Siemens OP77A K1-K4 LED Control and System Key Mapping in WinCC

Overview

The SIMATIC OP77A is a 4.5-inch graphics-capable operator panel from the OP 73 / OP 77A / OP 77B family, used for small- to mid-range machine HMI in SIMATIC S7-200, S7-300, S7-400, and LOGO! environments. The OP77A replaces the legacy OP7 and is configured natively with WinCC flexible 2004 SP1 and later. This reference consolidates the four recurring configuration tasks encountered when migrating an OP7/ProTool project to OP77A/WinCC flexible:

• Driving the K1–K4 LED indicators embedded in the system keys
• Mapping system-key events (ACK, ENTER, ESC, HELP, INFO, PRINT) to PLC tags
• Viewing and exporting the alarm history buffer
• Synchronizing date and time without a battery-backed RTC

All four tasks are located in the System Keys, Alarms, and Connections / Area Pointers editors of WinCC flexible — none of which exist in identical form in ProTool/OP7, which is the source of most migration friction.

Hardware Identification and Ordering Data

Use the Siemens Industry Online Support portal entry OP77A product support page to download firmware updates, manuals, and the WinCC flexible HSP (Hardware Support Package). The consolidated operating manual for the OP 73 / OP 77A / OP 77B family is the ba_op7xx_en_en-US.pdf document.

Project Structure in WinCC Flexible

When converting from ProTool to WinCC flexible, the project tree replaces the ProTool "Screens / Areas / Keys" hierarchy with the following nodes:

1. Project — overall configuration and device identification (panel type, firmware version)
2. Connections — MPI/PROFIBUS DP connection parameters (baud rate, station address, slot, area pointers)
3. Tags — internal tags (HMI-local) and external tags (PLC)
4. Screens — process screens, each containing templates and screen objects
5. System Keys — assignment of K1–K16 to functions, events, and PLC tags
6. Alarms — discrete alarms, analog alarms, alarm classes, history buffer
7. Schedules / Recipes / Reports — optional nodes (not all supported on OP77A)

WinCC Flexible Configuration Limits (OP77A)

Configuring the K1–K4 LED Indicators

The four soft LEDs are driven indirectly: a Boolean tag whose value toggles the LED state. In WinCC flexible this is configured under Project > Screens > <Screen> > Template or directly on a screen via Properties > Representation. Configuring the LEDs on the Template is preferred so every screen inherits identical LED behavior without per-screen repetition.

Step-by-Step: Drive K1 LED from a PLC Tag

1. Open WinCC flexible > Tags and create a new external tag, e.g. HMI_Tag_K1_LED, of data type BOOL, address DB1.DBX0.0 on the S7 PLC.
2. Open the Template editor (Screens > Template).
3. Insert the K1 key object from the toolbox (System Keys folder).
4. In the K1 properties, navigate to General > LED Assignment > Value source and select HMI_Tag_K1_LED.
5. Set the polarity: Active = 1 lights the LED when the tag is TRUE; Active = 0 inverts.
6. Repeat for K2, K3, K4 with separate tags (HMI_Tag_K2_LED … HMI_Tag_K4_LED).
7. Compile the project (Project > Compiler > Generate) and download to the panel.
8. Force DB1.DBX0.0 = TRUE in STEP 7 and verify the K1 LED illuminates within one polling cycle.

Address Mapping Table

How It Differs From ProTool/OP7

ProTool exposed the LED as a discrete key property and required a separate "LED area" configured on the PLC side. In WinCC flexible, the LED is a tag-driven property of the same screen object used for the key press; this eliminates the legacy LED area pointer but also means the LED no longer responds automatically to the keystroke — the PLC must set the LED tag explicitly in response to its own logic.

System Keys and PLC Function Mapping

System keys perform fixed HMI functions (ACK, ENTER, ESC, HELP, PRINT, INFO) but can also fire a configurable event that triggers a function list or sets a tag. Use this mechanism to bridge "ACK" or "ENTER" to a Boolean tag the PLC can read on its own scan cycle.

Standard System Key Functions on OP77A

Step-by-Step: Map ENTER to a PLC Tag

1. Create Boolean external tag HMI_ENTER_Request at MB10.
2. In the screen editor, click System Keys > ENTER > Properties > Events > Press.
3. Add function SetBit: tag = HMI_ENTER_Request, value = TRUE.
4. Add a second function SetBit with a 200 ms delay: value = FALSE (auto-clear pulse).
5. Compile, download, and verify in STEP 7 by reading MB10 with a VAT table.

STEP 7 STL Snippet — Latch ENTER Pulse in PLC

// FC 50 — Acknowledge ENTER key from OP77A
      A     M 10.0                // HMI_ENTER_Request (pulse, 200 ms)
      S     M 20.0                // Latched "ENTER received" flag
      A     M 20.0
      =     DB1.DBX1.0            // Echo to LED tag if needed
      BE

The same pattern applies to ACK, ESC, HELP, and INFO. For ACK specifically, the OP77A also automatically writes the acknowledgement to the alarm history buffer; the additional tag is only needed if your PLC application wants to react programmatically.

Alarm History Buffer: View and Export

The OP77A supports both an alarm log (persistent on the panel's internal flash) and the on-screen alarm view. To inspect the historical buffer on the device:

On-Device Steps

1. Insert an Alarm View screen object on a screen (toolbox > Controls > Alarm View).
2. In the alarm view properties, set Display > Show History = TRUE.
3. Define the filter: Active alarms + Acknowledged alarms + Cleared alarms with date/time range.
4. Press the ALARM button on the panel — historical entries appear in the same list as active alarms.

Transferring the Buffer to a PC

The OP77A logs alarms to the internal flash if logging is enabled under Alarms > Settings > Activate logging. To retrieve:

1. Connect the OP77A to the engineering PC via MPI/PROFIBUS DP.
2. Open WinCC flexible > Transfer > Backup/Restore.
3. Select Backup HMI data, choose Alarm log as the source.
4. Save the .csv file. The CSV contains columns: Date, Time, Status, Class, Number, Text, PLC tag.

Buffer Capacity

Date and Time Synchronization (No Battery)

The OP77A has no on-board battery backup for the real-time clock. After a power loss the RTC defaults to 01.01.2000 00:00:00 and increments from there until the next PLC sync. This is by design across the OP 73 / OP 77A / OP 77B family. The mitigation is to push the PLC clock into the panel at every startup and at periodic intervals.

Two Complementary Mechanisms

1. Date/Time Area Pointer (read-only from PLC to HMI): the OP77A polls eight bytes of PLC memory and updates its display clock on each cycle.
2. PLC Job (job mailbox): the PLC writes a job number to a configured word to force the HMI to read the PLC clock immediately.

Area Pointer Configuration

S7 DATE_AND_TIME Byte Layout (BCD)

PLC Job Numbers Reference

PLC-Job Data Block Layout (S7-300/400)

The job mailbox is one word, written by the PLC and read by the OP77A. The OP77A resets the word to zero on completion.

DATA_BLOCK "HMI_Job"
  STRUCT
   Job_No   : WORD;     // DB100.DBW8 — write job number here
   Reserved : WORD;     // DB100.DBW10
  END_STRUCT;
END_DATA_BLOCK

STEP 7 STL Example — Trigger Time Sync on OB100 (Warm Restart)

// OB100 - Complete restart
      CALL   "BLKMOV"               // Copy current DT into DB100
      SRCBLK := "RTC_OB1".TimeStamp
      DSTBLK := "HMI_Time".Date_Time  // DB100.DBB0..DBB7
      L     W#16#000E               // Job 14 = set date/time from PLC
      T     "HMI_Job".Job_No
      SET
      SAVE
      BE

When the OP77A detects the value 14 in the configured job pointer, it reads the eight bytes of the Date/Time area pointer and applies them to its internal RTC. The job pointer word is then cleared by the panel within one polling cycle.

Recommended Commissioning Sequence

1. Define the Date/Time area pointer at DB100.DBB0..DB100.DBB7 with BCD DT format.
2. Define the Job mailbox at DB100.DBW8.
3. Configure OP77A under Connections > Area Pointers: enable "Date/Time" and "Job mailbox".
4. In STEP 7, fill DB100 bytes 0–7 with the current local time in DT format on every PLC startup using SFC 1 (READ_CLK).
5. Write job 14 to DB100.DBW8 in OB100 and OB101.
6. Verify on the OP77A: power-cycle the panel and check that the displayed time matches the PLC clock within ±1 s of startup.

S7-300 SFC 1 Example — Reading the PLC Clock

// FC 10 — read PLC RTC into DB100 DT format
      CALL "READ_CLK"
       RET_VAL := MW100
       CDT     := "HMI_Time".Date_Time   // DB100.DBB0..DBB7
      L     W#16#000E
      T     "HMI_Job".Job_No             // Trigger OP77A time sync
      BE

Communication: MPI and PROFIBUS DP

Troubleshooting Matrix

Verification Procedure

1. LED test: Force DB1.DBX0.0 = 1 in STEP 7; K1 LED must illuminate within the configured update cycle (default 1 s).
2. System key test: Press ENTER on the panel and observe MB10 toggle TRUE for the configured pulse width (default 200 ms).
3. Alarm buffer test: Trigger a discrete alarm, acknowledge it, then open Alarm View with history enabled and confirm the entry is present with a valid timestamp.
4. Time sync test: Set the PLC clock to 12:00:00, power-cycle the OP77A, then verify the HMI displays the PLC time within 2 s of startup.
5. Communication test: Run WinCC flexible > Transfer > Diagnostics > Read Tags on at least 10 external tags; all must return correct values without timeout.
6. PLC job test: Write job 40 to DB100.DBW8 from STEP 7 and confirm the alarm log file timestamp updates on the panel within one polling cycle.

Migration Notes: OP7/ProTool → OP77A/WinCC Flexible

Migration Checklist

• Confirm WinCC flexible version ≥ 2004 SP1 supports OP77A target device
• Re-create LED area as four Boolean tags bound to K1–K4 properties
• Re-create system-key-to-PLC mapping as key events with SetBit functions
• Add Date/Time area pointer (8 bytes BCD DT) and Job mailbox (1 word)
• Configure SFC 1 (READ_CLK) in OB100 to push PLC clock to the panel
• Export alarm history to CSV for archival before any project reload

FAQ