FDX System	5.25" and 3.5" SDX Systems

Replacement Drives

After the power supply, the floppy drives are probably the most unreliable component in Memotech disk systems. When these drives fail, it is most unlikely that you will be able to source identical replacements - this is particularly true for the 5.25" drives.

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

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.

The most common 5.25" disk capacities that you are likely to come across are 500kByte, 1MByte and 1.6MByte

Density	Unformatted	Formatted (FDX)	Formatted (IBM)	Tracks	RPM
Double	500 kBytes	320 kBytes	360 kBytes	40	300
Quad	1.0 MBytes	640 kBytes	720 kByes	80	300
High	1.6 MBytes	640 kBytes	1.2 MBytes	80	360

A number of alternatives that have been found to work with the FDX are shown below, although there are probably numerous others that would work just as well. Legacy 5.25" drives in particular often have a bewildering array of jumpers to configure the drive's number of tracks, recording density, rotational speed, etc. etc. and the biggest stumbling block to getting alternative drives to work seems to be working out the right jumper settings to use on the drive.

 

Teac FD-55 [Teac FD-55 Series Brochure]

Other users have reported good results using Teac FD-55 drives with the FDX. There are a number of drives in the Teac FD-55 range which are suitable for use, including the ones in the table below :-

Physical Specifications - (Teac FD-55 Specification Document)
		FD-55E		FD-55F+		FD-55G
Recording Method		FM	MFM	FM	MFM	FM	MFM
Transfer Rate (kbits/sec)		125	250	125	250	250	500
Recording Density  (bpi)		2788	5576	2961	5922	4823	9646
Capacity*	Unformatted	250kB	500kB	500kB	1.0 MB	800kB	1.6 MB
(PC)	Formatted	160kB	320kB	320kB	640kB	600kB	1.2 MB
Track Density TPI (Track/Inch)		96		96		96
Cylinders		40		80		80
Tracks		80		160		160
Rotational Speed (RPM)		300		300		360
Average Latency (ms)		100		100		83.3
Average Access Time (ms)		94		94		91
Track-to-track Seek Time (ms)		3		3		3

I bought a Teac drive off ebay that was advertised as an FD-55GFV-16-U which in fact turned out to be a FD-55F-16-U⁺, fortunately, as the drive was for use in the FDX, I could not use the 1.2 MB capacity of a "G" series drive anyway.

I did have a problem with this drive though, the disk would spin and the head carriage would move when requested, but the heads would not load. According to the FD55 specification document that I have, there should have been a jumper option (SM) to enable the head load HS (drive select) and HM (motor on) condition straps. However, on my drive, there was no SM jumper, so the head load condition jumpers were ineffective and the heads would not load. There are many variations of FD55 and the specification document that I have likely pre-dates some of the later versions of the drive, so some of the details below are "best guesses".

The manual for the FDXC1 controller board states that the head load signal is on Pin 33 of the J3 connector on the board, corresponding to Pin 2 in the interface cable, and is tied to ground on the controller board. The head load signal in a "standard" 34 way floppy disk interface is on Pin 4 and is "active low", shorting this pin to ground will cause the drive heads to load with the motor on signal. I soldered a small wire on the drive PCB between Pin 4 of the FDD interface and ground, this allowed the heads to load as required under control of the motor on signal.

I also had a bit of luck in 2014 when someone was selling off spare parts from a very old helicopter flight simulator that had been uses to train pilots in Aberdeen. The bulk of the equipment went to a museum, but I managed to get part of the control console that had four floppy drives mounted in it. They all turned out to be Teac drives and, apart from one that was faulty (read errors), all have been successfully used with my FDX, with the disk controller pinned for 96 TPI, the drives work as both Type 03 and Type 07.

FD-55FV-13-U Part Number 19307200-13
Name	Meaning	Position	Description
DS0	Drive ID	0	FDX = B:, PC = A:
DS1	Drive ID	1	FDX = C:, PC = B:
DS2	Drive ID	2	FDX = D:, PC = n/a
DS3	Drive ID	3	FDX = E:, PC = n/a
FG	Frame Ground	closed	Connects frame to 0V
IL		open
U1	LED Turn on condition	open	In combination with U2 & IU
U2	LED Turn on condition	closed	In combination with U1 & IU
HL		open
IU	LED Turn on condition	open	In combination with U1 & U2
ML	Motor Start condition	open
DCRY	Disk Change / Ready	RY
XT	Unknown	open

FD-55FV-211-U     (2 of) Part Number 19307272-11
Name	Meaning	Position	Description
D0	Drive ID	0	FDX = B:, PC = A:
D1	Drive ID	1	FDX = C:, PC = B:
D2	Drive ID	2	FDX = D:, PC = n/a
D3	Drive ID	3	FDX = E:, PC = n/a
FG	Frame Ground	closed	Connects frame to 0V
IL	(Jumper pins not present)	open
U0	LED Turn on condition	open	In combination with U1 & IU
U1	LED Turn on condition	closed	In combination with U0 & IU
HL	Head Load condition	closed	(Follow Interface signal)
IU	LED Turn on condition	closed	In combination with U0 & U1
HS	Head Load condition	open	(HL used)
ML	Motor Start condition	open	(Jumper pins not present)
RE	(Strap not present)	open	(Used on FG model only)
RY	Disk Change / Ready	closed

 

 

Cumana Disk Systems for the BBC Micro

Probably the most readily available 5.25" disk drives for vintage micros are the single and dual drive systems made by Cumana for the BBC Micro. These come up frequently on eBay in the UK and are perhaps the easiest drives to source for use in a Memotech system - unfortunately, they are often fitted with beige bezels, but that's a minor issue and you could always re-spray the bezel black if you really wanted to.

Some examples of the Cumana disk enclosures are shown below, they used drives from different manufacturers, some of which had a plethora jumpers to configure various options for the drive. The Cumana documentation didn't provide any details of the jumpers and original manufacturer information is often hard to find, so the biggest challenge is often to configure a compatible drive to work with the Memotech disk controllers.

I picked up a double disk unit and a single disk unit off eBay - both of these used Chinon FZ-506 drives. The manual for the FZ-506 is available here. The Chinon FZ-506 supports capacities up to 1.6MB with densities up to 96TPI, but may be configured to support Memotech Type 03 (48 TPI, 500kB, 320k formatted) and Type 07 (96 TPI, 1MB, 640k formatted) disks.

The FDX disk controller has Shugart interface pin 34 tied to ground, this works in tandem with the jumper block to configure the drive as required.

Cumana produced a number of different single and dual drive versions. Some models featured a front mounted switch that could be used to switch between 40 and 80 track operation.
The dual drive versions were available with the drives mounted either one on top of the other, or side-by-side. Photo courtesy of : Martin Jones website
A bare Chinon FX-506 drive This is a standard drive and does not have the custom 40/80 track switch fitted to the front fascia as seen on the Cumana disk units.
The drives have a 13 position jumper block on the edge of the PCB that sets various configuration options. The jumpers are mounted below the PCB, the size and position makes them rather "fiddly" to change.
The table shows the setup that I have found to work with my FDX, the terminator and ID jumpers are set as needed.	13 12 11 10 9 8 7 6 5 4 3 2 1 o x o x o x - - ? ? ? ? ?

 

J1 Jumper Block
Position	Label	Function
1	TERM	End-of-line terminator enabled (set on the last drive on the cable)
2	DS0	Drive ID 0
3	DS1	Drive ID 1
4	DS2	Drive ID 2
5	DS3	Drive ID 3
6	IU	In Use
7	HL	Head Load solenoid enable
8	-
9	-
10	MS
11	MS
12	RDY	Function of interface pin 34 (Ready)
13	DKC	Function of interface pin 34 (Disk Change - used in IBM PC)

 

NEC Floppy Drive - FD1057

I obtained another Cumana BBC Drive, this one contained a NEC FD-1057 drive and I initially had some difficulty in getting it to work with the FDX. This is a very old drive and there seems to be very little information available on the internet about jumper settings etc.

The NEC FD1057 installed in a Cumana case If you open the full size photo, you can see the remnants of a  piece of sponge on top of the R/W heads. The drive had what appears to be a copper EMC shield glued to the heads, this had fallen off in transit. I have not refitted it but the drive seems to work fine without it.

NEC Drive Jumper Names & Functions

The table below shows information that I have put together from the few web pages and documents that I have found on the web, some of the documents relate to other NEC drives and I expect the pinning options to be similar to the FD-1057 drive, but please be aware that the data has not been reviewed by anyone else but me, so it may not be 100% accurate.

The jumpers marked in Green in the table below show the configuration that has been proven to work on an FDX.

The FDX disk controller was pinned for a 96 TPI drive and the floppy worked successfully as both Memotech Type 03 and 07 using a non-twisted floppy drive cable.

Name	Meaning	Position		Description
DX	Drive ID	0		FDX = B:, PC = A:
		1		FDX = C:, PC = B:
		2		FDX = D:, PC = n/a
		3		FDX = E:, PC = n/a
DCG	Disk Change / Ready	1
	Head Load Condition	HS	USE	Work together as shown below
		1	1	Drive select & head load 1 1
		1	2	Drive select
		2	1	Head load
		1	3	Drive select & motor on
		2	3	Motor on
MX	Drive always selected	open		Always seems to be open
MON	Motor ON Condition	1		Motor ON
		2		Head Load
		3		Drive Select
LED	Drive LED Configuration	2 & 5		Jumpers installed, others open
VC	Unknown	closed		Always seems to be linked
HDE	Rotational Speed	1-2		360 rpm (1.2M Bytes)
	(Jumpers not present)	2-3		300 rpm (360k Bytes)
DEN	Density Select	1		96 TPI (1.2M Bytes)
	(Jumpers not present)	2		48 TPI (360k Bytes)
LUS	(Jumper not present)	open		Unknown
M1	(Jumper not present)	open		Unknown

The drive that I received was BNIB and I don't think had ever been used previously. Although this Cumana enclosure does not have a 40/80 Track switch there is a jumper position that looks like it may had had wires soldered to it at some point. When I was researching this drive, Roy Bates on the StarDot forum suggested that this was where a 40/80 Track switch would be connected too. Cumana may have used this drive in cases that did have the 40/80 Track switch fitted. It is not an issue for use with my MTX disk systems though as the system can switch between 40 and 80 tracks using the CONFIG.COM program.

 

Replacing a 5.25" Drive With a 3.5" Drive

Since a 5.25" drive and a 3.5" drive have the same disk geometry, it is possible to replace an FDX 5.25" drive with a 3.5" one without much difficulty. 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). This means that the replacement drive can either be used as a Memotech type 03 (40 Track) or type 07 (80 Track) drive.

When my 5.25" disk drives failed, I was able to use Teledisk to create a 3.5" bootable CP/M disk from an image file of a 5.25" FDX type 03 (40 Track, DS/DD, 48 TPI) System disk on a PC with a 1.44MB drive and a 720K floppy disk. This wrote the FDX data into the first 40 tracks of the 80 track 3.5" disk (80 Track, DS//DD, 96 TPI) to read this disk on the FDX, the option configuration switch, SW6, for the relevant drive on the FDCX1 disk controller had to be switched ON.

5.25" drives use a card edge (CE34) connector.
3.5" drives use an IDC (ID34) connector. Therefore, the FDX floppy cable cannot be used to connect to a 3.5" drive.
ID34 to CE34 Adapter I could not find these in the UK and had to buy from ebay.com

A standard PC "universal" cable can be made to work, but with some limitations. The usual method of setting the floppy drive ID on PCs is to have all drives factory set to an ID of 1 and use the twist in a PC floppy cable to set the drive at the end of the cable to ID 0 - in fact, most modern 3.5" drives do not even have configuration jumpers to set the drive ID - they are factory fixed at ID 1.

Since the FDX floppy drive cable pin-out is different to that of a PC, the cable twist cannot be used to set the drive ID. The 3.5" drive must be connected before the cable twist and either be used as ID1 or have a link selectable ID. On a two drive system, this means that both drives must be connected before the cable twist and even if both types of connectors are available on the cable being used, it is unlikely that the distance between them will be enough to allow both drives to be connected and a 34-way IDC extension cable like this will probably be required. Alternatively, an IDC to Card Edge adapter as shown in the photo can be used to connect the original FDX floppy cable to a 3.5" drive.

A word about 3.5" disk media.

High Density (HD) 3.5" media is not the same as Double Density (DD) media - their characteristics are similar, but are NOT the same. This is why the HD drive has an additional hole in the diskette, opposite the write protect hole, this allows the drive to identify which media type is in the drive and adjust its write configuration accordingly. Whilst formatting a DS/DD disk to HD was possible (if an additional hole was punched in the disk), the format would often result in bad sectors, particularly towards the centre of the disk where the data density is highest- it was pretty obvious that you were taking a risk with your data. On the other hand, if you wanted to use an HD diskette to create DD diskettes, putting opaque tape over the density select hole will fool the drive into thinking that a DD disk had been inserted and will write to the disk at DD density, apparently with no problems.

When you do this though, you are writing to media of one type (HD) with a write current appropriate for lower density (DD) media. Most of the time, you will not notice any problems, but this technique is not as inherently reliable as using the correct media type. For important data, it is better to use DD media for DD formats - the slight problem being that 3.5" DS/DD media is pretty hard, if not impossible to find these days!

 

Replacing a 360k 5.25" (DD) Drive With a 1.2MB (HD) Drive

This is a little more complicated since, as well as having double the number of tracks (80) as a 360k one (40), a 1.2MB floppy has a rotational speed of 360 RPM rather than 300 RPM. When running at 360 RPM a HD drive requires that the disk controller is capable of supporting a transfer rate of 500k bits/sec, fortunately, the FDX FDCX1 controller can support this data rate. When using a 360k disk operating in DD mode, the drive should operate at 250k bits/sec with a rotational speed of 300 RPM.

Early 1.2MB drives had a link selectable option to set the drive speed but more modern ones do not have this option, some IBM PC floppy disk controllers maintain the drive speed at 360 RPM and throttle the data rate to 250k bits/sec when using 360k disks. To use a DD floppy in an HD drive on an FDX, you will need to be able to set the drive to 300 RPM. If you want to try modifying a 1.2MB, 360 RPM drive to operate at 300RPM, this page from Dave Duffield how to do it, otherwise, make sure you get a legacy drive with a 300 RPM option.

There are potential problems with the choice of media when using HD drives with DD floppy disks.

A look at a typical floppy disk head should help show what the issues are.

The picture shows a floppy disk head arrangement from a 3.5" drive. The Read/Write head is followed by a "Tunnel Erase" head on either side of it to clean up the track and ensure there is good separation between adjacent tracks. Photo from Gough's Tech Zone

Problems can occur because a 96 TPI (80 Track HD) drive has narrower heads than a 48 TPI (40 Track DD) drive. If you write data onto a blank DD disk using a HD drive, a DD drive, should be able to read the disk - the data track will be narrower than had it been written on a DD drive, but it should be read OK.

However, if you write the same data from an HD drive onto a disk previously formatted on a DD drive, you are likely to get problems as the new, narrower, data track will be on top of the old, wider, track. The data can probably still be read on a HD drive, but problems will likely occur when trying to read the data on a DD drive. The HD Tunnel Erase heads will not have cleared out the old data from the wider track and the DD heads will likely read a mixture of the new, thinner, track data and the older, wider, track data. This could be a problem if you intend to use the disk in both HD and DD drives.

 

 

Reference : Some very useful information on floppy disk drives can be found at : http://www.retrotechnology.com/herbs_stuff/