal Drive SDX

For the MTX512S2, Memotech combined the SDX disk controller and a 3.5" floppy drive into a single unit which connected to the expansion connector on the left hand side of the MTX. Due to the extra height of the floppy drive, the profile no longer matched the contours of the MTX, but it was a much neater solution. The combined 80 Column and RS232 board was also installed in the MTX512S2.

The disk controller was combined with a Silicon Disk board, which allowed a Silicon/RAM disk up to 512kb to be configured.

As the disk drive did not have its own power supply like the external floppy disks, this controller module also required external power to be provided to supply the required voltages for logic circuits and disk drive operation. In this case, the SDX unit has a 6-pin DIN power connector like the MTX power input and needed a second MTX power supply unit.

The later version of the SDX released for the MTC512S2,

This case forgoes the wedge profile and has a squarer form that allows the 3.5" disk drive to be mounted inside the case.

Again, additional photos can be found on the SDX Photos page
The controller used here does not have any drive configuration switches, the ROM is hard coded to expect a DS/DD 80 track drive.

The controller could optionally be fitted with 512k of RAM as shown here which was typically used as a RAM disk.

Expansion Options

Memotech developed a combined 80 Column and RS232 board which could be attached to the internal edge connector of the MTX, this allowed the SDX system to be upgraded to CP/M with a similar package of software as the FDX, including NewWord and SuperCalc.


Supported Disk Formats

I believe that the disc controllers in the later version of the SDX was programmed with config codes "00" to "07" as per the FDX FDCX1, however, the SDX was hard coded to just allow types "03" and "07", i.e., DS/DD 40 Track and DS/DD 80 Track respectively.

All 3.5" disks are 80 Track, and apart from the very earliest, are double-sided and either double (DD) or high (HD) density. Higher capacity 3.5", HD disks, with a formatted capacity of 1.44mb were introduced in the late 1980s and became a standard (defined in ISO9520) in 1989. These disks have an additional hole in the case, opposite the write protect switch, to allow the drive to determine the disk density. You can force a HD disk to be treated as a DD disk in an HD drive by covering the media density hole with opaque tape (though this is not really a good idea as explained on this page).

A comparison of a number of PC formats and the SDX format is shown in the table below :-

Target hardware   SDX 02 SDX 03 SDX 07 IBM PC IBM PC IBM PC
Disk Size inch 5.25 3.5" or 5.25"  5.25  5.25 3.5
Bytes per Sector bps 256 256 256 512 512 512
Sectors per Track spt 16 16 16 8 9 9
Tracks per Side tps 40 40 80 40 40 80
Sides s 1 2 2 2 2 2

Formatted capacity =

 bps * spt * tps * s / 1024

kb 160 320 640 320 360 720

A 3.5" drive, with appropriate disk formatting, should be compatible with Type 03 (40 Track) or Type 07 (80 Track) config modes, but neither of these SDX modes are supported by the standard DOS or Windows formatting utilities. However, disks can be formatted for SDX use on a PC using low level disk tools such as Teledisk 2.15, available on the Tools page. Version 2.15 is preferred over 2.16 as it supports Direct I/O whereas 2.16 does not. However, this is NOT enough to allow the disks to be used by SDX - see the next section for the reason why.


SDX System Disks for Non-CP/M Systems

NB: ALL disks used with an SDX BASIC (non-CP/M) system need to contain a copy of the SDX disk system tracks, i.e., they MUST be system disks. When an SDX system is powered on or reset, the user should first enter a "ROM x" command from BASIC, where "x" is  the ROM ID of the disk system ROM, which is usually "3", but may also be "5". Entering this command causes the SDX to read the CP/M system from the system tracks (0 and 1) and patch the data in RAM for use with SDX disk USER commands (FORMAT, SYSCOPY, LOAD, SAVE).

If the disk in the drive cannot be read or does not contain the system tracks, the MTX will report the pretty unhelpful "DISC ERROR" message. This message can also indicate that the disk format (Type 03 or Type 07) does not correspond to the configuration set on the disk controller's hardware bit switches.

This point is key to being able to use your SDX, so it's worth restating, you cannot use a non-system disk to start an SDX ! It follows then, that if you obtain an SDX controller and disk drive, you can not get it working without access to a pre-formatted and system enabled SDX disk. With access to such a disk, you can then, and only then, create additional disks as required with USER FORMAT and USER SYSCOPY.

Without this knowledge, it is very easy to conclude that the controller or disk drive is faulty or the disk cannot be read, whereas you may just have an incompatible floppy in the drive.


SDX System Hardware Configuration

As described above, the SDX controller must be compatible with the disk drive used. For the later version, only a DS/DD 80 track drive can be used. For the earlier version, the appropriate bit switch pack must be configured for the drive type being used.






Disk Interface

As with the FDX system, the pin numbers for connector J2 on the FDC03/05 board are derived from their correspondence with the conductors in the ribbon cable - conductor 1 is marked with a red stripe and makes contact with pin 1 on the connector.

Quoting from the FDX manual "Unfortunately, this scheme is not used by disc drive manufacturers who number pins from the opposite end." (Perhaps they should have said "we numbered the pins the opposite way around from everyone else in the industry?)

The FDX manual goes on to say, ""Don't worry about plugging the connectors in the wrong way round - no electrical harm will be done, but any discs installed may have their data and format corrupted." The connector has alternate (even numbered) pins grounded, the table below gives designations for the remaining pins.

What this means in practice is that the ribbon cable needs to be connected with a twist such that Pin 1 at one end connects to Pin 1 of the interface and Pin 1 at the other connects to Pin 34 on the disk drive (or vice-versa).

Connector Pin-Out
 (not used) 
11   WRITE GATE (write enabled)
15   STEP 
17   DIRECTION (head step direction)
27   INDEX 
31   (not used)  
 Pin 33 and  all even numbered pins are tied together at 0V 

This information suggests that the SWx-1 DIP switch setting (Head Load solenoid present on drive) is superfluous, with this line grounded, even if the drive does have a head load solenoid, the heads should be loaded as soon as the drive is powered on.

The SWx-4 DIP switch setting may be of limited use too. As might be expected, older drives, particularly 5.25" ones have the slowest step rates, with newer, especially 3.5", drives having faster step rates. However, I not seen a noticeable difference when this setting is changed, ever on 5.25" drives. It may be that, provided that the drive can move the heads faster, the read speed does improve for larger files that span multiple tracks, but the difference is probably negligible.

The table below provides a cross reference between the SDX pin-out and the standard used by just about everyone else

Pin Name



(Original Shugart Interface)



Modern PC


Pin Description
---      Head Load (non-Shugart) 333 ---  
2 /REDWC  Reduced Write Compensation (8" only) --- 2 Density Select
4 /INU  In Use (non-Shugart) --- 4 (Not used)
6 DS3  Device Select 32 29 6 (Not used)
8 /IDX  Index 27 8 (As Shugart)
10 /DS0  Device Select 02 25 10 Motor Enable A
12 /DS1  Drive Select 12 23 12 Drive Select B
14 /DS2  Device Select 22 21 14 Drive Select A
16 /MTRON  Motor On 19 16 Motor Enable B
18 /DIR  Head Step Direction 17 18 (As Shugart)
20 /STEP  Step 15 20 (As Shugart)
22 /WDATA  Write Data 13 22 (As Shugart)
24 /WGATE  Floppy Write Enabled 11 24 (As Shugart)
26 /TRK00  Track 0 9 26 (As Shugart)
28 /WPT  Write Protect 7 28 (As Shugart)

 Read Data 5 30 (As Shugart)
32 /SIDE1  Head Select 3 32 (As Shugart)
34 /RDY

 Ready (non-Shugart) --- 34 (As Shugart)5

Odd numbered pins are connected to ground


Notes :

 1 Direction indicates that the signal direction is from the controller to the drive

 2 Legacy drives from different manufacturers may have ID select numbered 0 to 3 or 1 to 4

 3 Pin 33 on the SDX FDC board is connected to ground, leaving this signal always ON, see the SDX schematic

 4 A PC Floppy Disk interface only supports the use of two drives

 5 I don't think this is actually used on a PC

Drive ID Selection

The original design of the floppy disk drive used jumpers on the drive to set its address which would be selected by controller pins 6, 10, 12 & 14. Most modern drives, particularly 3.5" drives for PCs, do not have ID jumpers and are factory configured to have an ID of 1. (It makes for quicker assembly when the drives do not need to be individually set up by the PC manufacturer.)

The table explains how the twisted floppy cable on a "modern" PC is used to perform drive selection when only two drives are present, the cable between the first and second drive connectors has pins 10 to 16 reversed between the connectors. Both drives should be set to an ID of 1, the drive connected to the first (untwisted) plug would therefore be Drive 1 (PC Drive "B") and the drive connected to the second connector, after the twist, would be Drive 0 (PC Drive "A"). This picture, from the PC Guide website shows a typical Universal (supporting both 3.5" and 5.25" drives) PC floppy cable - a full explanation can be found on the PC Guide Floppy Interface Cable webpage.

Drive Rotational Speed

The details on drive IDs become particularly relevant if you are looking for a modern drive to replace an original Qume drive. If that is the case, another consideration is the rotational speed of the drive. The data transfer rate between the drive heads and the host controller is a function of the media density and the rotational speed of the drive. For the QumeTrak 142, the rotational speed is 300RPM and the transfer rate is either 125 kbit/s (single density) or 250 kbit/s (double density).

The rotational speed of the Qume drive is common to all legacy 360 kb 5.25" drives as well as all 3.5" drives, but "modern", HD 1.2MB, 5.25" drives have a rotational speed of 360RPM. This potentially means that an older disk controller would not be able to handle the higher data rate (500 kbit/s) from a "modern" drive. Some drives, for example most Teak drives, have a link selectable speed option for 300 or 360RPM, most newer 5.25" HD drives do not.

If you want to try modifying a 1.2MB, 360RPM drive to operate at 300RPM, this page from Dave Duffield how to do it.




mailto: Webmaster

 Terms & Conditions