Configuring S7-1200 KTP400 HMI I/O Field for Motor Control

1. System Overview and Problem Statement

A common entry-level automation task combines a Siemens SIMATIC S7-1200 CPU 1212C AC/DC/RLY with a SINAMIC KTP400 Basic Panel to monitor an analog temperature sensor and start a motor when the measured value exceeds an operator-defined setpoint. The challenge for engineers who are new to TIA Portal is twofold: link a screen input element on the HMI to a real PLC tag, and use that tag inside a compare instruction that drives a digital output.

This reference walks through the full workflow: defining the PLC tag that holds the operator-entered setpoint, creating the HMI variable that mirrors it, placing an I/O field on the KTP400 screen, and writing the comparison logic that energises the motor contactor. All steps are written for TIA Portal V16 or newer and have been validated against the S7-1200 system manual and KTP400 Basic operating instructions available on the Siemens Industry Online Support portal.

2. Prerequisites

Before opening TIA Portal, verify that the following hardware, software, and signal-wiring prerequisites are met.

Confirm the firmware of the CPU by selecting Online → Accessible devices in TIA Portal. The 1212C AC/DC/RLY shipped from 2017 onward carries firmware 4.2 minimum; update to 4.4 if a feature such as OPC UA server or secure communication is required.

3. Hardware Topology and Signal Flow

PROFINET is the only supported path between the S7-1200 and the KTP400 Basic. The 1212C CPU has a single PROFINET port (X1) and the KTP400 Basic has a single PROFINET port (X1). Both must be on the same subnet, and the HMI must point to the PLC's IP address as its connection target.

4. Configuring the PLC Tags in TIA Portal

Open the project tree and select PLC_1 → PLC tags → Add new tag. Create the following symbols. The compare block needs the raw analog value, a scaled engineering value, and the operator setpoint.

Set SP_Temp_C retain so the operator-entered value survives a power cycle. Use a Real because most temperature sensors are scaled to a floating-point value.

5. Implementing the Compare Logic in the PLC

Add a new FC called TempControl and call it from the main OB1. The block performs three jobs: scale the raw analog word to engineering units, compare to the setpoint, and drive the digital output with a small hysteresis to prevent contactor chattering.

// FC "TempControl" (ST source view)
// Inputs:  AI_Temp_Raw  (Int)  - IW64
//          SP_Temp_C    (Real) - HMI setpoint
// Outputs: Motor_On     (Bool) - Q0.0

#AI_Temp_C  := NORM_X(MIN := 0,     VALUE := #AI_Temp_Raw,
                     MAX := 27648.0) * 100.0;   // 0-10 V = 0-100 °C

IF #AI_Temp_C >= #SP_Temp_C THEN
    #Motor_On := TRUE;
END_IF;

IF #AI_Temp_C <= (#SP_Temp_C - 1.0) THEN
    #Motor_On := FALSE;                         // 1 °C hysteresis
END_IF;

If the sensor is 4-20 mA into a 500 Ω shunt, replace the NORM_X range with MIN=0, MAX=27648 and scale to (NORM * 100) - 0 only after offset correction. The 1212C onboard AI is 0-10 V voltage only on channels 0 and 1; for 4-20 mA use an external resistor or a signal conditioner module such as the SM 1231 (6ES7231-4HD32-0XB0).

6. Configuring the KTP400 HMI Connection

Drag a HMI device → SIMATIC Basic Panel → 4" → KTP400 Basic color PN into the project. Open Devices & Networks, draw a PROFINET line between PLC_1.PNIE_1 and HMI_1.PNIE_1, then select the line to create an HMI connection automatically. Verify the IP addresses:

Open HMI_1 → Connections and confirm the partner is PLC_1 with the role IO device. The access point must remain S7ONLINE so that the panel is discoverable from TIA Portal during download.

7. Creating the HMI Variable that Mirrors the PLC Tag

Switch the project tree to HMI_1 → HMI tags and add the following tags. The acquisition mode must be Cyclic continuous at 1 s so that the operator's entry is written back to the PLC immediately.

8. Building the HMI Screen with the I/O Field

Open HMI_1 → Screens → Add new screen. On the toolbox, drag the I/O field element onto the canvas and configure the properties exactly as shown below. The I/O field is the only HMI control that lets the operator write a numeric value into a PLC tag.

Add a second I/O field bound to HMI_AI_Temp in Output mode to display the live temperature. Add a circle or text field that changes color based on HMI_Motor_On by configuring an Appearance → Animation on the element. Save the screen as Overview and set it as the start screen under HMI_1 → Runtime settings → Start screen.

9. Downloading to the PLC and HMI

1. Select PLC_1 → Compile → Hardware (rebuild all) and resolve any errors in the diagnostics pane.
2. Right-click PLC_1 → Download to device → Hardware and software. Accept the default and confirm with the displayed MAC address.
3. Right-click HMI_1 → Download to device → Software (all). Choose the visible HMI_1 node on subnet 192.168.0.0/24.
4. At the panel, accept the runtime transfer prompt. If a transfer password has been set, enter it.

10. Verification and Field Commissioning

Confirm the loop by injecting a known signal at AI2 with a 0-10 V calibrator and reading the displayed value on the panel. The following checks should all pass before handing the system over to operations.

11. Troubleshooting Matrix

12. Extending the Application

Once the single setpoint is working, the same pattern can be reused for proportional control. Replace the on/off compare with a PID_Compact instance (available in STEP 7 Basic V16) and bind the setpoint to SP_Temp_C. Add a second screen with a trend view to log AI_Temp_C at 500 ms intervals; the KTP400 supports up to 32,000 log entries. For remote supervision, expose AI_Temp_C and Motor_On through the integrated OPC UA server of the 1212C FW 4.4 and connect any SCADA client on the same subnet.