SITRANS T3K PA Inputs Showing Zero in HW Config
Category: Industrial Networking / PROFIBUS PA
Technology: Siemens S7-300 / SITRANS T3K
Article Type: Troubleshooting & Technical Guide
Overview
During the commissioning of a SITRANS T3K PA on a PROFIBUS segment, you may encounter a scenario where the hardware appears healthy (no "SF" or "BF" LEDs on the PLC), but the input data remains at 0x00. This guide addresses the common disconnect between the physical fieldbus data and the PLC memory registers.
Method 1: The Quick Fix (Peripheral Monitoring)
The most common cause is that the PLC addresses assigned to the SITRANS T3K fall outside the Process Image Partition (PII). By default, many S7-300 CPUs only update the first 128 or 256 bytes automatically.
Procedure: How to View Live Data
Open HW Config: Open your Siemens Step 7 project and enter the Hardware Configuration editor.
Locate Device: Click on the SITRANS T3K PA station on the DP/PA bus.
Monitor Periphery: Right-click the address area in the bottom detail view and select Monitor/Modify.
Switch View: Click the Monitor (glasses) icon. Crucially, ensure the display mode is set to "Periphery" (PI) rather than "Process Image."
Check Status Byte: Look at the 5th byte of the data string. If it is 0x80, the device is communicating correctly, and the "zero" you saw previously was simply a memory refresh issue.
Method 2: Direct Addressing in PLC Code
If the data is visible in HW Config but remains zero in your logic (LAD/FBD/STL), you must bypass the standard process image.
Standard Read (Fails if outside image):
L ID 256Direct Peripheral Read (Recommended):
L PID 256
Using the "P" (Periphery) prefix forces the CPU to pull data directly from the bus during that specific instruction rather than waiting for the automatic update at the start of the scan.
Method 3: Ensuring Data Consistency (SFC14)
Since the SITRANS T3K sends 5 bytes of data (4 bytes for the Real value + 1 byte for Status), a standard move command might split the data across two different bus cycles.
Recommended Approach:
Use system function SFC14 "DPRD_DAT" to read the 5-byte block. This ensures the temperature value and its status byte are "consistent"—meaning they were captured at the exact same moment.
Method 4: Physical Layer & Address Verification
If peripheral monitoring still shows zeros, check the following hardware settings:
Bus Speed: Ensure the DP side is set to 45.45 kbps (standard for many DP/PA couplers).
Termination: Verify a 100 \Omega resistor and 1 \muF capacitor are installed at the furthest end of the PA segment.
Address Conflicts: PA devices often default to 126. Ensure your T3K has a unique address (typically 3 or higher).
Segment Voltage: Use a multimeter to confirm the PA segment voltage is between 9V and 32V DC.
Common Hardware / Product Families
Sensors: SITRANS T3K PA (Profile 3.0)
Controllers: SIMATIC S7-300 (e.g., CPU 315-2DP)
Couplers: Siemens DP/PA Coupler (6ES7157-...)
Software: STEP 7 V5.x or TIA Portal
Verification Checklist
Is the "Monitor" mode in HW Config set to Periphery?
Does the Status Byte (5th byte) show 0x80 (Good)?
Is the GSD file version matching the device firmware?
Are the PLC addresses higher than the CPU's default Process Image size?
FAQ Section
Q: Why does the PLC show no errors if the data is zero? A: The PROFIBUS DP/PA connection is likely healthy at the "link" level, but the CPU is simply not configured to move that specific data into its active memory image.
Q: What if the Status Byte is 0x00? A: This indicates the device is either in "Initial Value" mode or "Out of Service" (OOS). Check the device configuration via PDM or acyclic messaging.
Q: Can I increase the Process Image size instead of using PID? A: Yes, in the CPU "Properties" under the "Cycle/Clock Memory" tab, you can often increase the size of the Process Image, provided the CPU hardware supports it.
Next Steps
GSD File Management – Ensuring correct device descriptions.
Troubleshooting Bus Termination – Deep dive into RC network requirements.
S7-300 Peripheral Addressing – Understanding memory limits.
Need further assistance? Contact our technical support team at [email protected] or visit the Siemens Industry Online Support portal.