"John Dann" wrote in message
...
On Tue, 22 Nov 2005 15:44:25 GMT, "Alan Gardiner"
wrote:

I would also go for a USB logger connection unless your PC is old and does
not
have a USB interface.


It's difficult to be definitive about serial vs USB. Obviously some
modern PCs don't have a traditional serial port and so can't acccept a
serial logger, at least not without using an external serial-to-USB
converter.

However, USB has at least 3 minor drawbacks:

1. While the great majority of users of USB loggers have no problems,
overall USB just doesn't seem as robust as RS232 and a small % of
users have difficulties maintaining a reliable connection. Maybe it's
because it's a shared protocol, where other devices or their drivers
can interfere. Maybe it's a function of the variety of USB chipsets
and their implementation. But it is occasionally an issue with USB
which virtually never happens with RS232.

2. RS232 cables can be easily extended to 15m (and beyond). For
reliable operation, USB is limited to ca 5m without special repeaters.

3. Traditional RS232 is a well-established protocol and there are
readily available fixes to cure eg ground loop problems, tunnel the
RS232 data across ethernet LANs and so on. While such solutions are
starting to appear for USB, they can still often be specialist and
more costly in practice.

In a typical data logging application where data rates are determined
by the internal design of the logger not the interface type, USB will
often no faster than RS232 and so typically USB speed isn't an
advantage.

John Dann
www.weatherstations.coi.uk

The reason I suggested USB is than the traditional serial port is becoming
obsolete, my recently acquired laptop is all USB and manufacturers can save
money by not including them on desktop machines.

Speed is certainly irrelevant as the data downloads are quite quick using a
serial port which is the method I use. Clearly your experience has shown
that USB does have drawbacks in reliability terms and that is definitely an
important fact.

I would assume that the USB connection is slightly easier to set up as the
detection is automatic but to be fair the serial connection is easy to setup
as well.


Alan