IMC CANSAS - Constraints

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134 Virtual Channels

4.5 Constraints

Virtual channels which aren't used are automatically omitted from calculations. Virtual channels which

are results of certain functions having the reduction factor 1 (i.e., the input channel's sampling rate is the

same as the result's) are automatically treated as an allocation (e.g. Maximum, Minimum, Average).

Certain functions automatically carry out re-scaling, which is necessary whenever differently scaled

channels are jointly subjected to computational operations (Addition, Subtraction, etc.) or to cancel out

an offset (e.g. for SQRT or high-pass filters). The re-scaling changes the factor and the offset. The factor

and offset are indicated on the virtual channel's Message Mapping Properties dialog. Re-scaling and its

effects on the factor and offset are not discussed in the function descriptions.

The re-scaling does, however, produce a loss of precision which must be taken into consideration. If two

input channels with substantially differing scaling are combined in an operation, the discrepancy can

become large indeed.

There are limits on the processing power. In particular, imc CANSAS modules supporting high sampling

rates (e.g. 10 kHz sampling rate for the digital data acquisition module DI16) will tend to exhaust the

available processing resources when performing complex data processing at the highest possible

sampling rate. However, by using data reduction (resampling and thus reduction of the data rate), even

processing which is very taxing for such modules (for instance, filtering) can be carried out at a slower

sampling rate. By contrast, imc CANSAS modules supporting only relatively slow sampling rates (for

instance, the differential amplifier module C12 with a 500 Hz sampling rate) can indeed perform 4th

order high-pass filtering on all input channels at the top sampling rate.

The number of virtual channels available is limited to a total of 100. The available channels depend on

the module, because a certain number of virtual channels are already used internally. For example to

calculate the temperature of a thermocouple, the cold junction compensation has to be calculated.

Therefore an internal channel has to be used, which is not visible to the user.

If one module has both messages configured with slow data rates and with high rates, it may occur that

the rapid messages are delayed in being transmitted. This happens whenever several messages having a

slow rate are transmitted in rapid succession, which of course takes up much time. This time amount can

also be (significantly) greater than the fastest data rate. The corresponding messages can contain virtual

or physical channels. The problem usually arises whenever very many channels or low Baud rates are set.

One possible remedy might be a higher Baud rate.

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