Path: utzoo!attcan!uunet!motcid!koch
From: koch@motcid.UUCP (Clifton Koch)
Newsgroups: comp.sys.ibm.pc.misc
Subject: Re: Drive Help
Message-ID: <5740@navy22.UUCP>
Date: 11 Dec 90 22:41:13 GMT
References: <90343.010956KDM101@psuvm.psu.edu>
Organization: Motorola Inc., Cellular Infrastructure Div., Arlington Heights, IL
Lines: 41

From article <90343.010956KDM101@psuvm.psu.edu>, by KDM101@psuvm.psu.edu (Kevin Maher):
> 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.

  The terminating resistors are used to pull control and data lines to a 
logic 1 level for the floppy cables.  The electronics use an open-collector
device to talk to the drives, meaning that the driver can only actively pull
a line down to a logical 0, and not up to a 1.  This is done so that several
drives can be daisy chained without having to use tri-statable devices.

  The correct termination scheme is to have the terminating resistors in the
last drive conntected to the cable only.  This insures that the drivers will
not be 
overloaded.  In practice, I've never had any problems with extra termination
resistors in place.

  On cables:  If you have a straight cable, then the drives should be jumpered
as DS1 and DS2 for the A: and B: drives respectively.  With a twist cable,
the drive before the twist is usually B: and the drive after the twist is
A:.  The norm is to jumper both drives as DS2, but this is controller 
dependant.  I've seen some controllers which require both drives to be jumpered
as DS1.

  Note: By DS1 and DS2, I mean the first two drive selects.  Some drives have
them marked as DS0 and DS1.

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