This is the hard disk controller you find in the Mega STE:

Except in real life, it’s mounted upside down:

And this causes a great deal of confusion on how to set the ACSI ID.

Actually, all existing sources I found on the web are wrong about how to set the ACSI ID in the Mega STE. Hence this post.

The important thing is that you have to:

  • either ignore the “1 2 3” and “ON” markings on the red dip switch box
  • or assume the red box is mounted upside down. The markings would be right if the dip switch box was turned by 180°

The only correct markings are the “1”, “2” and “X” that you see printed in white on the green board:

  • 1 is for the least significant bit
  • 2 is fot the second least significabt bit
  • X means the third dip switch does nothing!

That’s right, you only get 2 bits, so you can only select between 4 ACSI IDs, not 8!

I verified all 8 combinations by rebooting with a drive attached after each combination change:

Here are the 4 actual possibilities (again the rightmost swicth is ignored):

ACSI ID 0 (down, down, X) or (ON, ON, X):

ACSI ID 1 (up, down, X) or (OFF, ON, X):

ACSI ID 2 (down, up, X) or (ON, OFF, X):

ACSI ID 3 (up, up, X) or (OFF, OFF, X):

Summary

ACSI ID green board mark 1 green board mark 2 green board mark X red switch mark 3 red switch mark 2 red switch mark 1
0 down down ignored ON ON ignored
1 up down ignored OFF ON ignored
2 down up ignored ON OFF ignored
3 up up ignored OFF OFF ignored

So what is the third switch for?

According to this reverse-engineered schematic made by tOri , the 3rd switch is connected to an EPL16P8BP field-programmable logic array. It’s not easy to know what happens inside that chip…

Now as far as speculation goes:

  • It might be a leftover from the Megafile 44 controller which seems to have been a predecessor to the Mega STE conroller.
  • It might be useful in case a second hard drive is connected to the controller.

Bonus: how to remove the internal HD controller

If you remove the controller completely (because you’re swicthing from internal HD to external SD), you need to place 8 jumpers on the board like this:

These jumpers allow to connect the 8 data lines from the internal bus to the external ACSI port.

Note: I tried this when I made the photo but I had other issues that made it not work. I haven’t validated that this actually works but I will update this post when I do.