Campbell Scientific SDM-CVO4 4-Channel Current/Voltage Output Module - Manual (Page 21)

SDM-CVO4 4-Channel Current/Voltage Output Module
location is used to scale the output to 0-10000mV for voltage mode, or
0-20 mA for current output mode.
In most applications a specific range of a measured value is scaled to utilize the
full scale output of SDM-CVO4. For instance, a temperature sensor reading in
the range of -20
°C to +50°C would be scaled so at -20C the output is 0 mV
and at +50 °C the output is 10000 mV. To do this with the SDM-CVO4 you
would use the mathematical functions of the datalogger to calculate a scaled
value in a new input location, so that the value is -5000 when the temperature
is -20
°C and +5000 at +50°C.
The datalogger limits the output of the SDM-CVO4 so that even if the
measured value exceeds the intended range (i.e. the scaling value exceeds
±5000) the signal output is limited to the equivalent output at -5000 or +5000.
Therefore the datalogger instruction does not normally need to have extra
instructions to limit the scaling value range. However for displays or input
devices which require a current signal of 4-20 mA, the datalogger program
needs to include extra instructions to prevent the scaling value going below
- 3000 (see the examples below) and thereby setting the lowest possible output
to 4 mA.
When checking the output of the SDM-CVO4 against the scaling value you
should be aware that the output changes in discrete steps, e.g. 2.5 mV in
voltage mode. When the datalogger works out which of these discrete steps to
set for a given scaling value, a combination of floating point mathematics
followed by truncation to an integer value is made. This will sometimes result
in the transition from one output step to the next not happening at the exact
midpoint value of each 2.5 mV step, although it should happen within a limit
of ±0.5 in terms of the scaling value stored in the input location.
The number of repetitions, parameter 1, specifies the total number of
SDM-CVO4 output channels to be set. The address of the first SDM-CVO4 is
specified with parameter 2; multiple SDM-CVO4s must have consecutive
addresses. Parameter 3 is the starting input location containing the scaling
value for the first output channel of the first SDM-CVO4. Subsequent scaling
values must be contained in consecutive input locations immediately after the
first input location specified in parameter 3.
For example, two SDM-CVO4s can be used to output eight signals, which are
contained in input locations 15 to 22. There are eight repetitions, and so eight
(8) is entered for parameter 1. The SDM-CVO4s must have consecutive
addresses (e.g. 31 and 32), and so parameter 2 would contain 31 in this case.
Fifteen (15) would be entered for parameter 3.
TABLE 6-2. Description of Instruction 103
Par. Data
Number Type Description
01: 2 Reps — Number of analog outputs
02: 2 Address of SDM-CVO4 in base 4 (00 to 33)
03: 4 Input loc. holds scaling value for the output level
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