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SITRANS T3K PA input values remain zero

PUBLIC ARTICLE

SITRANS T3K PA input values remain zero in HW Config [C137]

Quick Fix

Likely Fix: Ensure you are monitoring the Peripheral Inputs (PI) directly rather than the Process Image Partition. In Siemens Step 7, use the "Monitor/Modify" tool with the address prefix PIW or PID to bypass the CPU scan cycle limitations.

Error Message

Observed Behavior:
The SITRANS T3K PA is visible in the hardware configuration and the PLC shows no "SF" (System Fault) or "BF" (Bus Fault) LEDs, but all 5 bytes of the input data remain at 0x00 when viewed in a variable table or logic.

Related:

When This Happens

This typically occurs during the commissioning phase of a PROFIBUS PA segment. The hardware is physically connected, the DP/PA Coupler is active, and the GSD file is correctly installed, but the data does not seem to "flow" into the PLC registers.

Why It Happens

In many S7-300 configurations, the default Process Image (PII) is limited (often to 128 or 256 bytes). If the SITRANS T3K PA addresses fall outside this range, the CPU does not automatically update those values at the start of the scan. Furthermore, monitoring standard memory addresses in a Watch Table without specifying "Periphery" access will display the last known state in the image—which is zero—rather than the live data from the instrument.

What to Check

  • Bus Speed: Verify the DP side is set to 45.45 kbps if using a non-diagnostic coupler, or the appropriate rated speed for your specific coupler model.

  • Termination: Ensure the 100 \Omega resistor and 1 \muF capacitor are at the furthest end of the PA segment.

  • Input Addresses: Note the start address in HW Config (e.g., Input 256).

  • Monitoring Mode: Ensure you are using "Monitor/Modify" directly from the HW Config screen.

Step-by-Step Fix

  1. Open HW Config: Open your Step 7 project and go to the Hardware Configuration editor.

  2. Select Device: Click on the SITRANS T3K PA station on the DP/PA bus.

  3. Monitor Periphery: Right-click the address area in the bottom detail view and select Monitor/Modify.

  4. Enable Status: Click the "Monitor" (glasses) icon. Ensure the display is set to "Periphery" (PI) rather than "Process Image".

  5. Direct Addressing in Code: If you are calling the data in your PLC program (LAD/STL), use:
    L PID [Address]
    instead of
    L ID [Address]
    to force a direct read from the bus.

Engineering Notes & Tradeoffs

The SITRANS T3K PA provides 5 bytes of data: 4 bytes for the Floating Point value (Real) and 1 byte for the Status Byte. If the Status Byte is 0x80, the value is "Good". If the Status Byte is also 0x00, it indicates the device is not communicating or is in an initialization state, even if the bus is physically healthy.

Summary

To fix “SITRANS T3K PA inputs showing zero”:

  1. Verify the address is outside the default Process Image range.

  2. Use the "Monitor/Modify" function directly in HW Config.

  3. Switch the monitoring view from "Process Image" to "Periphery".

Checking the periphery ensures you are seeing live bus data rather than an unmapped memory buffer.

Related Tags

PROFIBUS-PA
S7-300
SITRANS
DP-PA-Coupler
Process-Image

Related Topics

🔍 PRO Preview

The PRO article explores the nuances of the PROFIBUS PA Status Byte, how the DP/PA Coupler handles transparency, and why standard DP diagnostic tools might miss a "frozen" zero value on a PA segment.

Inside the PRO Article

  • Root Cause Variants

  • Deep Technical Explanation

  • Diagnostic Methodology

  • Advanced Fix Strategies

  • Edge Cases

  • Prevention Strategy

PRO ARTICLE

PRO: SITRANS T3K PA input values remain zero in HW Config [C137]

Introduction

When a SITRANS T3K PA reports zeroed inputs despite a healthy bus status, engineers often check the physical layer first. However, the disconnect is frequently found in the data mapping between the PROFIBUS PA profile and the S7 CPU's memory management.

Executive Summary

The primary cause is typically a mismatch between the Periphery Input (PI) and the Process Image (PII). In S7-300 systems, addresses high in the range are not updated automatically. If the Status Byte (the 5th byte) is also zero, the issue may involve the DP/PA Coupler’s inability to pass data due to incorrect GSD configuration or bus timing parameters.

Root Cause Variants

  • Process Image Limit: The CPU scan cycle does not include the PA instrument's addresses because they exceed the OB1_PA_SIZE.

  • Status Byte Masking: The device is in "Initial Value" mode, sending 0.0 with a Status Byte indicating a "Bad" sensor state.

  • Coupler Transparency: The DP/PA Coupler is not properly translating the 31.25 kbps PA signal to the DP master due to noise or incorrect termination (RC network failure).

  • Slot Configuration: The GSD file allows for multiple modules; if the wrong "Value + Status" module is selected, the mapping will be offset.

Deep Technical Explanation

A SITRANS T3K PA follows the PROFIBUS PA Profile. It communicates using a 5-byte data structure.

  • Bytes 0-3: IEEE 754 Floating Point (Real).

  • Byte 4: Status Byte (Quality, Sub-status, and Limits).

If the CPU is a 315-2DP, the hardware gate for data exchange is open, but the software gate (the PII update) is closed. Unlike PROFIBUS DP, PA devices often have longer conversion times. If the device is not yet commissioned or in "Out of Service" (OOS) mode via acyclic communication (Slot 0), the cyclic data output is often forced to zero.

Diagnostic Methodology

  1. Check the Status Byte: Use a Variable Table (VAT) to monitor the exact address of the 5th byte (e.g., PIB 260). If it is 0x80, the device is healthy. If 0x00 or 0x4C, there is a configuration or sensor error.

  2. Verify Bus Current: A DP/PA Coupler typically provides 100mA to 500mA. Use a multimeter to ensure the PA segment voltage is between 9V and 32V DC.

  3. HW Config Monitor: Use the "Monitoring/Controlling" function in HW Config. This tool bypasses the standard CPU image and polls the periphery directly.

Advanced Fix Strategies

  • Direct PI Access: Use L PID and T commands in an FC or FB to ensure every scan gets fresh data, bypassing the PII entirely.

  • SFC14 "DPRD_DAT": For consistent data reading of the 5-byte block. Since 5 bytes is not a standard power-of-two (2, 4), using SFC14 ensures the float and its status byte are read in the same bus cycle.

  • GSD Revision Check: Ensure the GSD matches the firmware version of the T3K. Older T3K units may require specific profile versions (e.g., Profile 3.0).

Edge Cases

  • Duplicate Addresses: PROFIBUS PA devices often ship with a default address of 126. If multiple devices are on the bus at 126, the coupler may show a link, but data collision will result in zeroed inputs at the Master.

  • Coupler "Non-Transparent" Mode: Some older Siemens couplers require a fixed bus speed of 45.45 kbps on the DP side to function.

Prevention Strategy

  • Address Management: Always assign PA addresses starting from 3 upwards, avoiding 126.

  • Standardized Blocks: Use a standard "Analog Input PA" function block that reads 5 bytes and evaluates the status byte before passing the value to the HMI.

Summary

Zero values in a SITRANS T3K PA are rarely a sign of hardware failure if the PLC LEDs are clear. It is almost always a result of monitoring the Process Image instead of the Periphery, or the device being in a non-productive "OOS" state.

Related Errors

Related Tags

Diagnostics
Siemens-Step-7
IEEE-754
Process-Control
Fieldbus
Instrumentation

PRO Article Link: PRO: SITRANS T3K PA input values remain zero in HW Config

Classification

Technology: Industrial Networking / PROFIBUS PA
Type: TS – Troubleshooting
Suggested Space: Troubleshooting Library
Suggested Pro Series:

  • Advanced Debugging

  • Root Cause Analysis

  • Network Diagnostics