All good things come in threes: Why a measurement result alone provides little information

In order to ensure error-free communication within a BUS system, testing with one measuring device alone is usually not sufficient. The operating company of a new conveyor system experienced this, too.

One diagnostic device shows abnormalities, the other one does not

For the acceptance and testing of the PROFINET network, the operating company installed a PROFINET measuring device in the new system. The diagnostic device registers and stores all events in the network and provides information about the use, data throughput, update rates, telegram gaps and telegram jitter. This way, the device provides information about the quality of the system’s communication and flags any functional disorders at an early stage.

Even though the conveyor system ran smoothly, the PROFINET measuring device recorded several alarms within a few seconds (update rate, error telegrams, telegram gaps, jitter). The system manufacturer was called for help on site and analysed the network with another measuring device. However, their measuring setup did not detect any abnormalities – how is this possible?

A question of the right communication parameters

It is not so uncommon to detect significant abnormalities with one instrument and none using the other. This is caused by the fact that the devices measure different communication parameters (such as update rate, error telegrams, telegram gaps and jitter). According to the manufacturer of the PROFINET measuring device, these factors are decisive for the communication of the PROFINET network and a search for the causes of the error is therefore absolutely necessary. However, to make sure which anomalies are really responsible for the communication malfunction, a multi-part check is recommended.

How can I determine which measuring device is the right one for testing my network?

As a rule of thumb, a measurement result is correct if it has been confirmed with three different measurement methods. The different results must then be compared against the background of the measuring arrangement and the weighting of the measured values. Participation in our one-day PROFINET workshops provides information about which factors are important for the acceptance of a network and which measuring instruments are suitable in each case. In the workshop, we cover experiences from troubleshooting, previous concepts for acceptance measurements and gaps in existing standards and regulations. Together, we work out which test criteria make sense from a physical point of view during the acceptance of a PROFINET installation – and which do not.

What are the three measurement methods?

  • Wire Map Measurement
  • return loss
  • system error

Some cable certifiers allow you to change the measurement procedures. But beware, then you have nice measured values, but still no functioning system.

How was the customer’s problem solved?

When the plant operator measured through the TAP (terminal access point) with a cable certifier, a short-circuit in the system was detected at the TAP. After eliminating this problem using our TAP BS 130, no bus crashes and error telegrams were displayed any more, but many data frames got lost, which is an indicator that signals are lost in communication.

What is the reason for this?

Every device forwards the signal to the network with a certain delay. This causes a certain variation in communication cycles. The PLC, which controls the system, expects a signal after three times of an adjustable time constant: If the constant is for instance set to two milliseconds, the PLC expects a refresh of the signal at least every six milliseconds. If this does not happen, the system goes to fault. The next step is to have the line depth adjusted by an expert system builder and then to set the communication parameters accordingly.

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