Categorizing High-Density Digital Inputs into WinCC Event Logs Without Process Object Overruns

In SIMATIC PCS 7 projects requiring event recording for thousands of digital inputs (such as recording 0->1 and 1->0 transitions for 2,000+ signals), instantiating dedicated APL monitoring blocks (MonDiL) consumes 2,000 Automation Station Process Objects (AS RT PO), causing substantial license costs. Engineers avoid license overruns by multiplexing digital signals through ALARM_8P blocks (allocating 8 inputs per single block instance) or recording transitions directly via WinCC bit archiving and cyclic event logging.

Process Object Licensing Rules for Digital Signal Monitoring

In SIMATIC PCS 7, software licensing strictly regulates the number of active technological objects operating within the automation stations:
* AS Runtime Process Object (AS RT PO): Any CFC block that possesses message capability (integrating system message functions like ALARM_8P), operator control and monitoring capability, and typically interfaces with field I/O consumes exactly 1 AS RT PO.
* Standard APL digital monitoring blocks (MonDiL, FB1847) provide full faceplate integration, alarm limits, maintenance bypass, and event generation, but consume 1 AS PO per instance.
* Signal channel driver blocks (Pcs7DiIn, Pcs7DiIT) do not consume AS POs, but do not natively generate WinCC operator event messages.
* Physical CPU 410 hardware capacity is established by the System Expansion Card (SEC), which is distinct from software AS RT PO licenses.

The Architectural Problem: 2,000 Inputs vs. MonDiL PO Consumption

When a client requests that 2,000 digital signals (e.g., MCC auxiliary contacts, breaker statuses, field switch positions) record both rising ($0
ightarrow 1$) and falling ($1
ightarrow 0$) edges in the WinCC event log:
* If the engineering team instantiates 2,000 MonDiL blocks, the CFC compiler books 2,000 AS RT PO licenses from the Engineering Station license pool in Automation License Manager (ALM).
* For medium or large plants, this could consume or exceed the entire AS PO license allocation, triggering expensive licensing expansion requirements.
* Because raw digital transitions represent informational events rather than complex closed-loop process units, utilizing full MonDiL instances is architecturally inefficient.

Cost-Effective Solution 1: Multiplexing via ALARM_8P Blocks in CFC

To minimize PO consumption while retaining controller-stamped microsecond event generation:
1. Group the 2,000 digital signals into sets of 8.
2. In the CFC charts, connect raw channel outputs (Pcs7DiIn.OutSig) into standard system message blocks: ALARM_8P (FB19) or custom message wrappers.
3. Each ALARM_8P block accepts 8 binary inputs (SIG_1 through SIG_8) and handles independent message generation, acknowledgment, and associated values.
4. License Impact: Grouping 2,000 inputs into ALARM_8P blocks requires only 250 block instances:
$$ ext{PO Consumption} = rac{2,000 ext{ inputs}}{8 ext{ inputs/block}} = 250 ext{ AS POs}$$
This yields an 87.5% reduction in required AS RT PO licenses (250 POs instead of 2,000 POs).
5. In the message configuration dialog of ALARM_8P, configure the message class to Event (or Operating Message), set message texts to include equipment tag identifiers, and enable messages for both incoming and outgoing signal edges.

Cost-Effective Solution 2: WinCC Bit Archiving and Tag Logging Events

If the 2,000 signals are purely supervisory and do not require controller-side alarm prioritization:
1. Pack the 2,000 digital inputs into 64 Double Words (DWORD, 32 bits each) in an S7 data block inside the AS.
2. In WinCC OS Tag Management, create 64 external 32-bit unsigned integer tags mapped to the AS data block.
3. In WinCC Alarm Logging:
* Create bit messages triggered by individual bits within the 64 DWORD tags.
* Assign the message class to System, without acknowledgment or Operational Message.
4. License Impact: Controller AS RT PO consumption is 0 POs, because no message-capable blocks are instantiated in CFC. Only standard WinCC OS archive tags/variables are utilized.

Auditing and Verifying PO Consumption in SIMATIC Manager

To verify license compliance and actual PO consumption:
1. Open SIMATIC Manager and select the top-level project object.
2. Navigate to Options -> PCS 7 License Information.
3. Select Process Objects (AS).
4. Review the PO ledger:
* Configured POs in Project
* Available POs in ALM
* Remaining PO Reserve
5. Confirm that the optimized architecture keeps project consumption safely within your purchased license entitlement.

Verification

In CFC dynamic test mode, toggle a digital input channel from 0 to 1, then back to 0.
In WinCC OS runtime, open the Alarm / Event List:
Verify that an event message appears immediately with class Event.
Confirm that the message text displays the correct tag name and state transition ($0
ightarrow 1$).
Confirm that returning the input to 0 generates the corresponding outgoing event ($1
ightarrow 0$).
Check SIMATIC Manager License Information: confirm that the AS PO count incremented only by the expected multiplexed block count.

Limitations and Operational Precautions

Multiplexing signals through ALARM_8P does not provide individual APL faceplates; operators cannot click the signal to open a dedicated diagnostic pop-up window.
When configuring WinCC Alarm Logging bit messages, high-frequency chattering digital inputs can saturate OS alarm buffers; always implement debounce filtering in the AS channel drivers.
Commercial procurement requirements should always be validated against the official Siemens Industry Mall and authorized distributor quotation before ordering software expansion packages.

Engineer FAQ

What defines an AS Process Object in SIMATIC PCS 7?
A technological block that contains operator control/monitoring capability, integrates alarm generation, and links to field I/O.
Does Pcs7DiIn consume an AS Process Object?
No, signal channel driver blocks like Pcs7DiIn and Pcs7AnIn do not consume AS Process Objects.
How many digital inputs can a single ALARM_8P block monitor?
A single ALARM_8P block monitors up to 8 independent binary signals simultaneously.
Does using ALARM_8P consume OS Process Objects?
In WinCC OS, process objects are counted based on OS tags/variables mapped from the AS; check OS License Information in WinCC Explorer.
Can I achieve 1-millisecond sequence of events (SOE) timestamping with ALARM_8P?
Yes, when used with high-feature digital input modules (e.g., ET 200M/ET 200SP HA) supporting hardware timestamping, ALARM_8P preserves 1 ms timestamps.
How do I configure ALARM_8P messages to generate events without requiring operator acknowledgment?
In the message configuration dialog, set the Message Class to "Operating Message" or "Event", which are unacknowledged by design.
Why is packing bits into DWORDs advantageous for supervisory SCADA systems?
Packing 32 digital signals into a single DWORD minimizes communication telegram count and reduces tag count across the plant bus.
Where in ALM can I check how many AS RT PO licenses are currently installed?
Open Automation License Manager, select the primary hard drive or license server, and locate the key SIMATIC PCS 7 AS RT PO.
What happens if the project PO count exceeds the ALM license quantity during compilation?
The CFC compiler issues a license warning. Downloading to the CPU is permitted, but the license deficit is permanently logged.
Does a redundant S7-400H or AS 410H pair require two sets of AS RT PO licenses?
No, a redundant AS pair consumes AS RT PO licenses only once from a common project pool.

Technical References

SIMATIC PCS 7 Licenses and Configuration Limits Manual (V9.0 / V9.1)
SIMATIC STEP 7 V5.6 System and Standard Functions Manual (ALARM_8P / FB19)
Siemens Industry Online Support Entry: Process Object Optimization for Digital Signals (Entry ID 298785)
SIMATIC PCS 7 Advanced Process Library (APL) Function Manual