Configuring WinCC I/O Field Limits for 0-10 Bar Pressure Tags

Configuring WinCC I/O Field Limits for 0-10 Bar Pressure Tags

This reference walks through the configuration of an I/O field in a WinCC HMI project to display and modify a pressure setpoint sourced from a SIMATIC S7 PLC programmed with SIMATIC Manager (STEP 7 V5.x). It covers the exact Process Value and Tag Process Value fields of the I/O field dialog, the tag-level low/high limit settings that enforce the 0-10 bar ceiling, the analog input scaling math that links the SM 331 raw count to engineering units, and the typical fault paths that prevent an operator from writing a new setpoint at runtime.

Overview

An I/O field in WinCC flexible is the only object that both reads and writes a process tag at runtime. The configuration of a pressure setpoint involves three distinct layers, all of which must be aligned:

1. PLC tag defined in the STEP 7 symbol table (REAL in a data block).
2. HMI tag defined in WinCC flexible that points to the PLC address and carries the engineering-unit limits.
3. I/O field configured on the screen that links to the HMI tag, with a mode (Input, Output, or Input/Output) and a format pattern.

If any of these three layers disagree on data type, address, or access direction, the operator will see "####", a frozen value, or a write that never reaches the PLC.

Prerequisites

• SIMATIC Manager with STEP 7 V5.5 or higher.
• WinCC flexible 2008 SP5 (or WinCC V7.x for PC Runtime) with the same S7 connection configured.
• S7-300/S7-400 CPU (e.g. 315-2 PN/DP) and an SM 331 analog input module (e.g. 6ES7 331-1KF02-0AB0) wired for a 0-10 V or 4-20 mA pressure transmitter.
• Pressure transmitter with 0-10 bar nominal range (e.g. SITRANS P, Siemens 7MF1565).
• An HMI connection of type S7ONLINE set up between the WinCC project and the CPU.
• Online access to both the PLC and the HMI for downloading and verification.

Understanding the I/O Field Dialog

The I/O field is the central object used to enter and display process values in WinCC flexible. The settings for its position, geometry, style, color, and font are configured in the Layout tab; its tag binding and operating direction are configured in the General tab. In WinCC Unified, the same concept exists as the I/O field (RT Unified) object.

Mode (Input / Output / Input/Output)

For a pressure setpoint, the I/O field must be configured as Input/Output. If the field is set to Output, every entry attempt fails silently and the value reverts to whatever the PLC currently holds.

Process Value vs. Tag Process Value

Two distinct identifiers appear in the I/O field configuration dialog under WinCC flexible:

• Process Value - the symbolic name or absolute address on the PLC side. In classic WinCC flexible, this can be entered as an absolute address (for example, DB 101 DBD 0) or as a fully qualified symbolic name (for example, "Pressure_SP" from the STEP 7 symbol table). It is the address that the HMI reads from / writes to on the controller.
• Tag Process Value - the HMI-internal tag name visible in the WinCC flexible project tree. The HMI tag is mapped to the PLC address through the connection. Renaming the HMI tag does not change the PLC address, only the name that WinCC flexible uses internally.

Step-by-Step Configuration

Step 1 - Define the PLC Tag in STEP 7

1. Open SIMATIC Manager and your S7 project.
2. Open the S7 program, then Symbols.
3. Add a new symbol:
   

   Symbol	Address	Data Type	Comment
   Pressure_SP	DB 101.DBD 0	REAL	Pressure setpoint, 0.0 - 10.0 bar

4. Open DB101 and confirm that DBD0 is declared as REAL with a default value of 0.0.
5. Save, compile (S7 Program → Compile and Download Objects), and download the hardware configuration to the CPU.

Step 2 - Scale the Analog Input to 0-10 Bar

For a 0-10 V sensor on the SM 331, configure the measuring range as Voltage ±10 V (DIP switch setting "E" on most 8-channel SM 331 modules). For a 4-20 mA sensor, configure the range as 4-Wire Transducer or 2-Wire Transducer (settings "B" or "C") and wire the loop accordingly.

The SM 331 returns a raw count of 0 to 27648 for the unipolar 0-10 V / 4-20 mA range. Convert raw count to engineering units with the following linear formula:

Pressure_bar = (Raw_Count / 27648.0) * 10.0

Implement the scaling in a STEP 7 FC (here FC100). Example Structured Text (SCL) source:

// FC100 - Scale SM 331 raw value to pressure in bar
// Inputs : IW512 (INT)   - raw AI value from SM 331 channel 0
// Outputs: DB101.DBD0 (REAL) - scaled pressure, 0.0 .. 10.0 bar
// Temp   : #tScale (REAL)

#tScale := INT_TO_REAL(IW512) / 27648.0 * 10.0;

// Optional: clamp to 0..10 bar in case of sensor over-range
IF #tScale < 0.0 THEN #tScale := 0.0; END_IF;
IF #tScale > 10.0 THEN #tScale := 10.0; END_IF;

DB101.DBD0 := #tScale;

If you ladder-logic only, use FC105 (SCALE) from the STEP 7 Standard Library → TI-S7 Converting Blocks:

|         CALL  FC  105                                  |
|         IN   :=IW512                                  |
|         HI_LIM:=10.0                                  |
|         LO_LIM:=0.0                                   |
|         BIPOLAR:=FALSE                                |
|         RET_VAL:=MW200                                |
|         OUT  :=DB101.DBD0                             |

Step 3 - Create the HMI Tag in WinCC Flexible

1. Open the WinCC flexible project.
2. In the project tree, right-click Tags → New Tag.
3. Configure the tag properties:
   

   Property	Value
   Name	Pressure_SP
   Connection	S7ONLINE (your S7 connection)
   PLC address	DB 101 DBD 0
   Data type	REAL
   Length	4 bytes
   Acquisition cycle	1 s
   Access method	Absolute access (recommended for first commissioning)

4. Open the tag Properties dialog and switch to the Limits section:
   

   Property	Value
   Lower limit	0
   Upper limit	10
   Start value low (scaling)	0
   End value high (scaling)	10
   Scaling	Linear

5. Compile and download the HMI project.

Step 4 - Configure the I/O Field on the Screen

1. Open the screen that contains the pressure indicator.
2. From the toolbox, drag an I/O field onto the canvas.
3. In the configuration dialog, fill in the General tab:
   

   Field	Entry
   Tag	Pressure_SP
   Mode	Input/Output
   Process Value	Pressure_SP (PLC-side symbolic name) or DB 101 DBD 0 (absolute address)
   Tag Process Value	Pressure_SP (HMI-internal tag name)
   Format pattern	999.9 (one decimal place)

4. Switch to the Limits tab and confirm:
   

   Property	Value
   Low limit	0
   High limit	10

5. Switch to the Appearance tab and select 3D or Flat as required.
6. Click OK, save the screen.

Step 5 - Optional: Add a Bar Graph for Visual Indication

For a richer operator view, add a Bar object on the same screen:

1. Insert a Bar from the toolbox.
2. Assign the same tag (Pressure_SP).
3. Set the bar range to 0 (low) and 10 (high) bar.
4. Color the bar segments: green 0-7 bar, yellow 7-9 bar, red 9-10 bar.

Verification

1. Download the WinCC project to the HMI panel (or start WinCC RT).
2. On the HMI, navigate to the pressure screen.
3. Confirm the I/O field displays the current value of DB101.DBD0 from the PLC.
4. Click the I/O field, enter 7.5, press Enter.
5. In STEP 7, open a VAT or watch table on DB101.DBD0 and confirm the value is now 7.5 (REAL: 0x40F00000).
6. Try entering 15.0 - the HMI should either reject the value with an error beep, or clamp it to 10.0 depending on the limit-behavior setting.
7. Force DB101.DBD0 := -1.0 from STEP 7 to verify the lower limit. The HMI should display 0.0.
8. Cycle power on the PLC to verify the setpoint retention behavior (see Edge Cases below).

Troubleshooting Matrix

Edge Cases and Field Tips

Negative Pressure or Compound Ranges

For a sensor measuring -1 to +10 bar, configure the SM 331 to bipolar ±10 V (range "D" on most modules). The raw count is -27648 to +27648. Use bipolar scaling:

Pressure_bar = (Raw_Count / 27648.0) * 10.0

Set FC105 (SCALE) with BIPOLAR := TRUE and LO_LIM := -1.0, HI_LIM := 10.0. On the HMI tag, set the lower limit to -1 and upper limit to 10.

4-20 mA Sensors with Live Zero

For a 4-20 mA sensor, raw count is 0 at 0 V / 0 mA and 27648 at 20 mA. The 4 mA offset does not subtract from the raw count; the standard FC105 / FC106 scaling formula already covers it. If the sensor is calibrated to 0-10 bar with 4 mA = 0 bar, the same formula applies:

Pressure_bar = (Raw_Count / 27648.0) * 10.0

Reject vs. Clamp Limit Behavior

WinCC flexible offers two limit behaviors on the I/O field:

• Reject (default) - the HMI rejects the input and emits an error tone. The PLC value is unchanged.
• Clamp - the HMI accepts the input but writes the limit value (0 or 10) to the PLC.

Configure this in the I/O field properties → Limit behavior. Use Reject for setpoints to prevent accidental over-pressurization due to clamping.

Persisting the Setpoint Through Power Cycle

Data blocks in STEP 7 are volatile by default. To retain DB101.DBD0 across a power cycle:

1. Open DB101 properties in STEP 7.
2. Check the Non-Retain / Retain setting for the area covering DBD0.
3. Alternative for non-retentive CPUs: use SFB 81 (RDP) and SFB 82 (WRIT_DBL) to store the value to a load memory area.

WinCC V7 vs WinCC flexible vs WinCC Unified

For the modern I/O field object in WinCC Unified, refer to the official TIA Portal help: I/O field (RT Unified) - WinCC Unified.

Sample VAT Verification (STEP 7)

Create a Variable Table (VAT) with the following rows to verify end-to-end behavior:

Open the VAT online, force IW512 := 13824 (which equals 5.0 bar after scaling) and confirm the HMI bar reads 5.0. Then enter 8.0 in the HMI I/O field and confirm DB101.DBD0 := 8.0.

FAQ