QUESTION:
What options to I have for diagnosing errors in the DP master system during the commissioning phase? 

ANSWER:
Errors can occur on the PROFIBUS DP network for the following reasons:

• Bad wiring / installation of the bus cable
• Incorrect configuration of the DP masters and the DP slave 
• Incorrect configuration of the associated PROFIBUS DP network
• Incorrect addressing of the communication in the program of the CPU

In the following we will present the diagnostics tools available for the various error sources.

Bad Wiring/Installation of the Bus Cable

BT200 BUS TESTER

The BT200 Bus Tester is available for checking the wiring. The following tests can be made with the BT200 Bus Tester when the PROFIBUS cable is not in use:

Checking the PROFIBUS cable:

• Wire breakage
• Shield breakage
• Short circuits between wires and between wire and shield 
• Location of breakage/short circuit of data cables 
• Recognition of reflections that are causing faults 
• Incorrect wiring 
• Length of wiring installed 

You can also use the BT200 Bus Tester for diagnostics when the network is in use:

Checking the availability of slaves:

• List of reachable slaves 
• Selective addressing of separate slaves 

Checking the RS 485 interface of the master and slave:

• RS 485 driver 
• Power supply for line termination 
• RTS signal 
• Display of the PROFIBUS DP address

SIGNAL LEVEL MEASURING WITH AN OSCILLOSCOPE

Using an oscilloscope you can display the signal level on the PROFIBUS. This should have a rectangular form. A specialist should be employed to evaluate the image.

Configuration of the DP Master and the DP Slave

To check the configuration of the DP master and the DP slaves connected to it you can monitor it in the HW Configuration both offline and online. The configuration can be downloaded from the CPU for offline analysis.

If you analyze the configuration online in the HW Configuration, you can identify failed or defective modules. In the case of modules that support diagnostics it is also possible to detect imminent channel faults (e.g. wire breakage).

Fig. 1: Online view of the HW Configuration

If modules fail, then the event is logged in the diagnostics buffer.

Fig. 2: Diagnostics buffer of a CPU with slave failure

PROFIBUS DP Network

Some PC interfaces for PROFIBUS provide you with diagnostics. You can find them via "Set PG/PC Interface" or "Set PC Station".

Here you can read out the actual bus parameters and you are presented with an overview of the PROFIBUS nodes available.

Fig. 3: SIMATIC Net diagnostics

AMPROLYZER

The Amprolyzer software is also available for analyzing the PROFIBUS network with the following functions.

• Lifelist with all the PROFIBUS nodes on the bus 
• Overview diagnostics with the current operating states of the nodes 
• Bus statistics with the number of events such as timeouts and message repetitions 
• Automatic detection of transfer rate 
• Message recording with trigger and filter options with respect to events and message content including time stamp 
• Storage and export of message records in Excel format

Information on the Amprolyzer product is available in Entry ID: 18818699.

Program of the CPU

A frequent source of error is also the STEP 7 program designated for data communication. Incorrect I/O access (e.g. I/O access via T AW20) to nonexistent addresses can be easily detected via the diagnostics buffer of the CPU. 
If system functions (e.g. SFC14/15) are used for data communication, then the success of data transfer can be determined via their return values. You can observe the return values via the variables table or the online view of the LAD/FBD/STL editor.

When using certain modules, extraordinary programming of accesses might be necessary. When using CP342-5, for example, it is not possible to access the I/O via load/transfer commands or system functions SFC14/15. Here you have to use special functions (FC1 "DP_SEND" and  FC2 "DP_RECV"). But also in this case you can determine success of data transfer from the return values of these functions via the variables table or the online view of the LAD/FBD/STL editor.

Note:
Users of S7-SCL or CFC also have the option of online monitoring of return values.

More information on the procedure for troubleshooting is available in Entry ID: 19379085.