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.