Recently I had a problem with an NMRNET connection between a pc
and my MSL300 when I replaced the pc. This connection had been
working previously but only with one specific interface card
that we had made. It should be noted that the Aspect and the pc
are connected by a 50 foot shielded Inmac cable, #H148-1. The
symptoms observed were that we could send and receive ascii
files and send data from the pc to the Aspect but could not
receive a complete binary data file from the Aspect because the
handshaking between the computers would be lost randomly. We
decided to check the signals using a digital scope and found
that the data out lines from the Aspect were noisy and even
worse one of the control lines, poutpls, suffered from
cross-talk.
Part of the noise problem arises because the datain lines to the
PIO card are not buffered so we added pull-up resistors, 1000
ohms, to the data lines 3,4,5,6,7,8,9,and 10 on our interface
card. In our case it was most convenient to add a resistor
network across the 37 pin AMP 745115-2 connector. See a
description of the connector between the PIO card and the Aspect
in the file connect.asc which can be found on my anonymous ftp
site cecnmr.mps.ohio-state.edu If you promise not to laugh I
have the beginnings of a web page at
http://cosy.mps.ohio-state.edu which also has a pointer to the
ftp site.
To minimize the cross-talk on the poutpls line we matched the
impedance of the cable by adding a pull-up resistor, 220 ohm,
and a pull-down resistor, 330 ohm, to the poutpls line before
the signal is inverted, that is, on side of the interface card
going to the Bruker, pin 15 on the 25 pin connector.
If any of you have had problems when using long cables, you
might consider adding these changes to your interface. --
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Charles E. Cottrell NMR Facility Manager
Campus Chemical Instrument Center Phone 614/292-0489
The Ohio State University Fax 614/292-1532
176 W. 19th Avenue
Columbus, Ohio 43210
ccottrel@postbox.acs.ohio-state.edu
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