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From: phils@tekigm2.UUCP
Newsgroups: comp.sys.amiga
Subject: Re: PALs,jumpers and expansion on the Amiga 1000.
Message-ID: <2141@tekigm2.TEK.COM>
Date: Sun, 6-Sep-87 00:14:51 EDT
Article-I.D.: tekigm2.2141
Posted: Sun Sep  6 00:14:51 1987
Date-Received: Sun, 6-Sep-87 20:18:31 EDT
References: <8709010613.AA05631@cogsci.berkeley.edu> <1367@ihlpm.ATT.COM>
Reply-To: phils@tekigm2.UUCP (Philip E Staub)
Organization: Tektronix, Inc., Beaverton, OR.
Lines: 54
Keywords: Two on the Bus

In article <1367@ihlpm.ATT.COM> nevets@ihlpm.ATT.COM (Steven R Ringwood) writes:
>In the end I used wire-wrap wire, a solding iron, and an old alligator clip.
                   ^^^^^^^^^^^^^^
>
...
>
>Did it work, for me YES.
>
>Before making this change i could not get by 2Meg Starboard II and
>Supra 30Meg harddisk to work together.
>
>After the 'fix', they both work but only if the Supra is first on the
>bus for some reason.
>
...
>	
>Steven Ringwood
>ihnp4!ihlpm!nevets

I'm glad to know that Bryce's fix worked for you, but personally, I'm a bit
surprised to find it to have been effective with such small wire. Presumably
the intent of the fix was to decrease the impedance of the ground path
back to the power supply common, in light of inadequately sized conductors
on the circuit board. The additional grounding you get from wire wrap wire
is questionable.

May I make a suggestion? Try using some larger wire, or even some small
ground braid (enclosed in some form of suitable insulation). This *may*
clear up your position dependency, although I rather suspect that the real
reason is that the disk controller's circuitry is more susceptible to the
degraded signal timings available at the end of the bus.

Another comment: wiring the grounds in series the way Bryce described 
may result in reduced effectiveness for the device at the end of the chain. 
This is due to the same effect which causes the problem in the first
place: the higher the impedance, the greater the voltage drop across that
impedance for a given current returned to common. Putting all of the devices
on the same circuit causes coupling of noise from all of the PALs to each
other.

If the impedance of the ground is low enough (read: large enough wire) the 
noise level may still be low enough, and it won't matter. However, if you 
use small wire, it may be more effective to connect each PAL *individually* 
to your good, solid ground point.

Again, congratulations for getting multiple devices running on the expansion
bus, and hope these suggestions are something you can use.


-- 
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Phil Staub                     "I do NOT approve. I merely said I UNDERSTAND."
tektronix!tekigm2!phils                                              - Spock
phils@tekigm2.TEKSpock