4 Apr
1998
4 Apr
'98
5:56 a.m.
Basically I've got a really stupid question, does
the external SCSI bus
HAVE to be terminated? If so any idea's on how to go about doing that
without finding a DEC terminator?
Yes, it needs termination, unless there's nothing connected *and the cable
between the controller and the last socket* is extrememly short -- which it
often isn't. This rule applies to SASI, SCSI (aka SCSI-1), SCSI-2, and SCSI-3.
Otherwise, you'll get signal reflections, which, at best, will limit the speed
the bus can run at. There should be a terminator at each end of the bus.
There are two kinds of terminators: active and passive. In passive types, each
signal has a 220 ohm resistor to +5V and a 330 ohm resistor to ground. They
hold the voltage on an idle line at about 3 volts, and (assuming the power
supply impedance is negligible) form an effective impedance of 132 ohms. The
ideal impedance depends on the cable, but it's generally about 110 ohms. It's
this terminating impedance that prevents reflections.
Active terminators do the same job, but each signal line is connected via a 110
ohm resistor to a 2.85V supply. These are a somewhat better impedance match
and often have a better high-speed response, so they're recommended for faster
SCSI busses.
Being exceedingly stingy, I built an active terminator using a pile of
miniature resistors, a voltage regulator, a couple of capacitors, and a plug.
Oh, and an LED for show. I don't believe any electronic device should be
LED-less, and if it flashes, so much the better :-) I would recommend buying
a terminator, though; building mine was quite fiddly and time-consuming.
I'd like to be able to run it without the
terminater, since I don't have
one, but would like to know if it's possible before I go digging up the
rest of the stuff I need to see if it works.
If your bus ends at a 68-pin socket (which is a DEC special, not part of the
SCSI spec, unless it's Wide SCSI), it might be relatively hard to find a
terminator plug. But most devices have provision for terminating resistors on
the the device itself. All the RZ drives and CD ROM drives I've seen do.
Usually these are in the form of three single-inline resistor packs, each with
8 pins, and containing six pairs of 220/330 ohm resistors. Toshiba CD-ROM
drives use two 11-pin networks, though. They cost about 40 pence each from
electronics suppliers in the UK; I expect they cost about the same in the
States.
--
Pete Peter Turnbull
Dept. of Computer Science
University of York