Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 Fluke 1/4/84; site fluke.UUCP Path: utzoo!watmath!clyde!floyd!vax135!cornell!uw-beaver!ssc-vax!fluke!pwl 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 {uw-beaver,decvax!microsof,ucbvax!lbl-csam,allegra,ssc-vax}!fluke!pwl