Re: RamFAST and 45 second message

[toddpw@frappe.ugcs.caltech.edu (Todd P. Whitesel)]

"Someone@Somewhere" <eaeu899@aldebaran.oac.uci.edu> writes:

>On Sun, 25 Jun 1995, J. L. Walters wrote:

>Well, that's another one of those mysteries of SCSI: Where to put the 
>terminators.

It is only a mystery because, unlike Thin Coax Ethernet, SCSI is slow enough
to let you get away with breaking the rules when the bus is short and the
equipment is not very noisy.

>To my knowledge, SCSI terminators simply reduces the line noise to 
>increase reliability. (Am I right?) :-)

Only in a really oversimplified sense.

Terminators, when properly placed, eliminate the component of line noise
that comes from reflections when the signal hits each end of the bus and
bounces back.

On an ideal SCSI bus, There are _exactly_ two terminators and they are at
the _endpoints_ of the bus, and there are no extra wires between the bus
itself and the interface circuits on every device that is connected.

In practice, this is very difficult. Terminators consume a noticeable
amount of power, and many hosts do not terminate their end of the bus
or (like the DMA SCSI card) terminate it but don't supply power to the
terminator. And there is internal vs. external termination on peripherals.

Good drives with two SCSI connectors run the ribbon cable from one connector
to the drive and then to the other connector. This way the signals speeding
down the wire only have to face a "fork" in the wire at the drive's 50 pin
connector, and the drive vendor can keep the extra length down by putting
the interface circuit just a few millimeters away on the circuit board.

With a short SCSI bus (2' or so) you can usually get away with no terminator
or only one. However at about 3' and up you are asking for errors and crashes
if you don't have two, each connected to the device at each end of the chain.
This is usually close enough to the "ideal" setup to work reliably.

As usual, your mileage may vary. This is based on my own experience with
SCSI on various GS's over the years. We have lots of Thin Coax Ethernet
at work, and those simply don't run without two terminators at each end
and half a meter between transmitters. The marketing guy we just hired
was trying to hook up his PC to the network and managed to create a 20
foot fork in the cable; we couldn't transmit any long packets on that
cable until I marched up there and removed the fork. Drives that use
a shorter ribbon cable and create a 6 inch fork in the SCSI cable are
the SCSI equivalent to this, and they are more dangerous because SCSI has
pathetic error detection compared to Ethernet.

Todd Whitesel
toddpw @ ugcs.caltech.edu