Start with this checklist if the PLC has a blinking red error lamp and you suspect an IO-Link fault.
- Verify that access rights for each port are returned to PLC control.
- All field devices are connected to the IO Link master(s).
- Each field device is connected to the assigned channel.
- TIA Portal’s IO-Link master submodules are properly assigned.
Log into the SIG300 using a USB cable and then point a browser to 169.254.0.1.
The default password for the “Service” login is “servicelevel”.
The default password for the “maintenance” login is “main”.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Troubleshoot It → Networks
Difficulty: 
Technician — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 06 Mar 2026
Figure 1: Hardware used for the IO-Link troubleshooting along with a Siemens comms error.
Access Rights Errors in the IO-Link Master
Incorrectly configured access rights are a painful source of self-induced errors. As part of troubleshooting we log into the IO-Link master, change the port to local control and then forget to return control back to the PLC.
For example, consider the SICK SIG300 Profinet IO-Link Master. The access rights are shown in Figure 2. The last four boxes in the first column must be selected for unrestricted PLC access.
Figure 2: Image of the SICK SIG300 IO-Link Master port access rights configuration.
Tech Tip: The access rights are assigned on an individual port basis! Check them all.
Physical connection Errors in the IO-Link Master
Unlike a simple Ethernet switch, IO-Link ports are not generic. Instead, each port is dedicated to a specific field device. This is no different than the physical connections to rack and slot IO such as the ET 200SP with each wire connecting to a specific I/O pin.
In the fog of 3 AM troubleshooting it is very easy to:
- Swap sensors
- Leave a sensor disconnected
- Fail to tighten an M12 connector leading to intermittent connections
Software Configuration Errors
From TIA Portal, the IO-Link master is handled using a rack and slot configuration. This is shown in Figure 3, where we see the top-level SICK SIG300 representation (left). The “modules” are shown in the device overview (right). For this configuration:
- Port S1 is configured as a digital output
- Port S2 is dedicated to the Banner SD50 display
- Port S3 unused
- Port S4 is dedicated to the SICK W10 distance sensor
- Ports S5 to S8 are unused
Note that each port is mapped to a specific (fixed) memory address within the PLC. This is why the IO-Link field devices connectors cannot be swapped. The PLC would respond with gibberish if, for example, the W10 and the SD50 were swapped.
Figure 3: Configuration of the SICK SIG300 from within Siemens TIA Portal.
Parting Thoughts
Leave a comment if you encounter a problem with your IO-Link system.
- What were the symptoms?
- How did you solve the problem?
- What were the dead ends that consumed time?
Best wishes,
APDahlen
Continue Exploring Industrial Control Systems
If this discussion was helpful, you may also want to explore:
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- Guide to Troubleshooting Industrial Control and Automation Equipment
- Understanding the IO-Link IO Device Description (IODD) File
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About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.