Activating Unused Analog Channels in SIMATIC PCS 7 Without CPU Stop - Configuration in RUN (CiR)

In SIMATIC PCS 7, an analog channel that was deactivated in HW Config cannot be activated in controller runtime merely by modifying driver block parameters such as ACC_MODE or mode word 16#0203 in CFC. When a channel is marked 'Deactivated' in HW Config, the module firmware physically turns off the internal converter circuitry and the CPU operating system excludes the channel from the Process Image Partition (PIP). To activate deactivated channels without stopping the CPU, the system must utilize Configuration in RUN (CiR) on an S7-400 controller. Alternatively, standard engineering practice mandates configuring all spare channels as active during initial plant commissioning, managing unused points purely via CFC channel drivers.

Why Deactivated Channels Cannot Be Activated via CFC Parameters

In running SIMATIC PCS 7 facilities, commissioning engineers frequently encounter situations where an additional field instrument (such as a 4–20 mA pressure transmitter or control valve positioner) must be wired into an existing analog input (e.g., 6ES7331-1KF02-0AB0) or analog output (6ES7332-5HF00-0AB0) module.

If the spare channel was set to 'Deactivated' during initial project design, engineers sometimes attempt to force the channel alive online by modifying the module driver block (MOD_D1, MOD_HA, or Pcs7AnIn) in CFC—for instance, setting ACC_MODE = 1 and forcing the channel mode parameter to 16#0203 (4–20 mA current). However, the physical module produces no output current and reads raw zero inputs.

This occurs because block parameters on software channel drivers cannot override the hardware parameter data records (DS1) stored in the module's onboard micro-controller, nor can they alter the CPU's internal process image tables.

Hardware Parameter Records vs Process Image Partitions

Understanding the architectural layers of SIMATIC S7-400 I/O processing clarifies why software-only forcing fails:

System LayerOperational MechanismState When Channel is Deactivated in HW ConfigCan Be Modified via CFC Online?Module Hardware FirmwareOnboard ADC/DAC multiplexer and analog power supplyMultiplexer skips channel; internal ADC stage powered offNo (Requires HW parameter download)CPU Operating System (OS)Process Image Partition (PIP) cyclic update tableMemory address excluded from cyclic I/O read/write OBsNo (Governed by HW Config compilation)System Driver Blocks (MOD_*)Evaluates diagnostic interrupts and module healthReports channel state based on hardware parameter recordYes (Parameters modified, but hardware ignores)APL Channel Driver (Pcs7AnIn)Scales raw peripheral word (PEW) to REAL (PV_Out)Reads raw zero from inactive process image; outputs Bad qualityYes (Logic changes, but physical signal absent)

When a channel is deactivated in HW Config, the CPU's operating system does not read or write the peripheral memory address. Software blocks attempting to access that address encounter dead memory.

Activating Spare Channels Without CPU Stop Using CiR

To activate channels on a running automation station without interrupting continuous production, engineers must follow one of two architectural approaches:

Configuration in RUN (CiR) Workflow (Dynamic HW Download):
- Prerequisite: The S7-400 hardware configuration must already contain a CiR Object on the PROFIBUS DP master line or PROFINET IO system, reserving communication bandwidth and memory buffers.
- In HW Config, change the channel setting from 'Deactivated' to 4..20 mA.
- Assign the required process image partition (e.g., PIP 1 for cyclic OB35 execution).
- Select PLC -> Download to Current Project -> Selected Station -> Configuration in RUN (CiR).
- The CPU accepts the hardware update while remaining in RUN mode, downloads new parameter records to the module, and expands the process image.
- Place the APL channel driver (Pcs7AnIn or Pcs7AnOu) in CFC, interconnect the address, and execute a standard Download Changes Only.
Best-Practice Proactive Provisioning (Standard DCS Guideline):
- In greenfield or planned outage engineering, always configure 100% of physical channels as active in HW Config (e.g., parameterized as 4–20 mA), even if currently unused.
- For unused input channels, deactivate wire-break diagnostic alarms or jumper the terminal pins to eliminate nuisance faults.
- Leave the software channel drivers deleted or dormant in CFC.
- When a new instrument is wired during plant operation, an engineer simply adds the Pcs7AnIn block in CFC and performs a Delta download in RUN, activating the channel in seconds without touching HW Config.

Verification

Verify successful online channel activation and signal processing through these diagnostic steps:

Module LED Inspection: Inspect the physical analog module front panel. Verify that zero red fault LEDs (SF, EXTF) are illuminated.
Variable Table (VAT) Direct Read: In SIMATIC Manager, create a Variable Table. Enter the absolute peripheral address (e.g., PEW 512 or PAW 512). Verify that injecting a 12 mA test signal displays raw integer 13824 ($\pm 10$ counts).
CFC Online Test Mode: Open the CFC chart containing Pcs7AnIn. Confirm that PV_Out reads the live engineering value (e.g., 50.0%) and signal status Bad remains 0 (16#80 Good Quality).

Limitations and Operational Precautions

When altering hardware configurations on running controllers, observe these strict operational precautions:

Never Execute Full HW Download in RUN Without CiR: Attempting to download hardware changes to an S7-400 system that does not support or have CiR enabled will abort or forcibly prompt to switch the CPU into STOP mode, causing catastrophic plant trips.
Measuring Range Module Orientation: For modules featuring physical measuring range cubes (such as SM 331 8x12 Bit, 6ES7331-7KF02-0AB0), ensure the hardware jumper cube matches the parameter setting (A, B, C, or D). Modifying parameters without rotating the physical cube results in measuring errors or hardware damage.
Single Station vs Redundant Station: In S7-400H redundant systems, CiR downloads execute seamlessly by switching roles between master and reserve CPUs without process disturbance.

Engineer FAQ

Why does ACC_MODE exist on the MOD block if it cannot activate channels?

ACC_MODE forces the software driver block to adopt manual operating settings for testing or simulation. It manipulates internal block logic but has no physical bus access to reprogram module hardware ASICs.

Can I use SFC55 (WR_PARM) to activate a deactivated channel dynamically?

While SFC55 can write parameter records to certain intelligent modules, it cannot expand the CPU's Process Image Partition (PIP). If the operating system excluded the address during startup, the CPU will still fail to update the process image cyclically.

What happens to an active analog input channel if no field wire is connected?

If configured as 4–20 mA, an open terminal reads 0 mA (< 3.6 mA), triggering an open-circuit / wire-break Control System Fault (CSF). To prevent alarms on spare active channels, jumper terminal inputs or disable wire-break diagnostics in module properties.