Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!ames!ptsfa!well!msudoc!umich!itivax!m-net!russ
From: russ@m-net.UUCP
Newsgroups: sci.electronics,comp.periphs,comp.sys.apple,comp.sys.atari.8bit,comp.sys.cbm,comp.sys.misc
Subject: Re: 6526 CIA chip problems?
Message-ID: <1086@m-net.UUCP>
Date: Wed, 25-Mar-87 18:38:00 EST
Article-I.D.: m-net.1086
Posted: Wed Mar 25 18:38:00 1987
Date-Received: Mon, 30-Mar-87 03:58:15 EST
References: <1097@cavell.alberta.UUCP>
Reply-To: russ@m-net.UUCP (Russ Cage)
Organization: M-NET, Ann Arbor, MI
Lines: 33
Keywords: Flash!, C64, 1541, disk drive enhancements
Xref: utgpu sci.electronics:432 comp.periphs:297 comp.sys.apple:884 comp.sys.atari.8bit:400 comp.sys.cbm:278 comp.sys.misc:469
Summary: Too many pull-ups or the OC drivers can't drain enough current.

In article <1097@cavell.alberta.UUCP> gail@alberta.UUCP (Gail Miklos) writes:
>
>An interesting problem:
[description of how either drive works alone, but how interconnected
drives fail, and a powered-off drive really mungs things.]

This seems like it's not too hard:

1.)	When both drives are connected, the open-collector drivers in
	one drive suddenly have to sink the current from not one but
	two sets of pull-up resistors in order to pull the lines down
	to logic 0.

2.)	When one drive is off, its pull-up resistor is connected to
	ground (or close enough); the lines can't be pulled up to
	logic 1, and errors result.

Possible solutions:

1.)	a.)	Try disconnecting one set of pullups.
	b.)	Failing this, try increasing the current drain capability
		of the line driver by buffering it, so that it can drive
		the line even with multiple pullups connected.

2.)	a.)	Connect diodes (germanium, not silicon) in series with
		the pullup resistors so that they can't sink current
		when the drive is off.
	b.)	Don't try running with one drive off!
-- 
My employer gets all its opinions from me.  (My employer *is* me.)
                                   	|  Russ Cage, Robust Software Inc.
     The Moral Majority runs Iran.   	|      ihnp4!itivax!m-net!russ
NSA food> CIA DIA KGB rocket ammunition AK-47 dynamite atomic TEMPEST Hail Eris!