I am used to the problem that if there is too much load (i.e. too
low of a resistance) on a TTL circuit that it simply loads it
down and the signal levels decrease, eventually to the point
where they do not provide enough signal to the next device(s)
they drive. I have never heard of one burning out.
73 Stan, N3HS
-----Original Message-----
From: writelog-bounces@contesting.com
[mailto:writelog-bounces@contesting.com] On Behalf Of Barry
Merrill
Sent: Friday, May 30, 2003 10:59 PM
To: 'George Johnson'; writelog@contesting.com
Subject: RE: [WriteLog] RE: User Port appears to have destroyed a
DellInsprion8100
George, W1ZT provided me with some very interesting facts about
LPT ports:
"The parallel port runs on 5 volts, but at the high logic level,
it is only intended to source about 1-2 mA with the original IBM
spec.
The new motherboard ICs probably skirt that and can safely
source
only about 0.5 mA. The output voltage is typically about 3
volts.
So the W1WEF interface and 1200 ohm resistor will draw:
(3v minus 0.8v ) for the transistor BE junction, or 2.2
volts,
divided by 1200 ohms equals about 2 ma.
You could still key your drive transistor if you increased your
resistor
to 10K which is what I use. Then the port would likely
"survive".
At 2mA it was probably going to fail if not sufficiently cooled.
With a 10K resistor, the current would be about 0.2 ma, quite
safe.
And when the port is not sending it is active 'low' and nothing
is being
driven."
So I then spent another two hours with Dell Technical Support,
and was
unable
to find anyone at Dell who could tell me what is the current
capacity of
their LPT port on the Inspiron 8100/8200 notebooks.
(well, one techie said "3 to 30 volts", and one hardware
engineer I did
get
after the 7th queue came back with "100 to 200 bits per
second", and
his
final answer was "you can't draw any current thru an LPT port
- it's
TTL".
So I then searched the internet to see if any USB-to-LPT port
devices
provided a current specification, and found none that did.
I did find one extensive reference that cited a wide range of
current
capacity
in different manufacturers port hardware, giving examples of
source/sink
capacities of 20/20 20/4 8/12 and 10/20 ma, but the document was
undated
and,
I think, old.
I also found a reference suggesting a minimum resistance of 4.7K
for
external circuits, implying that less than 4 ma was desirable.
But it was easy and wise to taking W1ZT's advice; I replaced the
original 1200 ohm resistor in the W1WEP LPT1 connector with a
10K resistor, and the Dell Inspiron 8200 with User Port enabled
keys the radios without a problem, and I feel safer using 1/8th
of the original current that might have caused the demise of
the 8100!
Barry, W5GN
Merrilly yours,
Barry Merrill, W5GN
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