From rwallaceatworld.std.com Wed Aug 24 16:31:27 PDT 1994
Article: 2845 of vmsnet.pdp-11
Path: nntp-server.caltech.edu!netline-fddi.jpl.nasa.gov!elroy.jpl.nasa.gov!usc!math.ohio-state.edu!howland.reston.ans.net!europa.eng.gtefsd.com!MathWorks.Com!mvb.saic.com!info-pdp11
From: Roger N Wallace <rwallaceatworld.std.com>
Newsgroups: vmsnet.pdp-11
Subject: Re: RX33 jumper settings needed!
Message-ID: <Pine.3.89.9408241710.A19114-0100000atworld.std.com>
Date: Wed, 24 Aug 1994 18:16:42 -0400 (EDT)
Organization: Info-Pdp11<==>Vmsnet.Pdp-11 Gateway
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Lines: 43



On Wed, 24 Aug 1994, Dan Lanciani wrote:

> I realize this is slightly off topic, but has anyone else noticed
> that many floppy drives no longer have even a terminator jumper (let
> alone a whole removable terminator pack)?  Is the assumption that a
> PC controller can handle two sets of parallel resistors?  (For that
> matter, can it?)  It's getting real hard to replace drives in older
> systems that had, say, a string of four.  (1/4 of 150 ohms is just
> too low. :)
> 
> 				Dan Lanciani
> 				ddlatharvard.*
> 

	As I recall, the original ST-412 / SA-850 / SA-540 interface standards
allowed cable lengths up to 20 feet with the proper 220/330 ohm "voltage 
divider" termination (about 130 ohms equivalent empedance) at the end of the
run.

	The PC industry has apparently decided that 20-foot cable runs 
are quite rare.  New TEAC floppies are shipping with "1000 ohm" terminations.
Effectively, this "kinda sorta" terminates a short run of cable well enough
to keep ringing down to the point where data errors don't occur.  The 1000-
ohm terminations are soldered in place.  You can daisy-chain up to the max
four drives allowed by the standard without having the impedance get too low.
The end user is spared the nasty task of trying to remember which drives 
should have terminators in/out.  Saves time at the factory also:  all PC 
floppies are (quasi) terminated, and set to DS1 (of 0,1,2,3).

	If you have to use one of the new floppies in a "real" computer with
a long cable run, you may have to add the correct 220/330 ohm terminator to
the last drive in the chain.  I have added these on a Sony 3.5" floppy that
I run as a "mini" RX-33 (writing 15 of 18 sectors per track) paired with a 
"real" RX-33 (Teac FD55-G) on an RQDX3 controller in a PDP11 system. 

	The hard-wired 1000-ohm terminators are not "correct" in that they
do not properly match the impedance of the interface cable.  However, in
short runs, one can cover up a multitude of sins -- and even with the 
mismatch the drives work OK.

						Roger Wallace