Path: utzoo!attcan!uunet!jarthur!usc!zaphod.mps.ohio-state.edu!think.com!linus!linus!usenet
From: usenet@linus.mitre.org (Linus's Usenet)
Newsgroups: comp.sys.ibm.pc.misc
Subject: Re: Drive Help
Message-ID: <127240@linus.mitre.org>
Date: 11 Dec 90 21:31:28 GMT
References: <7065@plains.NoDak.edu> <90343.010956KDM101@psuvm.psu.edu>
Reply-To: carlson@gateway (Bruce Carlson)
Organization: The Mitre Corporation
Lines: 51

>The resistor in question is called a terminating resistor.  Now, as far as
>I know, the resistor was only used on the original PC/XT systems.  The
>cable that connected the floppies was a simple straight ribbon.  The resistor
>was used to "tell" the controller that the drive with the resistor was the
>A drive.  (at least on a true IBM PC . the B drive is the first drive on the
>cable and the A is the last)  On the AT computer however, the floppy cable
>was designed with a few wires twisted.  This twist is what determined which
>drive was A or B.
>If you're using this drive in a computer with a simple straight ribbon cable,
>you will need to find a group of jumpers near the connector.  Put a jumper
>over the pins for DS1 for drive A or DS2 for drive B.  On an AT, I guess all
>you need to do is put it on DS1 and the cable does the rest.
From: carlson@gateway (Bruce Carlson)
Path: gateway!carlson

>      If somebody out there knows that this is wrong, please tell me.

I think you are a little off track with your directions.  The DS1/DS2
(or it may be DS0/DS1 in some cases) pin selection is related to the
twist in the cable.  With a properly twisted cable you put the A drive
at the end of the cable and the B drive in the middle and you don't have
to worry about the DS1/DS2 pin (it is left on DS1).  However, this is
not related to the terminating resistor.

I have seen explanations of why the terminating resistor is needed and
I can provide some info from memory (with about 75% confidence).
The resistor always goes on the drive on the end of the cable (A: drive)
and its function is to absorb or damp signals on the cable and avoid 
reflections that will produce errors.  I have used PCs where neither drive
had a resistor and it still seemed to work, but I don't know if I was
living dangerously.

I had a TEAC 5 1/4 inch drive in my AST (AT compatible) computer and it 
had a terminating resistor.  However, the drive broke and the replacement
TEAC drive was constructed differently and did not have the same type of
DIP resistor.  I don't know if the new drive uses jumpers to select
whether it has a resistor connected, or if there have been some other changes
to the drives.  I got no documentation with the new drive and haven't 
tried to call TEAC.

The other configuration item to look for between PCs and ATs is that one
of the cable pins (34 or 35) must be connected with a PC, but must not
be connected with the AT.  Most drives have a jumper setting to connect
or not connect the pin.  This pin is used to detect a disk change  and
if its configured wrong the drive may not work or you may scramble the 
FAT because the machine doesn't know you have changed disks in the
drive.  PC disk drives cannot sense when a disk has been changed so 
programs must recheck the FAT; AT HD drives can respond to a status
request and tell that the disk has been changed.

Bruce Carlson