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


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

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 labelled 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 I am aware that some external disk SDX systems used an FDC05 controller, but do not have any information on any differences between the two, or whether there was an FDC04, or other controller boards used.

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. As the disk controller required more power than the MTX PSU could provide through 5V line on the expansion connector, 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.


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.

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.

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.


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.

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

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

Formatted capacity =

 b/s * spt * tps * s / 1024

kb 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.











Disk Interface

As described in the Floppy Disk Controller manual, Memotech managed to use a pin-out that is pretty much exactly opposite to the original Shugart interface. The table below provides a cross reference between the FDX 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 32 294 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  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)6

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 FDCX1 board is only connected to ground, leaving this signal always ON, see the FDC schematic

 4 There must be an misprint in the FDX manual, it should be pin 29, not 28, the FDC schematic confirms this

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

 6 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