Configuring Siemens OP270 Screen Change via PLC Job 51

Overview

The Siemens SIMATIC OP270 is a 6-inch STN operator panel from the OP170/OP270 generation that runs ProTool V6.x (and the later ProTool/Pro RT) configuration software. The OP270 ships with function keys, numeric input keys, and a touch-sensitive membrane (depending on variant), and was designed primarily for S7-200/S7-300/S7-400 communication over MPI or PROFIBUS-DP. With the appropriate serial adapter and protocol selection, the OP270 can also exchange data with legacy S5 controllers such as the S5-928B.

A common requirement on the OP270 is to force the panel to display a specific screen from the PLC program without operator action. This is not done through the function-key events of the screen object itself; it is implemented through a PLC job mailbox, and the specific job that changes the displayed screen is PLC job 51.

This article documents:

• The structure of the ProTool area pointer and job mailbox.
• The exact payload of PLC job 51.
• How to configure the area pointer in ProTool.
• How to drive the job mailbox from an S5 CPU 928B using STEP 5 ladder/FBD/STL.
• How to verify the screen change in the ProTool runtime simulator.

Prerequisites

Before you begin, confirm the ProTool project on the OP270 compiles cleanly and that you can already download the project and go online with the S5-928B through the OP270's serial port. The screen-change function is purely a configuration overlay on top of an already-working link.

Understanding ProTool Area Pointers

ProTool does not let you bind HMI variables 1:1 to every PLC interaction. Instead, it exchanges blocks of contiguous PLC memory called area pointers. Each area pointer has a fixed meaning and a configurable start address in the PLC:

The job mailbox is the only one we need for screen change. It is a small block of words - typically 4 to 8 words, with word 1 being the job number and the rest being parameters that depend on the job. The OP270 polls the job mailbox each cycle and, when the job number changes, executes the requested job.

PLC Job 51 - Screen Selection

PLC job 51 is the dedicated job that selects the active screen on the OP270. The job is written by the PLC, executed by the panel, and then the job number is reset to 0 by the panel so the job is only fired once.

The job mailbox layout for job 51 is:

The full 32-bit screen number is the ProTool-internal screen index. The most common way to identify it is to open the screen in ProTool and read it from the Screen Number field in the properties dialog. By default, ProTool numbers screens in the order they are created, starting at 1, but the assignment can be edited.

Configuring the Area Pointer in ProTool

1. Open your ProTool project in ProTool/Pro CS.
2. In the project tree, right-click Area Pointers and select New.
3. Choose Job Mailbox from the list.
4. Set the Start address to the start of the mailbox in the S5, for example FW 10. With a width of 4 words the mailbox occupies FW 10 - FW 17.
5. Set the Length to 4 words (8 bytes). This is the minimum to support job 51; if you also plan to use date/time synchronisation (job 40/41) increase to 8 words.
6. Confirm with OK. ProTool shows the new area pointer in red on the area-pointer overview; it turns black once the configuration is consistent.
7. Compile and download the project to the OP270.

From the OP270's point of view, the panel will now poll the configured S5 mailbox on every cycle. The PLC owns the mailbox - the panel only reads it.

Driving the Mailbox from STEP 5 (S5-928B)

Below is a compact STEP 5 STL (statement list) example for the S5-928B that monitors the flag bit F10.0. When F10.0 = 0 the panel shows Screen 1 (AUTO); when F10.0 = 1 the panel shows Screen 2 (MANUAL). The example assumes the job mailbox starts at FW10 and that screen numbers 1 and 2 correspond to the two screens in the ProTool project.

  SEGMENT 1  -- build the screen-number parameter
  L   F 10.0            // screen select bit
  L   KF+0
  !=F  
  JC  M002              // F10.0 = 0 → AUTO_SCREEN
  // MANUAL_SCREEN branch
  L   KH 0002           // screen 2
  T   FW 12             // +2 (low word of screen number)
  L   KH 0000           // high word
  T   FW 14             // +1
  JU  M003
M002  // AUTO_SCREEN branch
  L   KH 0001           // screen 1
  T   FW 12
  L   KH 0000
  T   FW 14
M003  NOP 0

  SEGMENT 2  -- write job number 51 (W#16#0033) into +0
  L   KH 0033           // 51 decimal = 0x33
  T   FW 10             // +0 job number
  L   KB 1              // one-shot trigger
  T   FB 10             // optional: pulse flag for diagnostics

  SEGMENT 3  -- optional: clear the mailbox back to 0 after the panel
              -- has consumed the job (ProTool clears the job word
              -- to 0 itself; this segment is only for PLC-side
              -- housekeeping if you want to log or suppress re-fire)
  L   FW 10
  L   KH 0000
  ==F
  JC  M099
  // wait until the panel clears the job word
  L   FW 10
  L   KH 0000
  <>F
  JC  M099
  L   KH 0000
  T   FW 10
  T   FW 12
  T   FW 14
M099  NOP 0

Three things to check after loading this block:

• The job word (FW10) must oscillate between KH 0033 and KH 0000 on the S5 programming device's status display.
• The screen-number words (FW12, FW14) must hold the value of the active screen at the moment the job is written.
• The flag bit F10.0 should toggle on the S5 PG to confirm the conditional logic.

Detecting a screen change from the PLC

If the application also needs the PLC to know which screen is currently shown (for example, to suppress certain alarms while a manual screen is open), bind the screen number area pointer in ProTool. The panel writes the current screen number into a second S5 area on every screen change, so the PLC can read it back.

Step-by-Step Configuration Checklist

1. Open the ProTool project and add the Job Mailbox area pointer at a free word area (e.g., FW10, 4 words).
2. Confirm the start address in ProTool matches the STEP 5 program.
3. Make sure the screens in question are assigned explicit, stable screen numbers (open each screen, properties → Screen Number).
4. Compile the ProTool project and resolve any warnings about the area pointer.
5. Transfer the project to the OP270 via serial or PC-TTY adapter.
6. Edit the STEP 5 program: build the screen number from the PLC logic, write job number 51 to the mailbox.
7. Download the STEP 5 program to the S5-928B (PLC must be in STOP for the initial load, then RUN).
8. Observe the OP270 - the active screen should follow the PLC flag.
9. Run the ProTool runtime simulator in parallel to validate the job mailbox payload off-line.

Using the ProTool Runtime Simulator

ProTool ships a runtime simulator (ProTool/Pro RT) that runs the compiled project on a PC. The simulator can be configured to read the job mailbox from a connected PLC or, with a small extension, from a simulated memory block. This is useful to validate job 51 without disturbing the live system:

1. From ProTool CS, choose Tools → Start Runtime.
2. Select Simulation with PLC tags if you want the simulator to read from a tagged file (CSV) that contains the mailbox content.
3. Toggle the simulated flag, observe the screen change on the simulator screen.
4. Repeat for every screen you want to verify.

Verification

Troubleshooting Matrix

Reference: Area Pointer Width and Job Number Reference

The list above is a subset of the most-used jobs. For the full list, see the Siemens support entry 24229825 (formerly accessible at support.automation.siemens.com/WW/view/en/24229825) and the ProTool/Pro online help under Communication → Area Pointers → Job Mailbox.

Notes on S5-928B Specifics

The S5-928B belongs to the higher-end S5-135U/155U CPU family. Two practical points when wiring it to an OP270:

• The S5-928B communicates natively over the AS511 serial protocol (TTY, 20 mA current loop) on its programming port. The OP270 in this configuration must run the AS511 protocol variant in its connection settings.
• If the OP270 is wired to the S5-928B over PROFIBUS-DP using a DP/AS-i or external gateway, the panel sees the S5 as a PROFIBUS-DP slave and the area-pointer addresses must be re-mapped to PROFIBUS I/O. Job 51 still works the same way; only the transport changes.

For STEP 5 programming, the KH (constant hex) and KF (constant fixed-point) formats are the most efficient way to load the job number and the screen number. Avoid KY for jobs - the byte order would swap and ProTool would not interpret the word correctly.

FAQ