geller@rlgvax.UUCP (David Geller) (03/09/84)
Remember me? I used to post articles praising TEAC 1/2 height disk
drives for use with IBM Personal Computers to net.micro.pc. Well,
I'm happy to report that my TEAC 1/2 heights are STILL working
perfectly. Why the article? I just found out a few things that may
be of interest to those of you out there still contemplating pur-
chasing non-IBM disk drives for use with your PC.
TERMINATING RESISTOR
Up until yesterday I had never heard of the beast. (I'm not much
of an electronics jock) What I found out was that when you have disk
drives daisey-chained together (i.e. drives a: and b: on your PC) the
last drive in the chain is supposed to have a terminating resistor
attached to it (on the board). I found out about this thing a day ago
when someone called me asking about my experiences with the TEAC. I
told him that I had had no problems and that installation was fairly
simple - a few cabling attachments (power and I/O) and four screws for
the mounting (one is on top of the other). He then asked me about the
terminating resistor. What I said - I never did anything like that to my
drives. We ended the conversation with him thanking me for what I had
told him and me wondering what he had made mention of. Two of my fellow
workers then told me that it was VERY important for the terminating
resistor to be in place in the last drive and not anywhere else within
the I/O path. What would happen if they both had the resistors I asked -
afterall, this is how my system had been operating for many months
(perfectly I might add!!!). Fry, sizzle, die were their answers. Come
on I said. They told me that I should open up the ol' box again and look
for the beastie. Look for a socketted chip near the cabling area on the
drives board they said. I did and I think I found it. I THINK...
The "thing" I found is marked 1AM E3317 and is black with a
white underside and has fourteen (14) legs. Ichhhh - a bug!!! Well -
I removed the sucker from the a: drive and reassbled my system. All
well. I suspect, though, that I have made a mistake. Two things lead
me to this conclusion. The first is that my old configuration worked
just beautifully. I changed the config only to prevent impending
disastor. The second is that the a: drive is the last drive in the
chain - at least on my cable. I referenced page 11 of the IBM diskette
adaptor manual. The a: drive also has the twisted cable in the middle
of the ribbon. What's up?
Did I do the right thing? Will drives soon die? The thing I
pulled out - should I have. I eagerly await a response. I'm sorry if
I`ve confused any of you out there. I'm just playing around and always
learning new things.
THANKS IN ADVANCE
{seismo}!rlgvax!geller David Geller
Computer Consoles, Inc.
Office Systems Group
1760 Reston Avenue
Reston, VA 22090
703-471-6860sdyer@bbncca.ARPA (Steve Dyer) (03/10/84)
I have run two 5 1/4" drives daisy-chained with both containing a
termination resistor without any problem. Like you, I removed
the inner-most resistor only for peace of mind. What function,
exactly, does this perform?
--
/Steve Dyer
{decvax,linus,ima}!bbncca!sdyer
sdyer@bbncca.ARPApwl@fluke.UUCP (Paul Lutt) (03/13/84)
I'm surprised this question has not been answered yet. Floppy disk
drives in general use what are called "open collector" drivers for
their hardware interface signals. These drivers have the property
that they look either like an open switch or a switch which is
closed and connected to ground (0 volts). Information is passed
along this interface by the absence or presence of a voltage greater
than 2 volts. Now where does this 2 volts come from? The interface
driver itself can only be open or shorted to ground. The source of
this voltage is the terminating resistor, which is tied to +5 volts.
Without a terminating resistor connected to the interface line, there
is no reliable signal to be switched by the interface driver.
In a typical floppy disk system, there are a number of terminating
resistors used, tied to various signal lines. These resistors generally
reside in a package which looks very much like a 14-pin integrated circuit.
This resistor package resides on the floppy disk drive control board,
usually near the interface connector. These resistors are generally
placed in a socket, so that they may be readily removed. There should
only be ONE set of terminating resistors in a system. The interface
drivers are designed to switch only a limited amount of current. If
more than one set of terminating resistors is present, the current switched
by the driver may become excessive. The most likely result of this
would be marginal signal levels, which could cause erratic behavior.
No physical damage to the drive should result from this situation.
The bottom line is that for reliable operation, you should only have
one set of terminating resistors in a system, and these resistors
should normally reside on the disk drive which is farthest down the
interface cable.
Paul Lutt
John Fluke Mfg. Co
Everett, WA
{uw-beaver,decvax!microsof,ucbvax!lbl-csam,allegra,ssc-vax}!fluke!pwljones@fortune.UUCP (03/14/84)
#R:rlgvax:-179000:fortune:14700006:000:1521
fortune!jones Mar 13 19:40:00 1984
There are just a few points to remember when dealing with
daisy-chained peripherals:
1. Only one peripheral on each daisy-chain should have
the terminating resistor pack installed. All disk
peripherals come from the vendor with a terminating
pack in place because it is easier to throw the pack
away then to hunt one up. The vendor normally tests
each drive before shipment.
2. The termination resistor pack should be placed in
the last 'physical' location on the daisy chain.
That is the drive furthest from the controller.
3. Some people are blessed. You can ignore the basics
and your system may run just fine...perfectly in
fact. The reason is that sometimes you get more
than you pay for.
It is a fact of manufacturing that it is expensive to build something
exactly to specification. It is easier to make a part a little better
than required to minimize the testing and inspection. Of course,
you can't make it too much better or it is uneconomic to produce. So,
between those two bounds you get some systems which work even if you
stress them beyond specification and some that don't. If you have
more time than money, then give it a try. It may work. That is, of
course, if it doesn't damage anything. If you have more money than
time, then take out the insurance of doing it the recommended way.
Dan Jones
UUCP: {sri-unix,amd70,hpda,harpo,ihnp4,allegra}!fortune!jones
DDD: (415)595-8444 x 440
USPS: Fortune Systems Corp, 101 Twin Dolphins Drive, Redwood City, CA 94065