User Manual PROFview XL 3 Description of Functions and Measuring Principle
For this measurement to function correctly, it is merely necessary to set the baud rate of the PROFIBUS seg-
ment via the PBT application software and to connect the PBT to the plant. Subsequently, the device will de-
termine the address of a master automatically
2
and starts measuring the bus cycle time. Each time the token
is passed on, the time measurement is stopped and restarted at the same time. The resolution of the time
measurement is approx. 100 µs. The longest bus cycle time which can be detected is one second.
3.3.3 Illegal frames
The counter for illegal frames is - like the bus cycle time - a real-time monitoring function. The background to
this measurement is to be able to perform a protocol check of all frames transferred on the bus
3
. If a logic er-
ror is found in a frame, this counter is incremented by one.
As a rule, the counter of this display should always show "0". If, on the other hand, errors occur occasionally
or even frequently, this is due less to non-conforming PROFIBUS circuits of the PROFIBUS stations, but
rather to problems with the physical bus characteristics and the resulting transmission errors. But station ad-
dresses assigned twice (duplicate addressing) can also cause errors of this kind.
The illegal frame counter of the PBT counts max. 50,000 illegal frames. A frame is detected as illegal if at
least one of the following protocol errors has occurred:
unknown start delimiter (no SD1, SD2, SD3, SD4 or short acknowledgment)
incorrect checksum (FCS)
start, parity or stop-bit error
invalid end delimiter
invalid repeated SD2 (in SD2 frames only)
invalid length information (LE unequal to LEr; in SD2 frames only)
The error detection will only function correctly if the correct baud rate has been set for the PROFIBUS seg-
ments via the PBT application software. If this is not the case, the frame detection will not function correctly.
Consequently, any data transfer on the bus will increment the illegal frame counter.
Note:
Repeated or diagnostic frames according to the definition above are not registered by the illegal frame
counter.
3.4 Help functions
3.4.1 Master simulator
The PBT can now also perform measurements on PROFIBUS plants which are still in the phase of construc-
tion or start-up, even if the actual master station is not yet ready for operation. This function can be especially
useful if at first the correct realization of the bus cabling and of the installation of the individual stations is to
be checked.
2 If you have connected the PBT to a PROFIBUS plant with several master stations, the PBT will use the master address first detect-
ed on the bus for measuring the bus cycle time.
3 The PBT detects a frame as illegal if it does not comply with the protocol definition to IEC 61158-3. These illegal frames need not
necessarily result in frame repetitions by the master station, since the main causes of such errors are to be found predominantly in
the physical bus characteristics (e.g. signal reflections). Therefore, the signal waveform and the value of the illegal frame counter
can differ from measuring point to measuring point.
PROFview XL 9
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