termination and cabling question

[Mike Bilow]

Sheldon E. Newhouse wrote in a message to Mike Bilow:

 SEN> I have an AHA2940UW controller. There are three ports on the 
 SEN> contoller. Internal 68 pin, internal 50 pin, and external 68 
 SEN> pin.  

 SEN> I have the following setup.

 SEN>    tape drive on external 68 pin,
 SEN>    UW disk on internal 68 pin
 SEN>    scsi cdrom on internal 50 pin. 

You can't do this, at least not officially.  The 2940 series has one physical
SCSI bus, with the controller in the middle.  On one side of the controller is
its 68-pin internal connector.  On the other side of the controller are both
its 68-pin external connector _and_ its 50-pin internal connector.  You can use
_at most_ one connector from each side, meaning that you cannot use the 68-pin
external connector and the 50-pin internal connector at the same time.

 SEN> Auto-termination is enabled. There seems to be no need for 
 SEN> external termination.   

No.  The idea is that the controller is terminated if one side of it is not
used for peripheral devices, but unterminated if both sides are used.  This is
entirely in accord with the usual practice of terminating the physical ends of
the SCSI bus, once you understand how the physical SCSI bus is laid out as
explained above.

To be clearer, the controller is terminated if (1) the 68-pin internal
connector is not used, or (2) both the 68-pin external connector and the 50-pin
internal connector are not used.

A related concern is that using both the 50-pin internal connector and the
68-pin internal connector requires that the controller terminate the high part
of the Wide bus, but leave the low part unterminated so the device at the end
of the 50-pin cable can do it.

Here is the truth table for all supported configurations:

68-pin internal      68-pin external      50-pin internal      term (low/high)

   used                 not used             not used             on/on

   used                 used                 not used             off/off

   used                 not used             used                 off/on

   unused               used                 not used             on/on

   unused               not used             used                 on/on

(Obviously, there is a sixth legal combination where none of the connectors are
used, but that is pointless.)

 SEN> On Net Express' web site there is a statement about the need
 SEN> to have a linear topology instead of a branching topology
 SEN> for the cabling. They suggest that all devices should be on
 SEN> the same cable, so that, e.g. one should have

 SEN>   tape -> contoller -> internal scsi disk -> internal cdrom)

 SEN> Mine is 
 SEN>   tape -> controller -> internal scsi disk (all 68 pin
 SEN> connectors              |
 SEN>             \ /
 SEN>           50 pin internal cdrom

 SEN> For the first setup, I would need an adaptor since the
 SEN> internal disk is 68 pin and the internal cdrom is 50 pin. 

Your ASCII drawing did not come through legibly, at least not to me.  If you
are saying that you have nothing connected to the 68-pin external connector and
that you are using both the 68-pin internal connector and the 50-pin internal
connector, rest assured that this is a fully supported configuration and will
work in compliance with SCSI specifications.  If you are saying that you are
trying to use both the 68-pin external connector and the 50-pin internal
connector at the same time, then this is unsupported but can work.

If you leave the tape drive disconnected most of the time and only connect it
when you need to do a backup or a restore, you would most likely want to leave
the controller terminated all the time.  However, if auto-termination is
working for you, there is no reason to change this.

 SEN> It is working as is.  In private e-mail, they suggested that
 SEN> this is not scsi compliant and may lead over several months
 SEN> to damage to the hardware.  

You will not damage the hardware over time.  It is possible that allowing too
many termination paths will sink excessive current, but this will cause
problems (such as blown fuses on the controller) immediately if it happens, and
will also require a lot more over-termination than three devices.  I've never
seen this happen, in fact, although it is theoretically possible.

Over-terminating the SCSI bus is a fairly common technique to solve noise
problems, although it is certainly a violation of the specification.  Use your
head on something like this: it might be fine for your personal desktop
machine, but not for a network server that people depend upon.

Severe mistermination will show up as data errors and lockups far sooner than
it will show up as blown fuses, assuming you get as far as booting.

 SEN> I am seeking other opinions and/or experience.

 SEN> Would it help to put an active terminator on the end of the
 SEN> external tape drive?   Is it really better to simply get the
 SEN> adaptor for the internal 50 pin connector? 

No, an active terminator assumes that you are following the specification.  If
you are trying to do tricks, then you are better off with passive terminators.
Besides, it is working now, and the only way to play tricks with termination is
by trial and error.
 
-- Mike