Adaptec has pretty good support for end-of-life products, so you might be able to find info and downloads here:

http://www.adaptec.com/en-us/support/_eol/

Note that there are two different SCSI Host Adapters - ISA sections on this page, and each has some AHA-1542 variants listed, so make sure you know exactly which version you have (AHA-1542, AHA-1542A, AHA-1542B, AHA-1542C, AHA-1542CF, etc.). You can do this by paying close attention to the Adaptec BIOS screen at boot time. Some of these variants seemed to be short-lived with minimal support or information available while others were widespread and well-supported.

I know you were looking for help on booting a hard drive in another thread. I have an AHA-1540C, and the key to getting my particular model to boot a SCSI hard drive on an AST Advantage! 486SX machine was to make sure that I specified that I had no hard drives in the (proprietary) AST BIOS. Here are a few notes I took on configuration:

1. Record jumper settings for all add-in cards AFTER conversion Creative Labs CT2950 Card is PnP Adaptec AHA-1540C Card SW1: ON […]
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1. Record jumper settings for all add-in cards AFTER conversion
Creative Labs CT2950 Card is PnP
Adaptec AHA-1540C Card
SW1: ON
SW2: ON
SW3: OFF
SW4: OFF
SW5: ON
SW6: OFF
SW7: OFF
SW8: OFF

6. BIOS Changes:
<Hard Drives>
1. Changed Hard-Disk Adapter to Auto (from Auto)
2. Changed First Hard-Disk Type from Auto to None
3. Changed Second Hard-Disk Type from Auto to None

7. Hard Drive Partitioning

Logical Physical Total Partition
Drive Drive Space OS TYPE
A: 3 ½ inch Floppy Drive 1.44 Megabytes
C: Quantum Fireball SCSI Drive 510 Megabytes DOS FAT
C: Quantum Fireball SCSI Drive 8 Megabytes OS/2 BOOT
D: Quantum Fireball SCSI Drive 510 Megabytes DOS FAT
E: Quantum Fireball SCSI Drive 1000 Megabytes OS/2 HPFS
F: Quantum Fireball SCSI Drive 256 Megabytes OS/2 HPFS
G: Quantum Fireball SCSI Drive 744 Megabytes OS/2 HPFS
H: Quantum Fireball SCSI Drive 1000 Megabytes OS/2 HPFS
I: Quantum Fireball SCSI Drive 1740 Megabytes Linux EXT2
J: Quantum Fireball SCSI Drive 256 Megabytes Linux SWAP
K: Quantum Fireball SCSI Drive 1000 Megabytes Linux EXT2
L: Quantum Fireball SCSI Drive 1000 Megabytes Linux EXT2

The switches and possible settings on your particular card may be quite different.

I have never tried to get my Adaptec card to boot from the CD-ROM drive, so I can't offer any advice about that particular question.

I have 1542CF cards upgraded to v2.11 BIOS.
These could boot from the Win98SE install CDROM, and IIRC from a FreeDOS based boot CD as well. I have not tried others.
Hopefully, your issue is not CDROM related. Then with the proper SCSI BIOS setting you can boot sg...

As far as I know El torito was "the" standard for bootable CD-ROM discs.

I have a few 154x cards, but I never tried the CD-ROM boot. I did however use the PCMCIA equiviliant to boot a linux CD-ROM on a laptop computer. It would probably be best to download the manual. Maybe your drive needs to be configured to a specific device ID?

Stupid question, but are you using a known-good CD-ROM drive? If I have a computer that won't boot a freshly-burned disc, that's usually the first thing I suspect.

Also, is your CD-ROM set to ID 0? Most adapters default to booting that first. I don't remember if the 154x cards will automatically try other IDs, or just give up if they can't boot 0.

It is also evident that one checks the BIOS if bootable CDROM support is Enabled in the first place... I think Blake has done that as well 😀