I don't really understand this, but I believe (perhaps incorrectly)
that laptops generally use charge pumps powered by the low voltage
supply to generate the plus and minus 12 volts for the RS-232 signals.
Therefore they often have a maximum current that can be drawn or sinked
from all the pins, that is it is not current per pin but total current
that matters. If hooking things up differently or enabling the port
differently lowers the total load on the charge pump, that could make
everything work. You should be able to watch the voltage levels on the
pins with an oscilloscope and see what is going on. Perhaps this
indicates using a higer impedance circuit would work better on laptops
e.g. a Darlington or FET instead of the usual single 2n2222.
73 Kevin w9cf
"Dick Frey" <k4xu@arrl.net> wrote:
>
> I made up network cables for our annual foray into deep dark Modoc county
> for the CQP. Nothing fancy, just three 75 foot pieces of six pair telco wire
> with a DB9 female on each end and a junction box in the center which does
> the daisy chain connections. To check it out, I put all three computers on
> the desk with the pile of cables and hooked everything up. Bingo! It works
> like a champ.
>
> OK now, we want one station for CW only. Something Tree said about Com
> ports reminded me that pin 4 is used for CW keying and serial 1 could be
> used for keying in addition to network comms. Right on! Works great on the
> old Zenith 286 laptop.
>
> Just for fun, I swapped the CW-enabled plug to the com port on my company's
> Dell Latitude laptop which has NEVER been able to use its serial port for
> keying. But it worked too. Changing its config file to disable network
> com, I tried it again. The serial port was dead as usual. Hmmmm.
>
> Yesterday I used the Dell Laptop alone in the NAQP CW and serial keying. I
> turned off the beeps and ignored all the frantic warnings about network data
> being lost. Worked great. What if I short pin 2 to 3 on that serial CW
> keying plug. Will the messages go away...? I'd rather not risk blowing the
> port driver.
>
> Does this shed any light on the nature of the Dell Laptop serial problem?
=-------------------------------------------------------------
Kevin Schmidt, w9cf@ptolemy.la.asu.edu, http://fermi.la.asu.edu/w9cf
Department of Physics and Astronomy
Arizona State University, Tempe, AZ 85287-1504
(480) 965-8240 Fax: (480) 965-7954
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