Configuring the OP 177B Alarm Buffer in WinCC Flexible

The Siemens SIMATIC OP 177B operator panel retains a circular buffer of the last 256 alarm events for diagnostic review. Engineers commissioning or maintaining legacy systems in WinCC Flexible frequently need to surface this historical buffer on a dedicated screen, separated from the live alarm banner, so that operators can audit events that have been acknowledged and deactivated. This technical reference covers the OP 177B alarm buffer architecture, the distinction between Alarm View and Alarm Window objects, alarm class configuration, persistency options, and the verification steps required to confirm the buffer is being populated correctly on the panel.

Overview: OP 177B Alarm Buffer Architecture

The OP 177B (catalog numbers 6AV6642-0BA01-1AX1 monochrome and 6AV6642-0BC01-1AX1 color) runs Windows CE 5.0 and is configured with WinCC Flexible 2008 SP3 or later service packs. Inside the runtime, alarm events are stored in two layers:

• Active alarm list — events that have been triggered but not yet acknowledged. This list populates the standard Alarm Window that ships with the default template.
• Alarm buffer (history) — a persistent ring buffer that retains the last 256 entries of incoming, acknowledged, and cleared events. The buffer survives power cycles only if the panel was configured with the "Alarm persistency" runtime option enabled.

By default, the runtime writes incoming events to the buffer, updates the active list, and rolls the buffer when the 257th event arrives. The OP 177B does not provide a way to scroll through the buffer inside the standard Alarm Window — you must explicitly add a screen-level Alarm View with the "Display" property set to "Alarm buffer" (or to "Alarm events" for live tracking) so that the historical entries become visible.

Prerequisites

Before adding an alarm history view, confirm the following on the engineering station and on the panel:

1. WinCC Flexible 2008 SP3 (or SP5) is installed and the license is activated.
2. The OP 177B device type matches the project target. Right-click the panel in the project tree and confirm the version field reads "OP 177B 6" DP/PN" or "OP 177B 6" Mono DP".
3. The HMI device has a configured connection to the PLC (MPI, PROFIBUS, or PROFINET) and at least one alarm class is wired to a bit trigger in the HMI tag table.
4. Alarms have already been generated from a test PLC, or a simulation tag is forcing alarm bits so the engineer can verify entries land in the buffer.
5. You have at least one screen available to host the Alarm View, or a template available for an Alarm Window.

OP 177B Hardware Reference

The 256-entry buffer is a fixed panel capability — it cannot be extended through options, license upgrades, or firmware changes. Systems that require larger histories must use a PC Runtime (WinCC Runtime Professional) or migrate to a Unified Comfort Panel where the buffer size is configurable and can be backed by an SD card.

Alarm Class Configuration

Alarm classes govern the visual treatment of an event (icon, color, acknowledgment requirement) and act as the filter that the Alarm View applies. WinCC Flexible ships with four default classes:

To add or modify classes, open the project tree → "Alarms" → "Alarm classes" node. To restrict the Alarm View to a single class, set the "Alarm classes" property of the view to that class only; to render every event, leave the property set to "All classes".

Step 1: Confirm an alarm class exists for the events to show

1. In the project tree, expand Alarms → Alarm classes.
2. Verify the class required is present. If a unique process condition exists (for example "Quality"), right-click the node and select Add new class.
3. Set the acknowledge model: choose "Without acknowledgment" for informational buffers, or "With acknowledgment" if the operator must sign off each entry.
4. Compile the project. WinCC Flexible will refuse to compile the Alarm View if the selected class has zero configured alarms.

Alarm State Machine

Every event in the buffer carries a status token that the Alarm View renders in the "Status" column. The OP 177B transitions an event through three states:

The state tokens correspond to the filter syntax used by the Alarm View (described in the "Filter by Acknowledgment Status" section below). An event is held in the buffer as "incoming" until acknowledged, then transitions to "acknowledged", then to "outgoing" once the PLC trigger clears.

Adding an Alarm View to a Screen

The Alarm View is the screen object that exposes the buffer. When its "Display" property is set to "Alarm events", it renders incoming events as they arrive; when switched to "Alarm buffer", the same view scrolls through past entries that have already been acknowledged or cleared.

Step 2: Insert the Alarm View

1. Open the target screen in WinCC Flexible.
2. From the Tools toolbox, expand Enhanced Objects.
3. Drag the Alarm View object onto the screen.
4. Resize the object to occupy the bulk of the screen. The OP 177B's 320 × 240 display requires careful layout — keep the view at least six rows tall to render useful context.

Step 3: Configure the Alarm View properties

1. Right-click the Alarm View and select Properties.
2. In the General tab, locate the Display property.
3. Set Display to Alarm events for live tracking, or to Alarm buffer for historical scrolling.
4. Set Alarm classes to the class (or classes) to surface. For the standard "show me everything" buffer view, pick All classes.
5. Open the Columns tab and enable "Date", "Time", "Status", "Text", and "Class" columns. Disable "Number" if the operator does not need the cross-reference ID.
6. Open the Layout tab and set the row height to a value that fits the OP 177B's 16 px font — 24 px row height works on a 5.7" STN.

Step 4: Add a navigation button to refresh the view

1. From Tools → Simple Objects, drag a Button onto the screen.
2. Right-click → Properties → Events → Click.
3. Add the system function UpdateScreen with the current screen name as the parameter, or use ActivateScreen to jump to a dedicated "Alarm History" screen.

Adding an Alarm Window to a Template

If the buffer must surface across multiple screens, place an Alarm Window in the project template rather than the Alarm View. The Alarm Window is a passive object that floats above the active screen and is populated from the same buffer the Alarm View uses.

Step 5: Insert the Alarm Window in a template

1. Open the project tree → Templates and double-click the template assigned to the OP 177B (typically "Template_1").
2. From Tools → Enhanced Objects, drag the Alarm Window object into the template.
3. Set the window's Display property to Alarm events or Alarm buffer, depending on whether the window should show live or historical events.
4. Configure the column set identically to the Alarm View setup in Step 3.

Step 6: Bind the Alarm Window to the buffer

1. Open the Properties dialog of the Alarm Window.
2. Set the Alarm buffer property to Enabled — this is the toggle that tells the runtime to populate the window from the persistent 256-event buffer rather than the volatile active list.
3. Save the template and recompile the project.

Enabling Alarm Persistency

By default, the OP 177B's alarm buffer is volatile — when the panel reboots, the history is lost. To keep the 256 entries across a power loss, alarm persistency must be enabled in the runtime settings.

1. Open the project tree → Device → Runtime settings.
2. Locate the Alarms section.
3. Set Alarm persistency to Activated.
4. Rebuild the project and transfer to the panel.

When persistency is activated, the OP 177B writes the buffer to flash memory on every change. The flash write rate is the limiting factor: on the OP 177B, the panel can absorb roughly five to ten alarm transitions per second before the flash queue stalls. If the process generates alarms faster than that, the panel drops buffer entries and logs a system alarm on the "System" class. For high-rate alarm generators, disable persistency and rely on a SCADA historian or a CSV log on a network share to capture the events.

The same persistency concept is documented in the current TIA Portal help for Unified Basic Panels, which states: "You can enable or disable the retentivity of the alarm buffer under 'Alarm persistency'. The default setting is 'deactivated'." The principle is identical; only the menu path differs. Reference: TIA Portal — Unified Basic Panels: Alarm persistency.

Filter by Acknowledgment Status

A common need is to filter the buffer to show only events that have been acknowledged or only events that have not been acknowledged. The OP 177B's Alarm View supports this through the Status column filter:

1. Right-click the Alarm View → Properties → General.
2. Set the Filter property. The available filter tokens are:
   • * — all events (default)
   • + — incoming, not yet acknowledged
   • - — incoming, acknowledged
   • ! — incoming, acknowledged, cleared
3. Set the filter to ! to render the full audit trail of completed events — the equivalent of the "old alarms" view required in the original report.

Performance and Memory Footprint

Each alarm entry in the OP 177B buffer consumes approximately 256 bytes of RAM. The 256-entry buffer therefore reserves 64 KB of the 2 MB user memory for alarm history, leaving roughly 1.9 MB for tags, screens, and recipes. With the "Alarm persistency" option enabled, the same entries are mirrored to flash; the OP 177B flash partition dedicated to alarm logs is 256 KB and is reserved in addition to the user memory.

When persistency is enabled and the system alarm "Buffer overflow" begins to surface, the root cause is almost always a flash write bottleneck. The corrective action is either to:

• Disable persistency and accept the volatile buffer, or
• Configure a network CSV log file in WinCC Flexible (project tree → Alarms → Logging) and let the network share absorb the long-term history while the in-panel buffer remains the short-term view.

Exporting the Buffer for Off-Panel Analysis

The OP 177B does not support direct CSV export of the alarm buffer from a screen object. To extract the 256-event buffer for offline review, use one of the following options:

1. ProSave backup — use ProSave to back up the entire panel image, then read the embedded alarm log from the backup file on the engineering station.
2. WinCC Flexible's alarm logging — configure a circular log file (CSV) on a network share that the OP 177B writes to in parallel with the in-panel buffer. The log file can hold thousands of entries and is the recommended long-term storage path.
3. S7 Graph / S7-PDIAG — forward alarm transitions to a SIMATIC log that can be read from the PLC's web server.

Verification

After the Alarm View or Alarm Window is configured and the project is transferred to the OP 177B, verify the buffer is populating as expected.

1. Trigger a known alarm from the PLC (for example, flip a bit that maps to a configured discrete alarm).
2. Acknowledge the alarm on the panel.
3. Clear the trigger bit and wait for the "Outgoing" status to appear.
4. Navigate to the Alarm View screen. Confirm the event shows a status icon that combines "incoming", "acknowledged", and "outgoing" (typically a checkmark).
5. Cycle the buffer by triggering 257 additional events. The first event triggered in step 1 must be displaced and no longer visible.
6. Power-cycle the panel (if persistency is enabled) and re-check the Alarm View. The buffer entries must be intact.
7. From the engineering station, connect with WinCC Flexible "Online → Alarms" to read the live buffer state. The read must match what the screen shows.

Troubleshooting Matrix

Migration Notes: WinCC Flexible to TIA Portal

The OP 177B itself is not directly targetable from TIA Portal — the device type is not in the TIA Portal hardware catalog. If the project must move forward, the supported path is:

1. Open the WinCC Flexible project in WinCC Flexible 2008 SP5.
2. Run Project → Migrate to TIA Portal. The migration wizard ships with TIA Portal V16 or later.
3. Select a Unified Basic Panel (for example KTP700 Basic) as the target. The 256-event buffer maps to the Unified Basic Panel's "Alarm buffer" property, which is documented in the TIA Portal help as: "You can enable or disable the retentivity of the alarm buffer under 'Alarm persistency'. The default setting is 'deactivated'."
4. Re-import the PLC tags, then recompile.

The migration preserves the alarm text, the acknowledgment model, and the class structure. The OP 177B-specific quirks (the 256-entry hard limit, the flash write bottleneck) do not apply to Unified panels, which use a configurable buffer backed by SD card storage.

Frequently Asked Questions