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The Memotech MTX Series

SDX

   

Original Memotech "flyer" for the SDX

SDX Controller from ebay.de

 

The SDX System

The Memotech SDX was the successor to the high cost FDX disk system, the first version of the SDX consisted of a disk controller board fitted inside a brushed aluminium case with the same profile as the MTX, connected to the edge connector at the left hand side of the MTX. Originally supplied with either one or two 5.25" floppy disk drives, the controller could also be used with 3.5" drives in place of the 5.25" ones. A later version of the SDX had a single 3.5" floppy drive built into the controller module, which again connected to the MTX edge connector and could support a 512kb RAM/Silicon disk.

The original SDX was available in two versions :

  • SDX Disk BASIC - Added disk functionality to BASIC without requiring additional hardware, other than the SDX
  • SDX CP/M Package - Added full CP/M functionality, requiring the addition of an internal 80 Column board to the MTX

I bought an SDX controller from a seller on ebay.de in March 2013 and had it shipped from Germany to the UK. It was one of the original versions of the SDX and would originally have been supplied with either one or two 5.25" floppy disk drives. The drive originally supplied by Memotech also had a 5VDC output from its power supply to provide additional power to the SDX controller - there not being enough power available from the MTX PSU.

This SDX disk controller board originally supported a number of Memotech disk types, including, "03" (500k/320k) and "07" (1MB/640k) disk drives. These disk types have the same geometry as 3.5" disk drives, so I decided that a 3.5" disk was more desirable for my SDX than a 5.25".

NB: Memotech did not adhere to the usual convention for the orientation of the drive ribbon cable. Usually, Pin 1, denoted by the red stripe on the cable, and all other odd numbered pins, are connected to ground. On the SDX, the even numbered pins connect to ground, this means that the red index stripe on the IDC cable for the floppy drive must be connected to Pin 1 at either the controller or the drive(s), but not both - one or the other must be reversed.

 

Since the controller was sold without the disk drive, I needed to source a powered drive that could also supply power to the SDX controller. I already had an external, mains powered 3.5" disk drive for my Atari 520 STFM which seemed like a good choice for the SDX too, a Cusana Model CSA 354.
As luck would have it, there was an identical drive for sale on ebay while I was waiting for the controller to arrive, so I bought it. Here is a photo of the inside of the drive, showing the 240VAC PSU, the power switch and the round data cable which connected to the Atari ST.

You can see that the rear panel at the bottom right of the picture already has a slot cut out that will make it easy to get the ribbon cable out of the case and the 5VDC power cable for the SDX can exit through the opening for the ST data cable.

I believe that "normal" 3.5" disk drives require modifications in order to use them with an Atari ST, so rather than mess with this drive, I decided to keep is as a spare for my ST and use a PC drive that I was confident would work with the SDX. I chose a Sony MPF920 which I had already proved to work with my FDX disk controller as a Type "07" drive.
While I was waiting for the drive to arrive, the SDX itself was delivered. Up to this point, I did not know which PROM would be in the controller, the SDX controller can either be used to just give the MTX a disk based BASIC system, or with the addition of an 80 Column video board to the MTX, allows the computer to run CP/M 2.2.

On taking the cover off, I found that the installed PROM was the CP/M version., that saved me from having to replace it.

I had managed to pick up an MTX with an 80 Column board installed from ebay a couple of months earlier. Up to this point, I had not been able to test the 80 Column board as it requires the CP/M PROM to write to it.

When I got it, the 80 Column board had a very odd "end-user" modification on the video output, but I removed it and reinstated the original video output configuration.

Although I had not modified the disk drive PSU to provide 5VDC to the SDX, I thought that I would try the controller plugged into the MTX with the 80 Column board installed and see what happened.

Major good news! - both the 80 Column board and the SDX appear to be working, I just need to work on the drive now.

I cobbled together a Sony MPF920 that I had been using with my FDX, an ATX PSU, as well as my SDX controller and tried to boot the system using a Type "07" FDX System Disk. The system accessed the drive, but the disk could not be read.

I did not have any documentation to confirm the required DIP switch settings on the controller, but Andy's site had a photo of Jim Wills' SDX controller's (FDC03) DIP switches.

The position of the switches on Jim's board were the exact opposite of mine, so I tried resetting the ones on my (FDC05) controller board to be the same, i.e., all switches ON.

As you can see, the SDX could now boot successfully (from drive "C", as the 3.5" floppy drive did not have a link selectable address). I know how to fix this, but in the short term, this was enough to demonstrate that my new "jerry-built" SDX System works as it should!

I still have some minor changes to make, including permanently changing the Sony drive ID, fitting it into the enclosure and deciding whether modifying the PSU is actually worth it.

In the meantime though, I now have a functional SDX system!

 

After I created this page, I had some contact with Tony Brewer, the designer of the SDX disk controller. With Tony's help, I have created a circuit diagram for the FDC05 controller, the diagram also details the functionality of the DIP switches.

 

Changing the ID of a Sony MPF920 3.5" Drive
 
Like most newer 3.5" floppy drives, the Sony MPF920 does not have a jumper selectable drive ID, however, it is relatively easy to make the change. The drive is encased in a light steel shell which is removed by "popping" 4 clips on the sides of the drive to remove the top half and 3 tiny screws on the base to remove the bottom half.

Once the case has been removed, the PCB on the bottom of the drive can be accessed.

Located between the square BH9558KV ASIC and the data connector, you will see "SEL0" and "SEL1" labels on the PCB. These identify the locations where a 0 Ohm resistor is used to set the drive ID - which is in the "SEL 1" position by default.

Remove the resistor from the "SEL 1" position (JC31) and either move it to the "SEL 0" position (JC30), or, it is probably easier to just solder a small wire link between the "SEL 0" solder pads.

Whilst the concept is "relatively easy", in practice, you need a micro point soldering iron, good eyesight and a steady hand. I think that I probably have 1 out of 3!

- The 0 Ohm resistor is about 3mm long and half as wide, I didn't even try to re-solder it across the JC30 pads, instead, I tinned 3 strands of 0.2mm wire, soldered it over the pads and snipped the end off. The result is not beautiful, but it does the job.

   
   
   

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