Re: Another SCSI card question ...

[dempson@actrix.gen.nz (David Empson)]

John King <joki@eskimo.com> wrote:

> David Empson <dempson@actrix.gen.nz> wrote:
> > 1. Is the SyQuest terminated internally?  If so, you shouldn't have
> > needed to add a terminator to get it to work with the original Apple
> > card.

> The HS card came straight out of a new sealed box and was installed as it
> was shipped.

The Apple HS SCSI card might or might not be supplying termination
power.  Apple added a diode to the card in later ones so that it would
do this, but it wasn't part of the original design.  (You can add it
yourself if it isn't there already, but I don't have the details handy.)

> Also, it 'works' with and without this external termination pack. That is
> why I am asking because I would have thought it should not have worked with
> internal and an external termination (unless more at the end doesn't
> matter).

Double terminating one end of the SCSI bus will result in too much
loading on all the signal lines, which could make communication
unreliable.  This may be influenced by the type of terminator (active vs
passive).

Leaving the bus unterminated at one end can result in the transmitted
signals "bouncing" off the end of the bus and interfering with
subsequent data transfers, also causing errors.

If the termination is set up correctly at each end of the bus but no
device is supplying power for the terminators, they won't work properly.

It is often possible to have a SCSI bus work correctly despite incorrect
termination, but this varies depending on the devices involved.  In some
cases, "SCSI Voodoo" problems require rearranging the order of multiple
devices to get them to work together, despite all the rules being
followed, or breaking the normal termination rules in order to get
things to work at all.  This tends to happen with poorly implemented
SCSI peripherals.  Hard drives are usually reliable - it tends to be
devices like older scanners which misbehave.

> There is only a single device. I don't know if the SyQuest drive supplies
> term power.

If not, then you had better ensure that your SCSI card does.  Have a
hunt for the FAQs, which probably have information about this.

> > If you have multiple devices connected to an original Apple card, you
> > must ALSO terminate the first device (do not do this with the high-speed
> > card).
> 
> One device and HS card. How would I terminate the first device anyway?

With internal termination inside the first device, or a pass-through
external terminator connected between the first device and the cable
which goes to the SCSI card.

The original Apple SCSI card doesn't have any termination on the card.
The Apple High-Speed SCSI card has resistors, but might not be supplying
power for termination.