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.