S7-300 vs S7-1500 OB Priority Number Comparison Guide

Overview: OB Priority Concept Across SIMATIC Families

Organization blocks (OBs) are the program execution entry points in every Siemens SIMATIC controller. Whether the CPU is an S7-300, S7-400, S7-1200, or S7-1500, the operating system uses the OB priority class to arbitrate between blocks that become ready at the same time. When an OB is triggered (main scan, hardware interrupt, time-of-day, cyclic interrupt, time delay, error event, or startup), the operating system compares the new OB's priority with the priority of the OB currently active on the CPU. If the new priority is strictly higher, the current OB is suspended, its state is saved, and the higher-priority OB starts. Once the higher-priority OB completes, control returns to the suspended block at the point of interruption. If the new priority is equal or lower, the new OB is queued (or, in the case of cyclic interrupts, the event is dropped once the queue is full).

The rule is identical across all modern SIMATIC families: higher priority number equals higher priority. The official S7-1500 Comparison List for S7-300, S7-400, S7-1200, and S7-1500 from Siemens AG documents the common priority scheme, while the S7-1500 Cycle and Reaction Times Function Manual (Siemens entry ID 59193558) details the execution rules and configurability for the S7-1500 firmware. Three timing characteristics depend on OB priority: interrupt latency, preemption depth, and queue depth.

Quick Answer: Priority Direction Comparison

For the question "Does a higher OB number mean a higher priority?" the answer is the same for both S7-300 and S7-1500: yes. The two families differ in one important way: on the S7-300, the priority is encoded in firmware and cannot be changed; on the S7-1500, most OB priorities can be edited in TIA Portal hardware configuration.

S7-300/400 Fixed Priority Model

On the S7-300, every OB has a priority value that is hard-coded in the CPU firmware. STEP 7 V5.x does not expose a priority editor. The relationship between OB number and priority follows a strict pattern documented in the S7-300 CPU manuals:

• OB1 (main cyclic program): priority 1
• OB10-OB17 (time-of-day interrupts): priority 2 (each OB)
• OB20 (time-delay interrupt 0): priority 3
• OB21-OB23 (time-delay interrupts 1-3): priority 4-6
• OB30 (cyclic interrupt 0): priority 7
• OB31-OB38 (cyclic interrupts 1-8): priority 8-15
• OB40 (hardware interrupt 0): priority 16
• OB41-OB47 (hardware interrupts 1-7): priority 17-23
• OB55 (status interrupt, DP): priority 2
• OB56 (update interrupt, DP): priority 2
• OB57 (profile-specific interrupt, DP): priority 2
• OB60 (multicomputing / SFC 35 call): priority 9
• OB70-OB72 (I/O redundancy error): S7-400 only, priority 25
• OB80 (time error): priority 26 (fixed)
• OB81 (power supply error): priority 26
• OB82 (diagnostic interrupt): priority 26
• OB83 (module removal/insertion): priority 26
• OB84 (CPU hardware fault): priority 26
• OB85 (program sequence error): priority 26
• OB86 (rack/station failure): priority 26
• OB87 (communication error): priority 26
• OB90 (background OB): priority 29
• OB100 (warm restart): priority 27
• OB101 (hot restart): priority 27 (S7-400 only)
• OB102 (cold restart): priority 27
• OB121 (programming error): inherits the priority of the active OB
• OB122 (I/O access error): inherits the priority of the active OB

An S7-300 user who needs a different priority for a cyclic interrupt has only one option: select a different OB number. If the application requires a cyclic interrupt at priority 10, the only choice is OB33. There is no way to set OB30 to priority 10 on an S7-300 CPU. The fixed model has the advantage of being fully deterministic across all S7-300 CPU models and firmware versions.

S7-1500 Configurable Priority Model

The S7-1500 firmware preserves the S7-300 priority order but introduces two major enhancements:

1. Configurable priority in TIA Portal hardware configuration. Many OB priority values can be edited from the CPU properties dialog.
2. Priority 0 disables an OB. Setting an OB priority to 0 prevents the operating system from ever starting it, even if the trigger event fires. The OB source remains in the project.

Configurable OB priorities on S7-1500 (TIA Portal V13+, firmware V1.0+):

• OB10-OB17 (time-of-day): 1-26, default 2
• OB20-OB23 (time-delay): 1-26, default 3-6
• OB30-OB38 (cyclic interrupt): 1-26, default 7-15
• OB40-OB47 (hardware interrupt): 1-26, default 16-23
• OB60 (multicomputing): 1-26, default 9
• OB70-OB72 (I/O redundancy, S7-1500R/H only): 1-26, default 25
• OB82 (diagnostic interrupt): 1-26, default 26 (firmware V2.6+)

Fixed OB priorities on S7-1500:

• OB1: 1 (main cyclic)
• OB55-OB57: 2 (status/update/profile-specific)
• OB80, OB81, OB83-OB87: 26 (error OBs)
• OB90: 29 (background)
• OB100-OB102: 27 (startup OBs)
• OB121/OB122: inherit (priority of the active OB)

Default OB Priority Reference Table

OB Execution Rules and Preemption Behavior

The priority class of an OB controls four behaviors in both S7-300 and S7-1500:

1. Preemption by higher priority. When the operating system starts an OB, it compares the new OB's priority with the priority of the OB currently running. If the new priority is higher, the current OB is suspended. The state (accumulators, AR1/AR2, DB register, status word) is saved on the OB priority stack. When the higher-priority OB ends, the lower-priority OB resumes.
2. Same-priority queueing. Cyclic interrupts (OB30-OB38) cannot be queued multiple times of the same OB; if the OB is still running when the next period elapses, the operating system reports OB80 with event identifier 0x0001 (cycle time exceeded). Hardware interrupts (OB40-OB47) can be queued to a depth defined per CPU; once full, additional events are lost and OB80 with identifier 0x0002 is reported.
3. Error OB priority. OB80-OB87 run at fixed priority 26. They preempt OB1 (priority 1) and most user OBs. The startup OBs (OB100-OB102) run at priority 27, the highest in the user code, and can preempt error OBs.
4. Inherited priority for synchronous errors. OB121 (programming error) and OB122 (I/O access error) execute at the same priority as the OB that caused the error. If the error occurs in OB1, OB121/OB122 run at priority 1. If the error occurs in OB35, they run at priority 7. OB121/OB122 cannot be preempted by an OB of the same or lower priority.

The background OB (OB90, priority 29) is special: it runs only when no other OB of priority 28 or higher is active. It is intended for low-priority housekeeping and self-test routines. The OB90 has no preemption relationship with the main cycle in the usual sense; it yields time only when the main cycle or any interrupt OB needs the CPU.

Process Image Partition Assignment and Priority

Both S7-300 and S7-1500 support process image partitions (PIP). Each PIP is updated when the OB assigned to it starts (inputs) and ends (outputs). The priority of the OB determines when the update occurs, and therefore the latency of input access. Common PIP assignments:

• OB1 (priority 1): process image partition 0 is updated at OB1 start and refreshed at OB1 end.
• OB35 (priority 12, 100 ms default): if PIP 1 is assigned to OB35, the inputs and outputs of PIP 1 are updated at OB35 start and end.
• OB40 (priority 16): if PIP 2 is assigned to OB40, the inputs of PIP 2 are updated immediately when the hardware interrupt fires, before OB40 code runs.

This means that for time-critical inputs, the OB priority is closely tied to the latency of input access. High-priority OBs with assigned PIPs provide the fastest deterministic access to physical inputs. In STEP 7 V5.x, PIP assignment is set in HW Config by selecting the OB and opening Properties. In TIA Portal, the setting is in the CPU properties under System and clock memory.

Disabling Interrupts in Code: SFC 39/40 vs DISIRT/ENIRT

S7-300/400 code uses SFC 39 "DIS_IRT" to disable all interrupts and SFC 40 "EN_IRT" to re-enable them. S7-1500 code uses the equivalent TIA Portal instructions DISIRT and ENIRT:

// S7-1500 SCL example
"DISIRT"();            // disable all interrupts
"DB_Output".value := 100;  // atomic update
"ENIRT"();             // re-enable interrupts

// S7-300 STL example
CALL SFC 39             // disable all interrupts
L 100
T DB1.DBD0
CALL SFC 40             // re-enable interrupts

The S7-1500 also supports targeted interrupt disabling by passing a specific OB number to DISIRT, allowing a single OB to be suspended without affecting the rest of the interrupt tree. This is more granular than the SFC 39/40 behavior on the S7-300, which disables all interrupts globally. When interrupts are disabled, any OB that becomes active is queued (within queue depth) and not lost. Cyclic interrupts that fire while interrupts are disabled cause OB80 (cycle time exceeded) when the disable window ends, because the cyclic period has already been missed.

Configuring OB Priorities in TIA Portal (Step-by-Step)

1. Open the project in TIA Portal (V16 or later recommended for full configurability).
2. Select the S7-1500 CPU in the project tree.
3. Double-click the CPU to open the Device view.
4. Open Properties → System and clock memory → Interrupts.
5. Click the row of the desired OB (for example, OB35 cyclic interrupt).
6. Enter the new priority value between 1 and 26 in the Priority column.
7. Optionally adjust the Phase offset and Period (cyclic interrupts only).
8. Compile the hardware configuration and download to the CPU.

The priority change is part of the hardware configuration and is stored in the CPU system data blocks (SDBs). It is not part of the OB source. To verify the change took effect, go to Online → Online & Diagnostics → Cycle / clock memory and confirm the active OB and its priority in the runtime statistics. If the change does not appear, recompile the hardware configuration and re-download the SDBs. Setting the priority to 0 disables the OB; the block remains in the project but the operating system does not start it.

STEP 7 V5.x OB Properties (S7-300 / S7-400)

To inspect the priority of an OB in an existing S7-300 program in STEP 7 V5.x (SIMATIC Manager):

1. Open SIMATIC Manager and the S7 project.
2. Navigate to the S7 station and select the Blocks folder.
3. Right-click the OB (for example, OB35) and select Object Properties.
4. The General – Part 1 tab shows the Priority field, the OB number, and the OB type.
5. The General – Part 2 tab shows process image partition assignment, local data stack size, and programming language.

To "change" the priority on an S7-300, the only option is to use a different OB number. For example, to run a cyclic interrupt at priority 12, use OB35. To run at priority 9, use OB32. The OB period is also fixed per OB number on the S7-300; adjusting the period requires CPU-specific SFCs (SFC 28 to set the period, SFC 30 to read it on CPUs that support the change).

Migration from S7-300 to S7-1500

When converting an S7-300 program to an S7-1500 using the TIA Portal migration tool, OB priorities are preserved by default because the S7-1500 default priorities match the S7-300 firmware values. Engineers porting programs do not need to reassign priorities for functional equivalence. Common migration considerations:

• OB30-OB38 default periods match. Verify each OB's period setting in the hardware configuration after migration; the S7-300 default for OB30 is 5 s, and the S7-1500 default for OB30 is also 5 s.
• OB55-OB57 (status, update, profile-specific interrupts) are S7-1500 OBs that do not exist on the S7-300. They are introduced when the migration target uses PROFINET IO with isochronous mode or PROFenergy.
• OB70-OB72 (redundancy error OBs) apply only to S7-1500R/H redundant CPUs and are not generated during a standard S7-300 to S7-1500 migration.
• OB82 priority configurability (1-26 range, firmware V2.6+) is a new option. The S7-300 OB82 always runs at priority 26. Engineers can lower OB82 priority on the S7-1500 if needed, but this is generally not recommended because diagnostic events should be processed at the highest user priority.
• OB101 hot restart is supported on S7-1500 CPUs from firmware V2.0. S7-1500 firmware V1.x does not support OB101; verify the target firmware version before migration.
• OB122 behavior on the S7-1500 can be set to either return a substitute value (S7-300 default) or stop the CPU, configured in TIA Portal hardware configuration under System and clock memory → I/O access error.
• OB80 time error reporting on the S7-1500 includes more event identifiers than the S7-300, particularly for PROFINET IO and Web server events. Update error handling code accordingly.

Verification and Diagnostics Procedures

After changing OB priorities, verify the running behavior with these steps:

1. Online OB monitor. In TIA Portal, go to Online & Diagnostics for the CPU → Cycle / clock memory. The current active OB and its priority appear in real time.
2. Diagnostic buffer. TIA Portal → Online & Diagnostics → Diagnostic buffer. Filter on OB80-OB87 events. Each entry shows the priority (always 26 for OB80-OB87), the event identifier, and the timestamp.
3. OB80 event identifiers (most common):
   • 0x0001: cycle time exceeded (cyclic OB overrun)
   • 0x0002: hardware interrupt queue overflow
   • 0x0003: time-delay interrupt queue overflow
   • 0x0005: local data stack overflow
   • 0x0007: OB not loaded (e.g., OB80 missing from project)
4. Trace recording. TIA Portal Trace can record the OB system tag over time. Configure a 10-second trace, download, and trigger the OB transition. The trace confirms whether the configured priority is taking effect.
5. Cross-reference of priority override. TIA Portal → Show Cross-references on the OB block lists references including the SDB priority override value.

For S7-300 verification in STEP 7 V5.x, use PLC → Monitor/Modify with a VAT (variable table) for the OB priority tags, and PLC → Diagnostic buffer to read OB80-OB87 events. S7-300 does not expose the OB name in the online monitor; the active OB must be inferred from the priority class in the SZL partial list extract.

Troubleshooting Matrix

Safety OBs on F-CPUs (S7-300F / S7-1500F)

Both S7-300F and S7-1500F CPUs add safety-related OBs that run at fixed priorities. These OBs are part of the F-runtime and are not user-configurable. S7-300F safety OBs include OB35 (F cyclic interrupt, priority 12, 100 ms default) and the F-error OBs at priority 26. S7-1500F safety OBs include OB30 and OB35 (F cyclic interrupts, configurable priority 1-26), OB80-OB87 (F error, priority 26), and OB123 (F programming error, inherits from the active F-OB). When migrating a safety program from S7-300F to S7-1500F, the TIA Portal safety migration tool automatically remaps the F-OBs. Verify the F-OB priorities match the S7-1500F default values in the TIA Portal safety administration editor. The F-shutdown OBs (OB82 safety variant) on the S7-1500F run at the maximum priority and cannot be modified.

Recommendations

Based on the comparison above, the following recommendations apply when designing or migrating a SIMATIC program:

1. Keep OB1 at priority 1 for both S7-300 and S7-1500. The main cyclic program is the lowest-priority user code; interrupt OBs should preempt it.
2. Use the firmware default OB numbers for the priority you need. On the S7-300, choose OB30 for priority 7, OB33 for priority 10, OB35 for priority 12, and so on. There is no need to edit priorities on the S7-300.
3. On the S7-1500, leverage the configurable range for performance tuning. If a cyclic interrupt handles a time-critical task, raise its priority to 20-25 to ensure it preempts most user OBs.
4. Use priority 0 to disable an OB on the S7-1500 instead of deleting the OB source. This preserves the code and the trigger event association in the hardware configuration.
5. Never modify the fixed-priority OBs (OB80-OB87, OB100-OB102, OB90). The operating system relies on these values for error handling and startup sequencing.
6. For safety programs on F-CPUs, do not modify F-OB priorities. The safety integrity level depends on the certified F-runtime behavior.
7. During migration from S7-300 to S7-1500, re-evaluate the priority scheme. The S7-1500 configurability is an opportunity to optimize interrupt handling that did not exist on the S7-300.

FAQ