At 12:56 07/25/04, Brad Loomis wrote:
>I just upgraded my motherboard to an ASUS A7N8X-E Deluxe. New processor
>and memory. My system consists of two IDE hard drives, first is the
>boot drive, three UW SCSI HDDs on an AHA-2940UW controller. There are
>also two Plextor CD drives on the 50 pin connector. When I assembled
>all this I got a post and all the pre OS load info, but when Windows
>XP Pro was to load, just a black screen, monitor light green. After
>wondering if the video card was goofy, and messing around I finally
>got everything to boot by disconnecting the SCSI devices. I loaded XP
>on the boot drive, clean install, got all things going for the most
>part, no issues at this point. However if I try to connect the 3 UW
>hard drives, I'm back at the black screen on boot up. The CD drives
>all work ok on the 50 pin connector. All devices show up in the SCSI
>BIOS boot sequence. No termination jumpers were changed, basically
>nothing changed other than the MB, processor, and memory. Anyone have
>any ideas why when I connect the UW leg I get no OS load? No I am not
>using all three of the connections, I know that isn't allowed.


Hi Brad

The most likely problem is improper termination. (If the two CDROM
drives and the three SCSI hard drives are detected in the Adaptec
SCSI-Select BIOS screens, then the SCSI IDs have to be okay.)

The Adaptec 2940UW should have its termination set to Automatic
in the SCSI-Select BIOS Setup). The correct non-automatic
settings would be "Low OFF, HIGH ON" with hard drives and CDROM
drives all present and "Low ON, High ON" with just the CDROM drives
present. Automatic is best because you don't have to remember to
change the termination if you remove one of the SCSI cables while
troubleshooting.

Please check that the CDROM drive at the end of the narrow cable
(the 50 wire cable) is terminated (set by jumper on the drive
itself) and that the CDROM in the middle of the 50 pin cable is
NOT terminated. If there is a mistake with termination on either
CDROM drive, things might appear to work until the hard drives
are added to the SCSI bus. This is because the CDROM drives
transfer data (use lower signal frequencies) than do the hard
drives. A slower bus would be more tolerant of errors, making it
less likely that Windows would hang. But with hard drives present,
the signal frequency is likely to be twice or four times as high
and faulty termination could prove fatal.

If one of the Plextor CDROM drives is an UltraPlex, I'd put that
one at the end of the narrow cable so that this end of the SCSI
cable would be sure to have Active termination. (Non Ultra SCSI
CDROMs probably only have passive termination on-board. If hard
drives are present on the bus, you want active termination at
both ends of the bus.)

The hard drive at the end of the 68 pin cable must be terminated.
The other two SCSI drives must have termination OFF. If more than
one is terminated, or neither one, or the wrong one terminated,
the resulting problem would likely hang Windows when it scans the
SCSI bus while loading.

Be aware that Adaptec states that the total cable length for Ultra
SCSI should be less than five feet (1.5 meters) when more than
three devices are on the bus. (You are trying to run with five
devices.) The total cable length is the length of the narrow cable
plus the length of the wide cable. More devices means higher
capacitive load, which has the same effect as increasing the cable
length. (By the way, having less than a foot of cable between any
two devices can cause problems.) It's pretty hard to stay under
the five foot limit if you're using two cables. The way to find
out if your total cable length is a problem is to limit the speed
of all devices (in SCSI Select) to 10 MHz or even 5 MHz. If things
start working at lower frequencies, you'll know that you need
shorter cables to run everything at Ultra SCSI frequencies. By the
way, higher quality cables should tolerate higher frequencies.

Check to see that you didn't bend a pin inside any of the plastic
68 pin connectors on the wide SCSI cable. I've done that many times.
(You can probably straighten a bent pin with the flat end of a small
screw driver. I seem to remember that if an end pin is bent, the
device might still be detected by the SCSI card but not work in
Windows. It would be worse of course if a pin is bent enough that
it touches another pin. The results would depend on what pins were
touching.

Cables can be bad. I once bought two SCSI cables from a well known
Computer Super Store and both were bad. Try using a known good
cable.

I assume that you had pulled the 68 pin cable off the SCSI card when
you determined that the system worked with only the CDROMs present.
Pulling the hard drives from their individual connectors on the
wide cable would be unwise because it would leave a "tail" of cable
hanging off the card.

Feel free to ask for more help if nothing here helped.

Regards,
Bill

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