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From: pwl@fluke.UUCP (Paul Lutt)
Newsgroups: net.micro.pc
Subject: Re: TEAC drives (AGAIN!!!)
Message-ID: <545@vax1.fluke.UUCP>
Date: Tue, 13-Mar-84 11:06:26 EST
Article-I.D.: vax1.545
Posted: Tue Mar 13 11:06:26 1984
Date-Received: Thu, 15-Mar-84 00:30:11 EST
References: <1790@rlgvax.UUCP> <617@bbncca.ARPA>
Organization: John Fluke Mfg. Co., Everett, WA
Lines: 35

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

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