This repository contains the KiCad project and Gerber files for an adapter that converts the Atari ST external 14 pin floppy connector to a standard 34 pin IDC box header. The 14 pin floppy connectors are out of production for a long time and are extremely hard to get hold of and expensive nowadays. This adapter is a cheap workaround.
This floppy adapter is designed to be used in combination with my DSUB-19 Adapter for the Atari DMA/Harddisk port, they fit nicely next to each other. I also tried not to block other connectors, but I only have an Atari 1040 STF to check that, so your mileage might vary. Please check the situation on your Atari before ordering the PCBs!
When you are still looking for a way to provide power to your external floppy drive or Gotek floppy emulator, you might be interested in my Atari ST 5V Cartridge, which utilizes the cartridge port of the Atari ST to provide 5V to external devices.
This project is based on the work of Justin Davis who shared the information about the correct pins for the adapter in the comment section of his project page, on the work of the user "Atarian Computing" on the Exxos Forum who did the tedious task of measuring the locations of the pins in the connector, and on the work of Michael Rath who had the idea with the configurable signals on pins 10 and 14 of the box header. Michael sells his adapters on Ebay, so you might consider buying one from him and supporting his work.
The subdirectory "gerber-files" contains a Zip file you can use to order the PCB from PCB manufacturers like PCBWay or JLCPCB. Simply upload the Zip on their web page. I recommend to order the standard PCB thickness of 1.6mm and HASL finishing.
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14x Solder Pins.
These pins are used to recreate the 14 pin floppy plug. They are Mill-Max 4123-0-00-15-00-00-08-0. You can get these e.g. from DigiKey or Mouser.
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1x 2x17 Pin Straight Male Box Header 2.54mm pitch.
Used to connect the flat ribbon cable to the adapter.
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2x 1x3 Pin Headers 2.54mm pitch (optional).
Used to select the signals that go to pins 10 and 14 of the box header. These are only needed when you want to be able to configure your adapter for different setups, otherwise you can simply bridge the connections on the PCB by soldering in a piece of wire.
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2x Jumpers 2.54mm pitch (optional).
Used to select the signals that go to pins 10 and 14 of the box header.
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3x Self Adhesive Silicone Rubber Pads 5x2mm (optional).
These are used to prevent the pins on the back of the adapter from scratching the case of your Atari ST. They must be quite small, 5mm diameter and a height of 2mm. You might be able to get these locally in a stationery shop or a hardware store, but I got mine from AliExpress.
Start with soldering the 14 pins/nails to the PCB. It is crucial that you solder these pins straight to the PCB, otherwise they will not fit into the socket. I recommend to use the actual floppy socket on your Atari ST or on an old external floppy drive as a soldering aid. Stick the pins into the socket, and attach the PCB to the pins, e.g. by using an adhesive like Blu Tack. Solder the pins to the PCB, but be careful not to destroy your soldering aid by applying excessive heat or by dropping solder.
I myself did only have the socket on my Atari ST, and I did not succeed to solder the pins without removing the mainboard from the case - whenever I tried to put the PCB on the pins I pushed the pins too far into the socket. Removing the mainboard is quite a tedious task, but doing it this way is much easier and prevents you from damaging your precious Atari ST case. But nevertheless, the whole soldering process is quite finicky.
Solder the box header to the PCB. Mind the orientation, the notch of the header must face in the direction marked on the silk screen (downwards in the image below in direction to the pins).
Before soldering the box header, there is an optional step that I recommend to do. When you plan to use an original floppy cable from an IBM-compatible PC, you might realize that these cables are frequently keyed with a missing pin. On the box header it is the third pin from the left in the bottom row (see the images below). On the floppy side, it is actually the second pin that is missing, so you cannot install the cable in the wrong direction, but do not get confused - on the adapter side, you have to remove the third pin! Removing the pin is actually quite easy - simply use a pair of pliers to pull the pin out of the socket. Again, please do triple-check that you remove the third pin from the left on the row facing the notch (see the images below) - when you get this wrong, you might get a non-working adapter!
Even when the cable that you plan to use to connect your floppy to the adapter does not have this keying, I nevertheless recommend that you remove the pin, because it is easy to do, and doing it after soldering the box header to the PCB is nearly impossible.
To solder the box header to the PCB, I again recommend to use an adhesive like Blu Tack to stick it to the PCB. I also recommend to stick the floppy cable into the socket, because it happens easily that the plastic of the socket melts and the pins start to move when you apply too much heat while soldering, and this might ruin the whole adapter!
Next step is to solder the two pin headers used to configure the signals on pins 10 and 14 of the box header. How to configure the adapter is explained more in detail in the section Using the Adapter below. Of course using pin headers and jumpers is optional - when you do not need support for multiple configurations, you can also simply solder pieces of wire to the PCB to generate a fixed configuration. When soldering, make sure that you solder the pin headers straight onto the PCB.
The final step is to stick the silicone rubber pads to the adapter. This step of course is completly optional, but when you attach the adapter to your Atari ST, the pins on the back get in contact with its case and might scratch it. Simply sticking three rubber pads to the adapter as shown in the image below will prevent that from happening.
Carefully push the adapter into the 14 pin floppy socket. Use the cable to connect your external floppies to the adapter.
Before you can use the adapter, you have to configure it correctly for your specific setup. To understand this process, we have to dive into the theory of how floppies and floppy cables actually work. This is a simplified overview just for configuring the adapter, and I will leave out many details which are not important for that task.
On Atari ST machines as well as IBM-compatible PCs, you can attach up to two floppy drives, called drive "A" and drive "B". On the floppy connectors and the floppy cables, there are two signal lines which tell the floppies to which one of both the computer is currently talking to. These signals are called "Drive Select 0" (short "DS0") for drive A, and "Drive Select 1" (short "DS1") for drive B.
On the Atari ST, you have to configure the drives whether they respond to DS0 or DS1. There are little jumpers on the floppy drives that you have to set accordingly.
On PCs, a much simpler solution is used. PC floppy drives are generally configured to respond to DS1, and it is usually not possible to change this configration without using a soldering iron. There is a twist on PC floppy cables that exchanges the signals DS0 and DS1 (see image below). When you connect a PC floppy to a connector after the twist, it becomes drive A. When you connect it to a connector before the twist, it becomes drive B.
These differences between Atari ST and PC floppy drives and cables lead to a lot of confusion when connecting floppies or floppy emulators like Gotek drives. This is complicated even more by the fact that there are Atari STs without internal drives (e.g. 520 ST) and with internal drives (e.g. 1040 STF). When you have an Atari ST with an internal drive, you can only connect one external drive. The DS0 signal line on the external connector internally is actually connected to DS1, so DS0 and DS1 are the same signal. To connect two external drives to an Atari 1040 STF, you have to disconnect the internal drive and you have to modify the mainboard to provide signal DS0 to the external connector.
So, after all that background information, let us get back to the configuration of the adapter.
There are two pin headers with 3 pins, respectively.
The left one controls the signal on pin 10 of the box header. When you connect the left and the center pin with a jumper, the "Motor" signal is connected to pin 10. When you connect the center and the right pin, the "DS0" signal is connected to pin 10. Whenever you are connecting a floppy using a twisted cable, you have to use the "Motor" signal, otherwise you have to use the "DS0" signal.
The right pin header controls the signal on pin 14 of the box header. When you connect the left and the center pin with a jumper, the "DS1" signal is connected to pin 14. When you connect the center and the right pin, the "DS0" signal is connected to pin 14. Whenever you are connecting a floppy with an untwisted cable, you can ignore this pin header, i.e. you do not have to set a jumper, it does not matter. Otherwise, when using a twisted cable, you control which drive select signal gets sent to the floppy at the connector behind the twist. When the floppy is configured as "DS0", you have to send the "DS0" signal, and when the floppy is configured as "DS1", you have to send the "DS1" signal.
Let us have a look at some concrete scenarios. We will start with the simple case, an Atari ST with an integrated floppy (A) and one single external floppy (B):
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When you use a straight, untwisted cable:
The external floppy (B) can be configured either for DS0 or DS1. Pin 10 must be configured as DS0, so on the left pin header, you have to connect the center and the right pin with a jumper. Pin 14 is not used, so leave the right pin header without a jumper.
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When you use a twisted cable:
The external floppy (B) must be configured for DS1. Pin 10 must be configured as Motor, so on the left pin header, you have to connect the left and the center pin with a jumper. Pin 14 must be configured as DS1, so on the right pin header, you have to connect the left and the center pin with a jumper.
Now, let us have a look at an Atari ST without an integrated floppy and two external floppies (when you only attach a single external floppy, use the same configuration, and simply leave drive B away):
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When you use a straight, untwisted cable:
One floppy (A) must be configured for DS0, and the other floppy (B) must be configured for DS1. Pin 10 must be configured as DS0, so on the left pin header, you have to connect the center and the right pin with a jumper. Pin 14 is not used, so leave the right pin header without a jumper.
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When you use a twisted cable:
Both floppies must be configured for DS1. The floppy after the twist becomes drive A, and the floppy before the twist becomes drive B. Pin 10 must be configured as Motor, so on the left pin header, you have to connect the left and the center pin with a jumper. Pin 14 must be configured as DS0, so on the right pin header, you have to connect the center and the right pin with a jumper.
Note
When you use this adapter to connect actual floppy drives instead of Gotek emulators, you might notice that the motors of both floppy drives start to spin when you try to access any of the drives. This is actually correct and the expected behaviour!
Note
Also notice that the Atari ST only boots from drive A. When your Atari ST has a built-in floppy, that floppy is drive A. You won't be able to boot from the external drive, unless you do a hardware mod to the machine.
Atari ST Floppy Adapter (c) by Patrick Dähne
Atari ST Floppy Adapter is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
You should have received a copy of the license along with this work. If not, see https://creativecommons.org/licenses/by-nc-sa/4.0/.