Earlier versions of Adaptec AHA-2940AU BIOS suffered from several bugs and incompatibilities. These problems can be found in the knowledge base on Adaptec site. In my case, the key issue was lacking or incomplete support for booting from CD-ROM's in most but latest BIOS versions. I managed to find two BIOS images, as well as various instructions and recommendations which I compiled here. I know of no significant issues with either of these two images.

AHA-2940AU is not flashable, i.e. you will need to find someone with an EPROM writer. After writing the content of the image file into an EPROM, you will have to physically remove the old chip from the card and replace it with the new one.

There are two PCB versions of the controller. As far as I know, both will work with either BIOS. The only difference is that the later version, in my experience much rarer than the old one, comes with support for 64 KB EPROM instead of 32 KB. The pinouts of two EPROM chips are different so they are not exchangeable*. You can determine the version from the inscription on the ROM chip (usually hidden below the paper label): 27C256 is a 32 KB chip while 27C512 is a 64 KB one.


v1.32 this is a 32 KB BIOS image requiring 27C256 with access time of at least 150 ns.

v1.34.2 this is a 64 KB BIOS image requiring 27C512 with access time of at least 150 ns.

* Some cards with 27C256 can be modified to accept 27C512. The key issue is A15 located on the pin 1 of 27C512. I received the following advice:

"Just connect address line 15 from the eprom. If you look closely you'll see 2 solderpads on the pcb close to pin A15 from the eprom."

Pinout: 27C512 (Top view)
	INPUT 'A15'   1         28 POWER 'VCC'
        INPUT 'A12'   2         27 INPUT 'A14'
        INPUT 'A7'    3         26 INPUT 'A13'
        INPUT 'A6'    4         25 INPUT 'A8'
        INPUT 'A5'    5         24 INPUT 'A9'
        INPUT 'A4'    6         23 INPUT 'A11'
        INPUT 'A3'    7         22 INPUT 'O\E\'
        INPUT 'A2'    8         21 INPUT 'A10'
        INPUT 'A1'    9         20 INPUT 'C\E\'
        INPUT 'A0'   10         19 3-ST. 'O7'
        3-ST. 'O0'   11         18 3-ST. 'O6'
        3-ST. 'O1'   12         17 3-ST. 'O5'
        3-ST. 'O2'   13         16 3-ST. 'O4'
        POWER 'GND'  14         15 3-ST. 'O3'