Optimizing Module Driver Generation and Communication Job Limits in PCS 7 CFC

In SIMATIC PCS 7, selective module driver generation cannot be toggled on a channel-by-channel basis via a simple checkbox; however, the driver generator (GEN_MOD) inherently creates diagnostic and channel driver blocks only for hardware modules and subnets that are actively interconnected to technological blocks in Continuous Function Charts (CFC). Unused I/O modules declared in HW Config do not generate driver instances or consume controller communication jobs. When an S7-400 CPU approaches communication job capacity (Programmbegleitende Kommunikation - PBK limits), engineers can suppress driver generation for specific modules by disabling module diagnostic alarms in HW Config, disconnecting peripheral addresses from channel blocks, or optimizing cyclic OB execution assignments.

Module Driver Generation Architecture and PBK Communication Limits

In SIMATIC PCS 7, the Driver Generator (GEN_MOD) automated engineering tool connects field hardware configurations with technological control logic. When an engineer compiles CFC charts with the option Generate module drivers enabled, GEN_MOD reads the compiled hardware database (SDBs) and creates specialized driver blocks:

Signal Channel Blocks: Blocks such as Pcs7DiIn, Pcs7AnIn, Pcs7DiOu, and Pcs7AnOu that process individual field I/O channels.
Diagnostic Driver Blocks: System blocks such as MOD_D1, MOD_D2, MOD_HA, RACK, and SUBNET that monitor module power supplies, wire breaks, and communication errors across PROFIBUS DP and PROFINET IO.

Each diagnostic driver block utilizes system function calls (such as Alarm_8P / SFB35) to report diagnostic events to the WinCC OS. In S7-400 and S7-410 CPUs, these messages consume Communication Instances (PBK jobs - Programmbegleitende Kommunikation). If a system contains hundreds of distributed I/O modules, the cumulative PBK consumption can approach the CPU's maximum communication limit (e.g., 1800 to 4500 jobs depending on CPU model), prompting engineers to seek ways to suppress unnecessary drivers.

How GEN_MOD Evaluates Hardware and Interconnections

To optimize driver generation, engineers must understand the rules governing GEN_MOD execution:

Demand-Driven Generation: GEN_MOD only creates a MOD_... diagnostic driver block if at least one channel on that physical module is interconnected to a PCS 7 channel driver block (Pcs7AnIn, CH_AI, etc.) in a CFC chart. Spare, unreferenced modules configured in HW Config generate zero driver blocks and consume zero PBK resources.
Automatic De-duplication: If multiple channels on an 8-channel analog input card are used, GEN_MOD creates only one MOD_D1 block to service the entire card, minimizing communication overhead.
Suppression via Hardware Parameters: In HW Config, unchecking Diagnostic interrupt or disabling diagnostic alarms in module properties prevents GEN_MOD from allocating active alarm blocks for that station.

Driver Block Types, Communication Jobs, and Resource Allocation Matrix

The following table compares driver blocks, their communication job consumption, and resource optimization techniques:

Driver Block TypeBlock NumberPrimary FunctionPBK Jobs ConsumedOptimization MethodPcs7AnIn / Pcs7DiInFB1869 / FB1871Channel signal scaling & quality code0 PBK jobs (Uses Alarm_8P only if alarmed)Do not alarm channel drivers directlyMOD_D1 / MOD_D2FB107 / FB108I/O module health & wire break monitoring1 to 2 PBK jobs per physical cardDisable diagnostic alarms in HW ConfigRACKFB109Subrack & power supply monitoring1 PBK job per ET 200 rackGenerated once per rackSUBNETFB106Fieldbus line & master system monitoring1 PBK job per DP/PN networkConsolidate redundant linesOB_BEGIN / OB_ENDFB100 / FB101Execution cycle coordinator0 PBK jobsSystem overhead (mandatory)

Step-by-Step Procedure to Optimize and Restrict Driver Generation

To reduce communication job consumption and prevent PBK exhaustion in PCS 7:

Audit Active PBK Resource Consumption in SIMATIC Manager:
- In SIMATIC Manager, select the CPU and open PLC -> Diagnostic/Setting -> Module Information.
- Open the Communication tab.
- Inspect Configured PBK instances versus Maximum allowable PBK instances.
- If usage exceeds 85%, execute the following optimization steps.
Disable Diagnostics on Non-Critical Modules in HW Config:
- Open HW Config for the S7-400 station.
- For non-critical digital input cards (e.g., standard pushbutton panels or status indicator modules where wire-break detection is not required), double-click the module.
- Under the Inputs tab, uncheck Diagnostic interrupt and Hardware interrupt.
- Save and compile HW Config.
Remove Unused Channel Driver Blocks in CFC:
- Open CFC charts and delete channel driver blocks (Pcs7DiIn, Pcs7AnIn) whose field signals have been decommissioned or are spare.
- If signals are only needed for internal logic without diagnostic monitoring, read peripheral memory directly via word interconnections (PIWxxx) without channel driver wrappers.
Recompile with Driver Generation Enabled:
- In CFC, select Chart -> Compile -> Changes Only.
- Ensure Generate module drivers is checked.
- GEN_MOD automatically sweeps the project, identifies deleted channel links, and deletes orphan MOD_D1 diagnostic blocks.
- Re-check CPU module information; verify that PBK consumption has decreased.

Verification

Verify driver generation optimization:
1. Open the CFC Runtime Editor (Alt+F12); expand OB1 and cyclic interrupt OBs (e.g., OB35).
2. Confirm that driver runtime groups (@...) contain only active, required module diagnostic blocks.
3. Compile the entire program; confirm that compilation finishes cleanly without PBK capacity warnings.
4. In WinCC Runtime, verify that remaining critical modules correctly report rack failures and wire break alarms.

Limitations and Operational Precautions

Loss of Field Diagnostics: Suppressing module drivers on an I/O card prevents PCS 7 Maintenance Station from displaying module faults and inhibits wire-break detection.
Never Uncheck 'Generate Module Drivers' Globally: Disabling driver generation globally prevents updating Mode words and quality codes across the entire plant, corrupting process values.
CPU Sizing Limits: If a massive plant legitimately requires more PBK instances than the CPU supports, upgrade the controller (e.g., upgrading from CPU 416-3 to CPU 410-5H).

Engineer FAQ

Can I selectively check which modules generate drivers in CFC?

No. Driver generation is evaluated globally by GEN_MOD. However, GEN_MOD only creates drivers for modules actively connected to channel blocks in CFC.