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The Memotech MTX Series


FDX Disk Upgrade

(FDX Photos)

Dual Drive System

The first disk upgrade to be made available for the MTX range was the hugely expensive FDX, you can read an overview of the FDX on the Options page, but in short, it was a very large enclosure which contained an 6" card frame which held a bus interface card for connection to the MTX, an 80 column colour graphics card and a floppy disk controller card (FDCX1) as well as optional silicon disk boards.

There was also an option to install a hard disk in the FDX enclosure, making an HDX. The hard disk upgrade was considerably more expensive than the FDX, and I'm not sure if (m)any were actually sold. On this page, I am going to concentrate on the floppy disk drive options.

The FDX was originally fitted with 2 x 5.25", 500kb (unformatted capacity) Qume "QumeTrak 142" floppy disk drives, the Qume drives were mentioned by name in the earliest FDX adverts, but later systems used other drives, including the Epson SD-521 in which the motor is direct driven, rather than belt driven like the Qume drives are.

Single Drive System

A single drive version of the FDX was also released - confusingly, it was sometimes referred to as an SDX system. This budget system used the same case as the dual drive FDX, but lacked the 80 column board and did not supply power to the MTX like the dual drive version did. In this form, the machine could not run CP/M, but could be upgraded later if required. Although I have never seen one, based on Memotech's adverts. I believe that the Single Disk "SDX" version used the same Qume drive as the dual drive FDX. A number of potential problems with the QumeTrak 142 are described on the Maintenance page.

I think that the FDX like single disk system was a very short lived product, the SDX name was soon used for couple of small disk controller modules that plugged into the expansion port on the side of the MTX. (See the SDX disk option page for details ).

FDX Single/Dual Disk System Software Compatibility

Supported Disk Formats

The PROM on the disk controller card could support a range of different floppy drive formats by selecting the appropriate "config code". In the Version 03 bootstrap PROM, the type codes are :-

The default Config Code, at least on my FDX, was  Type 03, i.e., 5.25" DS/DD, 40 track with a formatted capacity of 320kb. (Andy Key has collated the complete details of the different config codes on his MTX Disk Types page.)

Drive Hardware Configuration Options

Specific disk drive options were configured using either packs of DIP switches or hard wired links in the 16 pin DIL sockets in positions 9E, 9D, 9C and 8E on the FDCX1 controller :-

  • Option switches SW2, SW3 and SW4 are used to configure write precompensation for the drives, this is mainly relevant for 8" disks which I'll ignore on this discussion, the normal position of these switches for 5.25" (or 3.5") drives is OFF.

  • Memotech supplied FDX drives were all double sided, as are modern replacements, so the normal position for SW5 is ON.

  • Memotech supplied drives for the FDX were 40 Track (48 TPI) , so the normal position for SW6 is OFF. (This switch should be changed if you add a higher capacity drive.)

  • My FDX was configured for a 6ms step rate, switches SW7=OFF and SW8=ON, but these may need to be reset of a different drive is installed.

It is worth mentioning that higher capacity drives can read/write lower capacity formats without any need to change the option switches on the disk controller. For example, a DS/DD 80 Track Drive should be configured with switches 5 & 6 ON, this corresponds to a Memotech Type 07 disk. In order to read lower capacity disks, the CONFIG.COM program is used to configure the disk controller to read the different format.

  • e.g., before accessing a DS/DD 40 Track drive in drive C:, you would enter CONFIG C:03

  • before accessing a SS/DD 40 Track drive in drive C:, you would enter CONFIG C:02, etc

NB :There are potential media issues with swapping media between drive types, see my MTX FDD page for the details

Although the disk controller manual only discussed 5.25" and 8" drives, this is probably because the disk controller was designed before the 3.5" floppy came into widespread use around 1984/85. All 3.5" disks are 80 Track, and apart from the very earliest, are double-sided and either double (DD) or high (HD) density (ignoring ED for the purposes of this discussion). 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 (again, see my MTX FDD page for an explanation of why this is not a good idea).

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

Target hardware   FDX 02 FDX 03 FDX 07 IBM PC IBM PC IBM PC
Disk Size inch 5.25 5.25 5.25  5.25  5.25 3.5
Bytes per Sector b/s 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 =

 b/s * spt * tps * s / 1024

kb 160 320 640 320 360 720
Unformatted Capacity   250kb 500kb 1MB      

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 FDX modes are supported by the standard DOS or Windows formatting utilities. However, disks can be formatted for FDX 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 connected to ground, leaving this signal always ON, see the FDC schematic.

    The R/W heads will be loaded as soon as the drive door is closed.

 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.

A standard PC twisted cable can not be used to swap the drive IDs with a Memotech disk controller - you need to use a straight through cable and set the IDs on the drives themselves.


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.


Power Supplies

Both the original FDX, and the budget single drive version, had an internal Astec AC8151 PSU fitted to provide +5VDC, +12VDC and -12VDC for the FDX disk drive(s) and the internal PCB card cage. The internal drives, disk controller and the 80 Column board required +5VDC and +12VDC. I don't think that the -12VDC line is used in the FDX, although it was connected to the card frame and given a dedicated bus line. I suspect that, since the PSU used was an "off-the-shelf" Astec product, Memotech connected it "just in case". I think that the most likely use for a -12V line in other applications would have been for RS232 "mark" (logic "1") and "space" (logic "0") voltage levels. The RS232 standard defines the "mark" (-5VDC to -15VDC) and "space" (+5VDC to +15VDC) voltages, with +/- 12VDC being typical.

FDX PSU Specifications

Manufacturer Astec
Model AC8151-01
Power 40 Watts

115V @ 1A

230V @ 0.5A


-12V @ 0.1A

+12V @ 2.02A,

+5V @ 2.5A


The FDX floppy drives use standard Molex connectors for power. A modern, PC type,  cable is colour coded as shown:-
+5V Red
+12V Yellow
Ground Black

I'm not sure whether Memotech had a standard for their internal wiring, but on the FDXs that I have seen, the colours are :-

+5V Red
+12V Pink
Ground Black

In addition, on my FDX, -12VDC from the PSU was connected to the PCB card frame using a Blue wire, although I don't think that this was actually used.

The original FDX also provided a low voltage AC power output for the MTX computer, this was achieved by installing the same  transformer as used in the MTX PSU inside the FDX, but without the standard case, i.e., just bolting the transformer to the chassis. As with the desktop PSU, this provided 22.5VAC, with additional taps at 18VAC and 9VAC with the regulation etc. being done on the MTX computer board.


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