
UM-0085-B09
DT80 Range User Manual
Page 68
RG
•
any channel's value (e.g. a measured temperature) may be assigned to a channel variable by using the
=
n
CV
channel option
•
certain special channel options (e.g. histogram) return multiple data values, which are written to a specified
range of channel variables. See
Multi Value Statistical Options (P78)
.
•
data values read from a serial sensor using the generic serial channel type (
n
SERIAL
) may be assigned to
.
•
CVs can be set or read by an external Modbus master device. See
.
•
CVs can be configured to follow the state of an alarm (e.g set to 1 if the alarm is active, 0 if it is not). See
.
Expressions
Some examples of using expressions to set CVs:
1..20CV=10.2
initialise multiple CVs
RA1S 1V 9CV=9CV+1
count the number of measurements taken
5CV(W)=3CV*SIN(21CV)+2CV*COS(21CV)
See also Expressions (P69).
=nCV Channel Option
The =nCV channel option allows a channel value to be assigned directly to a CV, typically so it can then be used in
further calculations.
This can be used to apply a complicated linearisation equation, e.g.:
1V(=2CV,W) 2CV(S9,"temp~K")=2CV/(LN(2CV+1))
This will measure a voltage and assign it to 2CV (note the W option – we are not interested in logging/returning the raw
voltage value). The value in 2CV is then plugged into the specified expression and the result stored back in 2CV. Finally
a span (
S9
, which must have been previously defined) is applied and the result is returned with appropriate name and
units.
An arithmetic operator may also be applied during the assignment, as follows:
Channel Option
Action
=
n
CV
n
CV
=
channel value
+=
n
CV
n
CV
=
n
CV
+
channel value
-=
n
CV
n
CV
=
n
CV
- channel value
*=
n
CV
n
CV
=
n
CV
* channel value
/=
n
CV
n
CV
=
n
CV
/ channel value
These allow a CV to be used as an accumulator, e.g.
RA1M 3C(+=2CV) 2CV("Total")
will report the number of counts received in each one minute period, plus the total counts, i.e.:
3C 192 Counts
Total 192
3C 77 Counts
Total 269
Storage Precision
Channel variables may be used to store values with magnitudes from 10-38 to 1038. However, it is important to be aware
that values are stored with 24 bits of precision, or approximately 7.25 significant digits when written in decimal form. This
means that whenever the DT80 displays a value, it will only be accurate to 7 significant digits. The 8th digit can be used
to distinguish values which are close together, but its value may be out by a few counts. Any changes in the 9th or later
digits will not be reflected in the stored value.
This is particularly noticable when incrementing large integer values. For example:
1CV=3600000
RA1S 1CV=1CV+1
1CV 3600000.0
1CV 3600000.8
1CV 3600002.0
In this example 8 significant digits are being displayed, so the last one is not accurate. It would be better in this case to
not display any decimal places, i.e.
1CV(FF0)=1CV+1
.
Once the magnitude of the CV value exceeds 224 (16,777,216), adding one to it will no longer cause its value to change.
A consequence of this is that if you manually count something using a channel variable (e.g.
1CV=1CV+1
) then it will
stop counting once its value reaches 16,777,216. Note that this only applies to manual counting using CVs – hardware
and software counters (
HSC
and
C
channel types) can count over the full 32-bit range.
Содержание DT80
Страница 29: ...UM 0085 B09 DT80 Range User Manual Page 29 RG The DT80 File System P114 ...
Страница 184: ...UM 0085 B09 DT80 Range User Manual Page 184 RG Figure 71 DT80 communications options ...
Страница 185: ...UM 0085 B09 DT80 Range User Manual Page 185 RG Figure 72 DT80 communications options integrated modem models ...