Instruction manual

Instruction Manual
760006-A
September 2001
Rosemount Analytical Inc. A Division of Emerson Process Management Appendices 11-13
Model NGA 2000 Platform
Analog I/O Module Range Con-
trol Discussion
The whole question of ranges in the NGA
2000 system is rather complex. Analyzers
themselves support four ranges, but these
are mostly software considerations only,
and mainly result in the application of dif-
ferent calibration and linearization pa-
rameters to the same sensor signals.
Certain Analyzers do make hardware
changes as a result of range changes,
and these are described in the appropri-
ate manuals and the NGA 2000 reference
manual.
The Analog I/O module can be made to
track the range of operation of the Ana-
lyzer, or to operate independently of it. It
can also control the range of the Ana-
lyzer, either automatically or as a result of
external input through the digital input
lines.
If the module is made to control the op-
eration of the Analyzer, allowance must
be made for the delay associated with the
LON communication system. It may take
as much as a second for the Analyzer to
respond to an external range change re-
quest. During this time the reading may
not relate to the requested range. Any
control system relying on an instantane-
ous range change will produce an error
during this time.
If this fact cannot be dealt with in the con-
trol system software, the Analog I/O mod-
ule should be set to operate
independently of the Analyzer. In this
mode, the output scaling corresponds to
the range requested by the digital input
line, but the Analyzer's range is not af-
fected. As long as the Analyzer's range is
wider than those of the Analog I/O mod-
ule, the system will operate correctly, ex-
cept that of course any specific calibration
or linearization performed on the other
ranges by the Analyzer will not be used.
In this case the response time to a re-
quested range change by a 2.3 V Analog
I/O module will be variable but always
less than 100mSec, typically 40mSec.
The preferred solution, of course, is to get
the output of the Analyzer directly from
the LON, where these considerations do
not apply, and the Analyzer can be oper-
ated on one or several ranges as desired.