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From: kaufman@Neon.Stanford.EDU (Marc T. Kaufman)
Newsgroups: comp.sys.mac.hardware
Subject: Re: IIfx upgrade and SCSI termination for int. hard disks
Message-ID: <1990Aug11.042741.22709@Neon.Stanford.EDU>
Date: 11 Aug 90 04:27:41 GMT
References: <3086@dftsrv.gsfc.nasa.gov>
Distribution: na
Organization: Computer Science Department, Stanford University
Lines: 21

In article <3086@dftsrv.gsfc.nasa.gov> jim@jagmac2.gsfc.nasa.gov (Jim Jagielski) writes:
>I'm interested in upgrading my Daystar 50 MacII to the IIfx, but the only
>real thing (besides bucks :) holding me back is this question of SCSI
>termination. I know that the IIfx upgrade "makes" you remove the termination
>from your internal disk and replace it with the IIfx terminator/filter.

I have been confused by this from the beginning, and now that the terminator
is documented (TN 273) I am even more confused.

It appears that the 'fx' termination differs from a standard external
termination in that it has extra capacitance from +5 to ground, to prevent
induced crosstalk between the signal lines (by stiffening the power supply
with the 2.2uf cap and bypassing high frequency spikes with the .01uf cap).
Now an internal terminator should be relatively close to a LOW impedance
source of +5, so the problem should never arise.  Why, then, must one
remove the internal terminators and replace them with external ones?

By the way: has anyone else noticed that the capacitors in TN 273 are installed
backwards?

Marc Kaufman (kaufman@Neon.stanford.edu)