Sampling Type: Simultaneous Vs. Multiplexed - Beckhoff EL3681 Documentation

5.4.10

Sampling type: Simultaneous vs. multiplexed

Analog inputs and outputs in Beckhoff devices can operate in two different ways in terms of time:
"simultaneous sampling" or "multiplex sampling". This so-called sampling type has a decisive influence on
the performance of such a device and must be taken into consideration when selecting a product, at least
when it comes to very complex timing control tasks. Whether an analog device operates simultaneously or
multiplexed can be taken from the respective device documentation.
This question is relevant for control tasks as well as for measurement tasks (DataRecording), if the timing of
the analog value acquisition is sensitive.
Note: The terms "simultaneous" and "multiplex" have been used for a long time and in many contexts, so
they have different meanings depending on the historical background and the subject area. In this chapter
and in relation to I/O, the terms are used as Beckhoff understands them as an I/O manufacturer for the
benefit of the user:
• If a test signal is applied electrically to all channels of a multi-channel device at the same time and the
measurements are evaluated in software, e.g. in TwinCAT Scope, and if no significant offset/delay can
be observed between the channels, then it is a simultaneously sampling device
• If an offset can be seen, it is a multiplex sampling device
• The easiest test to perform is with a square wave signal because an offset can then be easily
observed. However, the rare special case could occur (especially if the test signal is generated from an
EL2xxx/EL4xxx from the same IO line) that the square wave signal runs synchronously to the
EtherCAT for several minutes and then no offset can be seen.
Absolutely safe is a test with a sinusoidal signal, but then it must be considered that measurement deviations
(related to the amplitude) of the channels in the device are also represented as time offset!
Ideally, one should concentrate on the zero crossing.
• 1-channel devices are considered as simultaneous sampling by definition.
Explanation with the example "analog input": if a continuous analog signal is to be digitized and thus fed to
the further programmatic processing, it is digitized by a so-called ADC (AnalogDigitalConverter), e.g. with 16
bit resolution:
Fig. 147: Schematic representation of sampling with ADC converter
This represents an analog input channel that is functional in itself. It samples (measures) as often as desired,
e.g. 1,000 times per second, and thus sends 1,000 measured values equidistant in time (= at equal time
intervals) for further processing.
Often several channels are combined in one device, in this case the question arises about the sampling type:
simultaneous or multiplex.
1
) For experts: such a device could also be equipped with a multiplexing ADC, which works with sample-and-
hold on all channels. Then technically multiplex is built in, but from the outside the device works
simultaneously, because all channels are electrically read in at the same time.
Simultaneous
As in the 1-channel example, each channel can have its own ADC, shown here for 4 channels:
EL3681 Version: 3.0
Commissioning
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