2 Jan
2003
2 Jan
'03
7:01 a.m.
It has been my experience that any time you start seeing pulldown
resistors, you need to run away. Far far away. TTL and CMOS, with the
exception of certain parts, have very poor drive capability, but excellcent
sink capability. If he's really having a problem like this, then it may
call for an inverter, perhaps with a Schmidt triggered input.
But pulldowns... Bad juju...
--John
-----Original Message-----
From: cctalk-admin(a)classiccmp.org
[mailto:cctalk-admin@classiccmp.org]On
Behalf Of John Lawson
Sent: Thursday, January 02, 2003 15:50
To: Classic Computers Mailing List
Subject: Re: M200 interfacing continues...
On Thu, 2 Jan 2003, Peter C. Wallace wrote:
The resting state (0) will be determined by the M200, With
a pullup the signal
will be high when the M200 is not connected, but
I doubt if
that matters...
Certainly you are correct in the generic case, and I would
have thought
so to. However: from speaking w/Sellam on the phone
regarding this, it
seems that M200 is not 'zeroing' that pin well, so I suggested
pull-downs... without a resistor the pin floats around...
not exactly
sure on a molecular level whats going on, but it probaly has
to do with
competing power supplies... anyway, I'm curious to see if
changing to 5K
or 10K will solve the problem...
But certainly this whole subject is producing a good deal
of information
which should prove useful to the next person who has a
similar interfacing
task!
Cheers
John