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S7-300 CPU 31x -- Operation and maintenance -- Diagnosing errors
With diagnostics-compatible modules how do you evaluate a change of the diagnostics status in the user program of the S7-300?
When the CPU starts up how can you identify a previous stop event and process it in the program (even after an overall reset)?
Which organization blocks do you have to load into the CPU to ensure that the CPU does not go into the "STOP" status when a remote I/O fails?
What should you do upon display of the error message 33:8580 "MMC defective"
With MMC (Micro Memory Card) slotted, why does the STOP LED blink on the SIMATIC S7-300 CPU?
Which organization blocks do you need for error handling in STEP 7 V5.5?
How do you find out the status of the stations of a DP master system or of a PROFINET IO system?
Why are devices not correctly displayed in the topology view of the web server?
Capturing module and diagnostics data of the automation system
CPU switches to STOP when a PG/PC goes online via PROFIBUS
How do you get an overview of the SIMATIC automation system and what solutions are there for undesired operating states?
What do you have to do if the SIMATIC S7-300 CPU no longer goes into RUN mode after adding a CP or FM module?
Why does the EXTF LED light continuously on a CPU 300 (V1.0.0)/CPU 400 (V1.1.0) after occurrence of a diagnostics alarm?
Which information is entered in the diagnostic buffer of the SIMATIC S7 CPU in STEP 7 V5.x?
Memory reset request by the CPU
Why does the "SF DP" LED light on the CPU 315-2 DP (6ES7 315-2AF0x-0AB0; x = 0,1,2) although no DP configuration has been loaded into the CPU?
Why does the CPU remain in STOP after voltage recovery following complete loss of voltage for the entire system?
Why does the LED BF2 light with PROFINET CPUs S7-315/317-2PN/DP V2.3 even though no bus fault can be determined?
Why does my SIMATIC S7-300 CPU go into STOP with error 16#4548 "Stop by I/O administration"?
How can you avoid diagnostics problems when implementing dummy module DM 370?
Which information is entered in the diagnostic buffer of the SIMATIC S7 CPU in STEP 7 V5.x?
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System diagnostics is for recognizing, evaluating and displaying errors that occur in an automation system. For this there is a diagnostic buffer in each CPU and module that supports system diagnostics, in which detailed information on all the diagnostic events is entered.

With system diagnostics errors

  • are given a system-wide uniform number (cause)
  • with location and time of occurrence

by the operating system of the module and displayed as plain text in the medium shown. In addition, the previous history of an error is registered, because this error message is stored automatically in a diagnostic buffer without the user doing anything.

The basic functions of system diagnostics include all error events of the operating system and particularities of the run of the user program, which are stored in the diagnostic buffer with location, time, error number and additional relevant information.

In addition, the user can use the SFC52 WR_USMSG to enter user-defined diagnostic events (information about the run of the user program, for example) in the diagnostic buffer or send user-defined diagnostics messages to registered nodes (monitoring devices like PG, OP and TD, for example).

Diagnostic buffer
The diagnostic buffer provides the user with the following options:

  • Rapid detection of error causes and therefore increased plant availability.
  • In the case of plant stoppage, evaluation of the events just prior to the STOP to find the cause of the STOP.

The diagnostic buffer is a ring buffer with separate diagnostics entries. In the diagnostic buffer, the diagnostic events are displayed in the order of their occurrence. The first entry contains the latest event. When the buffer is full, the oldest entry is overwritten with each new diagnostic buffer entry. Depending on the CPU, the size of the diagnostic buffer is either fixed or can be set with parameters in the hardware configuration.

Items entered in the diagnostic buffer

  • Error events
  • Mode changes and other operating events important for the user
  • User-defined diagnostic events (with SFC52 WR_USMSG)

In STOP as few as possible events are entered in the diagnostic buffer so that when the plant stops, the user can find the STOP cause easily in the buffer. There, the operating system ensures that events are entered in the diagnostic buffer only if the event requires a user reaction (for example if the system requests an overall reset or a battery has to be changed) or important information has to be registered (firmware update executed or station failure, for example).

No user programs are processed in the STOP mode. Therefore, in STOP mode, no diagnostic buffer entries triggered by the user program are entered. 

Items NOT entered in the diagnostic buffer

  • Temporary error events
  • Statistics information or traces
  • Information about the quality of data or services
  • Cyclic OB start calls

Error events that occur cyclically are usually only entered the first time they occur and only again when the event of the outgoing error is recognized. This ensures that important entries are not so easily overwritten in the case of diagnostic buffer overflow. Using the Online Help the user can recognize the possible causes and appropriate remedies for each event from the diagnostic buffer. 

Practical evaluation of the diagnostic buffer
Typically you use diagnostic tools like the S7 System Diagnostics to evaluate the diagnostic buffer entries. You can use the user program to read out the diagnostic buffer, but compared with the process in the user program it is not adequate to derive from it control reactions in a polling process.

Increasing the diagnostic buffer to maximum number of entries in RUN mode
The maximum number of entries in the diagnostic buffer depends on the CPU type. In the diagnostic buffer you click the "Settings" button and make the following changes:

  1. Change the default setting from "Default set according to module property" to "Preselect".
  2. In the input field you enter the maximum number of entries (500, for example).
  3. Enable the "Save settings for this dialog box" option.
  4. Save the settings with OK.

Fig. 01

If you then click the "Update" button in the diagnostics buffer, the maximum number of entries are shown in the display. Using the "Save as..." function you can save the contents of the diagnostic buffer as ASCII text (in a file named "Diagnostic.txt", for example).

 Entry ID:14960968   Date:2013-04-18 
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