S7-1200 and S7-1500 Error OBs to Prevent CPU STOP: Reference

1. Overview

On SIMATIC S7-1200 and S7-1500 controllers, an unhandled fault at runtime typically forces the CPU into STOP. The traditional defense is to load error organization blocks (OBs) whose presence alone tells the operating system that the programmer has acknowledged the fault class. This reference catalogs the OBs available on S7-1200 and S7-1500 firmware, explains how TIA Portal handles them symbolically, and walks through a sample project in which two S7-1200 CPUs exchange data over Industrial Ethernet while every relevant error OB is loaded so that the CPU cannot drop out of RUN for a recoverable fault.

Engineers migrating from STEP 7 classic (S7-300/400) will notice two major differences:

• OBs are referenced symbolically (by name) inside TIA Portal; the underlying numeric identifier is generated by the system.
• Several OBs can coexist with the same number but different priorities (OB1 is the exception; it is single instance).

2. STOP Conditions and Why OBs Matter

An S7-1200/1500 CPU transitions from RUN to STOP when the firmware detects a class of event for which no matching OB is loaded. The diagnostic buffer records the cause; the user sees only a flashing ERROR LED and a SF fault LED on the CPU. The principal fault classes are:

If the corresponding OB exists in the program, the CPU logs the fault and continues. If the OB is missing, the CPU stops. Note that OB 85 in S7-1200 can be selectively suppressed by setting "Report of OB85 to PG/PC" in the device configuration; in S7-1500 the behavior is hardware-firmware controlled.

3. S7-300/400 vs S7-1200/1500 OB Architecture

For the engineer migrating a STEP 7 classic project, the only practical change is that the OB appears in TIA Portal as a block with a generated icon and an editable name; the firmware still treats OB80 as OB80. You do not need to renumber anything, but you must explicitly insert the block: an empty program does not include any error OB by default.

4. Available Error OBs in S7-1200 and S7-1500

The list below covers the OBs that affect RUN-to-STOP transitions. "Available" reflects firmware support as published in the Siemens function manuals. Verify against the SIMATIC S7-1200 Programmable Controller System Manual and the S7-1500 System Manual.

5. Adding OBs in TIA Portal

1. Open the project in TIA Portal V17 or later (V18/V19 are recommended for the latest S7-1500 firmware compatibility).
2. Expand the CPU in the project tree: PLC_1 > Program blocks.
3. Double-click Add new block. The dialog shows an icon group "Organization block".
4. Select the desired OB. The dialog filters to OBs that are valid for the CPU type and firmware.
5. Set the symbolic name (e.g., OB_TimeFault), the programming language (LAD, FBD, SCL, or GRAPH for some OBs), and the priority (defaults are sensible).
6. Click OK. TIA Portal inserts the OB with a generated start information block TEMP structure.
7. Implement the fault response inside the OB. For an OB that simply acknowledges the fault and logs the cause, a few lines of SCL are sufficient.

6. Symbolic vs Numeric OB Handling

Inside any FB or FC on S7-1200/1500, you cannot call an OB. OBs are activated only by the operating system. The symbolic name in TIA Portal is shown in:

• Program blocks tree
• Watch tables (as a row indicating the OB number and last execution timestamp)
• Diagnostic buffer entries (mapped to the symbolic name in TIA Portal V16+)
• Cross-reference (used like any other block symbol)

For PLC-to-PLC communication the receiving CPU must know the OB numbers only if it triggers them via RT_INFO, RD_SINFO, or similar system calls. The classic "scan the OB1 on the partner" pattern is replaced by the cleaner approach of reading the partner's tag values directly.

7. Programming Error OBs - Code Examples

The following SCL snippets illustrate the minimum useful body for each error OB. They are intended to be added to an S7-1200/S7-1500 project to suppress the STOP transition and to retain diagnostic information.

OB 80 (Cyclic time fault)

// OB_TimeFault (OB 80) - SCL
#OB80_EV_CLASS    := EV_CLASS;
#tempFaultTime    := OB_DATE_TIME;
#tempPriority     := OB_PRIORITY;
#tempOBNumber     := OB_OB_NUMBER;

// Increment a non-retain counter for trending
"dbTimeFaults".cntTotalFaults := "dbTimeFaults".cntTotalFaults + 1;
"dbTimeFaults".lastFaultAt    := #tempFaultTime;

// Optional: trigger a graceful degradation
IF "dbTimeFaults".cntTotalFaults > 10 THEN
    "alarmBuffer".consecutiveFaults := TRUE;
END_IF;

OB 82 (Diagnostic interrupt)

// OB_DiagInt (OB 82) - SCL
#tempIOState      := LADDR_STATE;  // 0 = OK, 1 = fault cleared, 2 = fault present
#tempChannelFault := IO_STATE;

IF #tempIOState = 2 THEN
    "dbDiag".pendingFaults    := TRUE;
    "dbDiag".faultedIOAddress  := LADDR;
    "dbDiag".faultDetectedAt   := OB_DATE_TIME;
ELSE
    "dbDiag".pendingFaults := FALSE;
END_IF;

OB 86 (Rack/station failure - PROFINET)

// OB_RackFault (OB 86) - SCL
IF EVENT_CLASS = 16#38 THEN // fault incoming
    "dbPN".stationFailed[STATION_ID] := TRUE;
ELSIF EVENT_CLASS = 16#39 THEN // fault outgoing
    "dbPN".stationFailed[STATION_ID] := FALSE;
END_IF;

OB 121 (Programming error)

// OB_ProgErr (OB 121) - SCL
// OB121_INFO_1 contains the block type that faulted
// OB121_INFO_2 contains the relative address
"dbProgErr".blkType      := B#16#0; // fill from BLK_TYPE if needed
"dbProgErr".faultAddress := INT_TO_WORD(OB121_MEM_ADDRESS);
"dbProgErr".faultAt      := OB_DATE_TIME;

OB 122 (I/O access error)

// OB_IOAccErr (OB 122) - SCL
"dbIOErr".laddr   := OB122_IO_ADDR;
"dbIOErr".blkNum  := OB122_BLK_NUM;
"dbIOErr".cause   := OB122_IO_LEN;   // zero-length access = slot empty

8. S7-1200 to S7-1200 Industrial Ethernet Sample Project

The following minimal project demonstrates two S7-1200 CPUs (firmware V4.5+) exchanging data over Industrial Ethernet (PROFINET) while running with all relevant error OBs loaded. The architecture is summarized below.

8.1 Configuration

1. Insert two S7-1200 CPUs in the TIA Portal project. Use the same firmware major version (e.g., both V4.5).
2. In Devices & Networks, connect the PROFINET interfaces of both CPUs. TIA Portal will create a PROFINET subnet automatically.
3. Assign fixed IP addresses: PLC_1 = 192.168.0.10, PLC_2 = 192.168.0.11, subnet mask 255.255.255.0.
4. Ensure the S7 connection is permitted. On S7-1200 V4.x the Permit access with PUT/GET option must be enabled in the CPU's protection properties, unless the connection is configured explicitly inside the project (TSEND_C / TRCV_C).

8.2 Data Exchange Block

// FB_Exchange (SCL) - call from OB1 on both PLCs
IF "dbConfig".partnerIP = '192.168.0.10' THEN
    // PLC_2 sends, PLC_1 receives
    iStatus := TRCV_C(
        EN        := TRUE,
        ID        := 1,
        LEN       := 20,
        DATA      := "dbShared".rxBuffer,
        CONNECT   := "dbConfig".cnRcv,
        COM_RST   := FALSE,
        DONE      => "dbShared".rxDone,
        BUSY      => "dbShared".rxBusy,
        ERROR     => "dbShared".rxError,
        STATUS    => "dbShared".rxStatus);
ELSE
    iStatus := TSEND_C(
        EN        := TRUE,
        ID        := 1,
        LEN       := 20,
        DATA      := "dbShared".txBuffer,
        CONNECT   := "dbConfig".cnSnd,
        COM_RST   := FALSE,
        DONE      => "dbShared".txDone,
        BUSY      => "dbShared".txBusy,
        ERROR     => "dbShared".txError,
        STATUS    => "dbShared".txStatus);
END_IF;

8.3 OB Coverage Checklist

9. Startup OBs (OB 100/101/102) Availability

A frequent confusion with engineers coming from S7-300/400: OB 100 is available on S7-1500 only. The S7-1200 does not support any of the classic startup OBs. On S7-1200, all startup-specific initialisation belongs in OB 1's first scan, which is detected with bit OB1_FIRST_SCAN (true on the first cycle after a STOP-to-RUN transition). On S7-1500, OB 100 runs before OB 1 on every restart and may initialise non-retentive tag values.

10. Verification and Diagnostics

After downloading the project to both CPUs, perform the following acceptance test:

1. Bring online. Right-click each CPU and select Go online. TIA Portal should report RUN for both, with the "Operating mode" diagnostic indicating "RUN, no faults".
2. Force the error class. Disconnect the PROFINET cable between the two CPUs while both are in RUN. The diagnostic buffer of each CPU should now contain entries "OB 86 Rack/station failure" with priority 16#1A. Because OB 86 is loaded, the CPUs remain in RUN.
3. Check the OB86 start information. In the project tree, expand the partner CPU, open the online block OB_RackFault, and use Monitor / Modify to read STATION_ID and LADDR. The values identify which PROFINET device dropped.
4. Recover. Reconnect the PROFINET cable. OB 86 will fire again (event class 16#39 - fault outgoing) and the watchdog bits in dbPN will clear.
5. Force OB 121. From OB 1, execute a temporary statement that reads DB_ANY(99) while DB 99 does not exist. The CPU will call OB 121 instead of going to STOP.
6. Inspect via web server. Open https://192.168.0.10 in a browser (S7-1200 firmware 4.4+) and navigate to Diagnostics > Diagnostic buffer. Each forced fault appears with timestamp, event ID, and OB number.

11. Troubleshooting Matrix

12. Frequently Asked Questions