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Commissioning
5.4.8
Temporal aspects of analog/digital or digital/analog
conversion
Analog output
The following information applies analogously to analog signal output via DAC (digital-to-analog
converter).
The conversion of the constant electrical input signal to a value-discrete digital and machine-readable form
takes place in the analog Beckhoff EL/KL/EP input modules with ADC (analog digital converter). Although
different ADC technologies are common, from the user's point of view they all have one common feature:
after the end of the conversion, a certain digital value is available for further processing in the controller. This
digital value, the so-called analog process data, has a fixed temporal relationship with the "original
parameter", i.e. the electrical input value. Therefore, corresponding temporal characteristic data can be
determined and specified for Beckhoff analogue input devices.
This process involves several functional components, which act more or less strongly in every AI (analog
input) module:
• the electrical input circuit
• the analog/digital conversion
• the digital further processing
• the final provision of the process and diagnostic data for collection at the fieldbus (EtherCAT, K-bus,
etc.)
Fig. 144: Signal processing analog input
Two aspects are crucial from a user perspective:
• "How often do I receive new values?", i.e. a sampling rate in terms of speed with regard to the device/
channel
• What delay does the (whole) AD conversion of the device/channel cause?
So hardware and firmware parts in toto. For technological reasons, the signal characteristics must be
considered to determine this specification: depending on the signal frequency, there may be different
propagation times through the system.
This is the "external" view of the "Beckhoff AI channel" system – internally the signal delay in particular is
composed of different components: hardware, amplifier, conversion itself, data transport and processing.
Internally a higher sampling rate may be used (e.g. in the deltaSigma converters) than is offered "externally"
from the user perspective. From a user perspective of the "Beckhoff AI channel" component this is usually
irrelevant or is specified accordingly, if it is relevant for the function.
For Beckhoff AI devices the following specification parameters for the AI channel are available for the user
from a temporal perspective:
1. Minimum conversion time [ms, µs]
This is the reciprocal value of the maximum sampling rate [Sps, samples per second]:
Specifies how often the analog channel provides a newly detected process data value for collection by the
fieldbus. Whether the fieldbus (EtherCAT, K-bus) fetches the value with the same speed (i.e. synchronous),
or more quickly (if the AI channel operates in slow FreeRun mode) or more slowly (e.g. with oversampling),
is then a question of the fieldbus setting and which modes the AI device supports.
For EtherCAT devices the so-called toggle bit indicates (by toggling) for the diagnostic PDOs when a newly
determined analog value is available.
136
Version: 3.0
EL3681
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