Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!floyd!harpo!decvax!mcnc!ecsvax!hsplab From: hsplab@ecsvax.UUCP Newsgroups: net.micro Subject: Terminating Resistors Message-ID: <2159@ecsvax.UUCP> Date: Thu, 15-Mar-84 00:45:09 EST Article-I.D.: ecsvax.2159 Posted: Thu Mar 15 00:45:09 1984 Date-Received: Fri, 16-Mar-84 07:42:46 EST Lines: 21 Terminating resisters on disk drive cables provide better wave (e.g., signal) transmission characteristics. This is probably of little importance in an IBM PC where the disk cable is less than a foot, but in some units where the disk drives are connected with up to ten feet of cable, the signal rapidly deteriorates. If the terminating impedance is wrong (e.g., infinite if un- terminated), the signals may reflect back and forth on the signal line similar to an undamped shock absorber. Thus the term standing wave. Some senior electrical enginneering courses discuss this in logic design; most do not. I found a valuable reference in a book by Matick, Transmission Lines for Digital and Communications Networks (1969) McGraw-Hill. The theory has not changed even though the book is old. Usually signals traversing on a cable requires consideration as a transmission line when the traversal time in the cable begins to approximate the timing of the logic gate. Most TTL gates have propagation times of 10-15 ns. Propagation in wire is approx. 1 ns/foot. Thus the ten foot disk drive cable should be treated as a transmission line and may require termination, depending on the equipment design. David Chou University of NC, Chapel Hill ...!decvax!mcnc!ecsvax!hsplab