Beckhoff EPP3504-0023 Short Manual page 99

Some notes and restrictions
• This principle can be implemented with two (as described above) or more terminals; the limit is the shift
time fineness of 1 µs.
• The terminals used must support Distributed Clocks. Oversampling is helpful, but not necessary. The
sampling methods simultaneous vs. multiplex must be considered; see corresponding documentation
with the question: "when the channels sample their values in relation to Distributed Clocks".
• Although this approach doubles the sampling rate of the signal under observation, the frequency
response and attenuation specified in the technical data for the terminal still apply! It is therefore not
possible to read signals that are twice as fast with twice the sampling rate. Sample: the EL3751 with a
sampling rate of 10 ksps can meaningfully (alias-free) read signals up to half the sampling rate =
5 kHz. This limit remains even with multiple parallel sampling! The attenuation of -3 dB at 3 kHz given
as an example also applies to the interlaced sum signal.
• Only one EtherCAT terminal can be functionally time-shifted as a whole by Distributed Clocks shift
time, not the individual channel of a terminal. The shift then affects all the channels of a terminal.
Therefore, for the given principle, two or more terminals/box modules must always be used; the
interlacing of two channels of the same terminal/box is not possible.
• The specified measurement uncertainty must be observed: the unavoidably different real measurement
uncertainty and thus the amplitude difference between the two terminals or their channels used on the
same signal can become visible as a noise component after interlacing. Therefore, terminals should be
used for this principle that exhibit a much smaller measurement uncertainty than is necessary for the
application. It is expressly recommended to carry out an explicit user calibration of at least the offset of
the two electrically interconnected channels in order to minimize this effect.
• Terminals with the same HW/FW version should be used.
Sample program
This setting, like the base time and the task cycle time, is already configured in the sample program:
Download TwinCAT 3 project / sample program 6a: https://infosys.beckhoff.com/content/1033/epp3504/
Resources/4867888523/.zip
In the following section, the simplest form of input value interlacing in Structured Text is initially shown with
oversampling = 1 for each input value: each of two elements of a field variable receives a value from a
terminal. The variable can be used for further processing and is shown here in the TwinCAT ScopeView. In
the EL3751 the programming instructions are assigned to a 100 µs task:
Variable declaration sample program 6a
PROGRAM MAIN
VAR
nSamples_1 AT%I* :DINT; // EL3751 input with no added shift time
nSamples_2 AT%I* :DINT; // EL3751 input with -50 µs added shift time
aCollectedResult :ARRAY[0..1] OF DINT;
END_VAR
Execution part:
// Example program 6a:
// 100 µs task
// ============================================================
aCollectedResult[0] := nSamples_1; // Put 1st Value of sequence into array
// Pattern: 1.1.1.1...
aCollectedResult[1] := nSamples_2; // Put n-th Value of sequence into array (2nd here)
// Pattern: .2.2.2.2...
// ============================================================
// Result pattern: 12121212... (--> see scope view dots)
For an input signal with sine 5 kHz and 2.5 V amplitude, for example, the TwinCAT ScopeView provides the
following results:
EPP3504-0023 Version: 1.2
Commissioning
99