Mounting the ACSI2STM inside a Mega ST

This document explains how you can to mount a Compact ACSI2STM card inside an Atari Mega ST.

While the ACSI2STM in its compact form can easily be plugged at the back of an Atari Mega ST, you may want to install it inside the machine and make the 3 micro SD card readers available from the outside.

This solution involves a 3D printed mount, an ACSI data cable similar to the UltraSatan one and a way to power the ACSI2STM.

There is no need to modify the case, the operation can be reversed without leaving any traces.

Safety warning first:

It is your responsibility to make sure you stay safe while working on your Atari
Disconnect the power before working on your Atari.
Never manipulate any cable or anything else while the power is on.

Any modification on the motherboard is your sole responsibility. This solution should not involve any modification to your motherboard anyway.

In case of problem, I decline all responsibility if you make an assembly error, or poorly done soldering.

1 - 3D printed mount.

The idea here is to be able to mount the ACSI2STM inside the MegaST and make the microSD card readers available at the back.
The MegaST has a small, removable plastic panel at the back to accommodate such use.

Print the mount on a 3D printer and fix the ACSI2STM on it using 2 M2 screws.

Remove the small blanking plastic plate and insert the mount.

You can download the 3D model from this link:

On Thingiverse: https://www.thingiverse.com/thing:6830532

2 - The ACSI cable

2.1 - The ACSI2STM compact has an optional 20 pins (2x10) IDC connector. This connector is not fitted by default, It is very easy to add one. I does require some soldering.
Here is what the connector look like:

And how it should look like once soldered onto the ACSI2STM card:

2.2 - The cable is a simple ribbon cable with a IDC 2x10 female connector on each side. The cable should be about 20cm long.
Longer is fine.

2.3 - On the MegaST side, you need an small adaptor in order to connect the ACSI2STM to the motherboard ACSI port. This adaptor is the same used by UltraSatan. It will convert from the IDC 2x10 connector to the 2x12 ACSI port

You can find more details about this adapter on Jookie's page here: http://joo.kie.sk/?page_id=224

The PCB's Gerber file is also available here .

3 - Power to the ACSI2STM

You still need to power the ACSI2STM card. There is no power coming from the internal ACSI port.
The easiest way is to start from a USB-C cable and cut on side:

Free the red and back wires. These will be your power wires.
Red is +5V and Black is the 0V

Ignore the green and white.

Just cut them to make sure they cannot touch anything.

You can then solder the red and black directly onto the PSU output.

Or like in this example, you can fit them into a 6 pint female header connector.

You can then connect your power cable on to the J18 header.
The J18 is normally used for hard drive.
It is located right next to the floppy drive power header.

Pin 1 is 5V

Pin 2 is 5V

Pin 3 is 0V (GND)

Pin 4 is 0V (GND)

Pin 5 is 0V (GND)

Pin 6 is 12V

Just pay attention how you connect it.
The red wire must be on a 5V source

and the black wire on the 0V (GND)

4 - Summary

To summarise, you will need the following:

A - An UltraSatan to ACSI adapter.

B - A ribbon cable with 2x10 IDC female connectors.

C - A 2x10 male IDC connector for your ACSI2STM card.

D - A mount you print on a 3D printer.
E - 2x M2 screws

And you should end up with something similar to this:

Once mounted inside your Mega ST, it should look like this:

Finally, place a CR2032 battery for the RTC (real time clock) and you care good to go.

5 - FAQ

Q: Do you sell this solution ready made?

A: No. The 3D mount would make the parcel thick and therefore too expensive to send by post. If you struggle to find some of the component, please use the contact form and we may find a solution. I stock some of them. But you will still have to solder/assemble the solution.

Q: Where can I find the required parts?

A: That will depend where you are. A local electronic shop should have them in stock.
Online electronic shops should also have them.

I personally get my parts from LCSC electronics here: https://www.lcsc.com

Here are the references I used (valid on December 2024)

2x10 IDC connector (C3405)
2x14 ACSI connector (C2897413)
Ready made 2x10 ribbon (C3000908)
USB-C cable: AlixExpress, local shop, spare cable, etc...
PCB Fabrication: JLCPCB https://jlcpcb.com/

Q: Can I use the cable made for UltraSatan?

A: Yes, the cable designed to mount the UltraSatan inside an Atari Mega ST is compatible.

Q: Can mount an ACSI2STM inside the MegaST and connect another one on the external port?

A: Yes you can. But you must make sure the IDs are not conflicting. By default, the card is on ID 0, 1 and 2. You need to set the second card on ID 3, 4 and 6 using a shunt jumper. But to be honest, with already 3 microSD slot, you should not require an additional card.