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What can I do with ProfiTrace 2?

18 May 2016

An analyser is an essential tool for fault-finding, monitoring and health-checking networks. It allows you to see the actual telegrams transmitted from device to device. Analysers for PROFIBUS are available from several manufacturers. However they vary enormously in capability, ease of use and price.

A basic PROFIBUS analyser should have the following features: 

You will see that ProfiTrace 2 is much more than a basic PROFIBUS analyser and contains many useful features that are not provided on other analysers: 

Now we will explore some examples of network problems that can be diagnosed and located with ProfiTrace Ultra. 

Communication faults

When we use ProfiTrace 2 to a working network, we do not need a spur line, this is because the hardware for ProfiTrace 2 (called ProfiCore) is a self-contained box of electronics that communicates with ProfiTrace 2 (running on your laptop) via a high-speed USB cable. Further the box, which fits in your pocket, takes its power from the laptop battery via the USB cable.  

ProfiCore Ultra
ProfiCore Ultra

Normally we use a small “tap connector”, which is actually a 100mm stub line to connect ProfiCore to the network. This may sound strange, but actually is very useful as we can see the very small reflection caused by the tap connector and with this make cable length measurements (see example below).  

ProfiCore Ultra connected
ProfiCore Ultra connected

 

 

 

 

 

 

 

 

OK, we can now run ProfiTrace 2 software on our laptop, but what should we look at? The staring point is usually to examine the “Live list”, which is an overview of the network showing the devices connected and their state: 

Diagnose part 1

Slaves and masters can be differentiated on the live list by the font colour. Don’t think that devices that flash between red and blue font are faulty; it just means that they are dual master/slave devices. Some PLCs and HMI devices behave that way.

Slave devices shown with a red background on the live list indicate a “parameterisation error”. This is often caused by the wrong device (GSD file) being configured in the controlling master. ProfiTrace 2 makes the diagnosis even simpler because the live list shows us the actual device identification (ID number) and model name together with the configured ID number and model name. Easy! 

PROFIBUS devices not only have diagnostics to tell us about communications faults, but also many devices also have “extended diagnostics” to tell us the details of peripheral errors. 

What is a peripheral error?

It is an error to do with the sensor, actuator or IO wiring. ProfiTrace 2 indicates extended diagnostics in the live list with a small flashing red indicator: 

Diagnose part 2

We can also capture the diagnostic telegram from the device using the extensive triggering capability of ProfiTrace 2:

Diagnose part 3
ProfiTrace 2 interprets device related diagnostics and also module and channel delated diagnostics

Reflections

Reflections are the number-one problem on high-speed networks such as PROFIBUS. Reflections can be caused by any electrical discontinuity along the PROFIBUS cable, such as stub line or the end of the cable. PROFIBUS uses termination networks to stop the reflection at the end of the line, but many things can go wrong! We teach what can go wrong on our one-day the Certified PROFIBUS Installer course.

Can ProfiTrace 2 help us find this type of fault? – you bet. The built-in high-speed oscilloscope can be easily triggered to show the waveform from each device connected to the cable (segment). We can see weak devices, corruption caused by double addressing and yes, reflections. 

Diagnose part 5
Reflections can be easily seen on the ProfiTrace oscilloscope (ScopeWare)

Not only can we identify the reflection but using a simple technique, we can quickly measure the distance from any device on the segment to the source of the reflection, so rapidly homing in on the problem. Without ProfiTrace 2, the location of the reflection is really difficult. Why? Because the devices that are most affected by the reflection are often those that are furthest away. Very confusing! With ProfiTrace 2 and a little training, you can learn to locate such faults along the cable to within +/- 1m. 

Diagnose part 6
Location of bent cable using the oscilloscope trace

Cable length measurement

We saw earlier that we normally connect ProfiTrace to our network via a small tap connector. This is actually a small stub line, which causes a small reflection. However, the reflection is of very small amplitude and will certainly not cause problems when running at less than 12Mbit/s. The reflection from this stub line can be useful for measuring the cable length to each device on the segment. Now, that is a bonus!

The technique is based on spotting the small reflection from the tap connector as we trigger the oscilloscope from each slave on the segment. 

Some Commissioning Engineers think that their job is done when they have no more “red lights”. Although the absence of a red Bus Fault (BF) shows that the slave is in data exchange with the master, we really do not know the quality of the communications. It is quite possible (and also quite common) that slaves have problems, even with no Bus Fault light showing. This is because PROFIBUS is very robust and keeps going in the event of occasional errors. The master will just repeat a request should no response be received within the required time. Only when the “retry limit” is exceeded, does the master give up and indicate that a fault has occurred. Even then the master will attempt to re-establish communications with the missing slave on the next bus cycle, and it may be successful, in which case we still do not see a red BF on the slave!

So you can see that we need to check that the communication is reliable and we do not have errors on the bus. ProfiTrace 2 to the rescue! ProfiTrace 2 has a large number of statistical measures that can show whether there are any faults on the bus and even rare errors will be caught in the net. The most important of the ProfiTrace 2 statistics include:

ProfiTrace 2 statistics work in the background, even when making other measurements. So, you can go back to check the statistics after say 10 minutes of checking other bus characteristics such as waveforms etc. The statistics page tells us whether any faults have been recorder since we initialised or reset ProfiTrace 2.

Diagnose part 7

 

Diagnose part 8
ProfiTrace 2 statistics

Health checking should also involve looking at the quality of the waveform transmitted by each device on the bus. This can be easily done using the triggered oscilloscope function in ProfiTrace 2. However, we must be aware that the signal we see on the screen is the signal at the point where ProfiCore is connected. Devices on the other side of a repeater will show a waveform on the oscilloscope, but this is actually the signal transmitted by the repeater and not the original signal transmitted by the device.

It is quite possible that the signal on the other side of a repeater is of poor quality due perhaps to interference or reflections, but the signal we see in the screen is good because the original signal is cleaned up by the repeater.

So, in summary, we should carry out a check of the waveforms on every segment (i.e. up to the repeater or optical link module (OLM). We should check the amplitude and quality of the waveform of every device on the segment. We should also check the repeater/OLM waveforms by triggering on a device which is on its far side. This should be done in addition to the statistics check described above. Note that our Certified PROFIBUS Installer course teaches the correct way to layout and install PROFIBUS networks to allow easy health checking to be carried out. 

PA segments

ProfiCore Ultra has an RS485 interface built in for direct connection to DP segments. So how can you do a health check about PA? The answer is to use the PA Probe Ultra. This device, which is part of the ProfiTrace pro kit, provides a transparent interface to the Manchester Bus Powered (MBP) signal that is used on PA segments and devices. The PA Probe Ultra not only allows us to read the telegrams and statistics directly from the PA cable, but also provides clever electronics to allow us to use the built-in ProfiTrace 2 oscilloscope to see the actual PA signal: 

Diagnose part 9

So now you can have a tool that can be used to do a thorough health check on the whole PROFIBUS system, including all DP and PA segments. Our PROFIBUS Commissioning and Maintenance course and our Certified PROFIBUS Engineer course both cover the details and practice of proper health checking on DP and PA systems.

Source: Verwer Training & Consultancy Ltd

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