I spent a lot of time in the last few days on upgrading my ST 520, and wanted to share that. But it was too long for a simple forum post, so i decided to write an article about it here.
Step 1: Preparation
In this picture, you can see the CPU in the middle. I have soldered a socket right on top of it. This is needed in order to add an IDE Adapter later. To the right you can see the IDE-CF-Adapter that i will connect to that. I just glued that on top of the ROM socket, and wired the 5V supply to a nearby spot. This means the ROM port will be blocked, since normal cartridges will no longer fit. But i don't have any cartridge anyways, and as far as i know, they were not used very often. Also, this is completely reversible. Above the CPU, where the Floppy controller used to be, i soldered in a socket. I didn't have the right one, so i just cut a 40-pin socket shorter. In between that socket and the CPU, there is one capacitor i had to change for a smaller type, because the original one was too tall to fit the HD-Module. You can also spot the single wire from the ACSI port, that is the needed IRQ signal for the IDE controller.
Step 2: HD-Floppies
The STm 520 has no internal Floppy drive, that means there is some extra effort required to be able to use HD (aka 1.44 MB) disks. Somehow, the HD-Detect signal must get from the drives to the HD-Module. If you are willing to limit yourself to a single drive, you can use the DS1 wire for that. But i wanted to have two drives. Luckily, there is a way to do that. Atari has used two ground pins on the Floppy connector. In this picture, you can see the floppy port from the inside of the ST. On the right side, one of the two ground pins is visible, and i just cut that and bent it up. Soldered onto that is the wire that gets the HD-signal to the floppy controller. That means that an original Atari Diskstation will no longer work on this port, because the floppy controller would always run in HD-Mode. But that is a compromise i was willing to take.
Here, i already plugged in the HD-Module with the Controller chip on top. This sits very close to the CPU, and i had to file off a bit of the PCB to make it fit. Space is very limited here, and there is no way the metal shield will still fit in here. But that is not required for the ST to work, and it is totally reversible. And my main concern was to keep the ST unmodified on the outside. Aside from a bit of yellowing on some keys, this ST is in perfect condition, including the original packaging and all. Therefore, i want it to look really original as well. But for several reasons, the HD-Mod was important to me. Firstly, the space on a 720 kB disk is very limited. With RAMTOS loader a TOS image, IDE- and ANSI-drivers, the disk is already almost full. Secondly, HD disks are not only twice as big, but also twice as fast, so the boot time is cut in half. And last but not least, those DD-disks are really hard to get these days, while HD disks are still available in many stores.
Step 3: IDE-Adaptor
The IDE adaptor i use here is a handwired one, based on the known c't design. The two empty sockets are for 74HCT245 bus drivers, but i never got those to work. That is not a problem in this case, because i will only use a very short cable. The HD-Module is now completely covered under the IDE adaptor, just the cable for the HD signal is visible on the left. The module i used here is very versatile, so it has more wires than i actually need. Since i put a bit more effort in the modification of the drives themselfes, one single wire would be sufficient here. But more on that later. As you can also see, i mounted a small fan to the top cover. Not permanently, i just used a bit of glue-gum or whatever it is actually called. That can be removed without residue if needed. It is obvious in this picture that not only the floppy controller, but also the CPU are completely covered now, and that makes passive cooling impossible. While testing, i encountered read and write errors on disks, but they completely dissapeared with this additional forced airflow.
A little creativity was required for routing the IDE cable. Directly next to the CF socket, there is one of the screw holes that hold the machine together. If you enlarge the picture, you can see that i also had to modify that a little bit. But since that is hidden inside the case, and has no influence on the function, i convinced myself to do it. This finishes the modifications inside the ST, all thats left to do is modify the actual drives themselfes.
Step 4: Floppydrives
Most people use the very simple method of just running a wire from the HD sensor inside the drive to pin 2 on the ribbon connector. While this works, it has some downsides to it. Because if done that way, the drive always pulls the HD signal low, as long as there is a HD disk in the drive. This means the HD-Module has to know when the drive is actually active, in order to not permanently overclock the floppy controller. It also makes it impossible to use DD and HD disks on the same cable. On top of that, Atari used a diffrent way to detect a disk change. In Ataris, the WP (write protect) line is pulled low not only when a write protected disk is in the drive, but also when the drive is empty.
Lucky for me, someone already thought this through and developed a circuit for that. It just needs a single 74LS03 chip, that is a NAND chip with 4 gates and open collector outputs. Only 3 of the 4 NAND gates are used here. One of them is free, in case one of the signals inside the drive needs to be inverted. I just glued the 74LS03 to a free spot on the PCB, and used strands of ribbon cable to wire it all up. It looks kind of messy here, bit this drive, a SONY MPF920, has a full metal cover on the bottom side. The chip fits under that just perfectly, and the wires will be protected while handling and mounting the drive. That is why i did not fix the cables any more. Many drives are open on the bottom, and in this case, i would have glued the cables onto the PCB as well. This little circuit generates the HD signal and the Atari-style disk change detection, all in one. The schematic for that, and another drive model as an example can be found here: http://atari.8bitchip.info/flomodam.html
Thats basically it, all thats still needed is a cable and case for the external disk drives. One could modify the original Atari Diskstation for that, by cutting the second ground pin there as well, and connecting it to pin 2 on the disk drive. With this modification, the drive will only output a HD signal when it is beeing accessed by the computer, so that the floppy controller is only overclocked when neccessary.
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