4 Issues that may be lurking in your PROFINET installation
PROFINET can be an excellent choice for process ethernet-based networks. The ease with which you can deploy and service PROFINET network makes it a very popular choice for many process networks. When done correctly, PROFINET is fault-tolerant and robust protocol. However, too often networks like PROFINET are not installed correctly which leads to premature failure. Some of the most common issues I’ve seen are listed below.
1. Duplex Mismatch
What is a duplex mismatch? I’ve written in greater detail about duplex mismatch in a separate article that can be found here, but I’ll give a general overview here. Most modern ethernet devices can use auto negotiation to configure it’s speed and duplex settings. PROFINET operates at 100 megabits per second at Full-Duplex. If both sides of a network connection are set to auto negotiate there are typically no issues. The mismatch generally happens when one side of the connection is fixed to a specific speed/duplex setting, but the other side is still set to auto. For example, a port on a network switch is configured to be at a fixed 100 Mbps Full Duplex, but the device connected to that port is set to Auto Negotiation. Auto Negotiation uses a set of electric pulses to advertise what capabilities are available to the device. When a device is fixed and the Auto-Negotiation feature is turned off, those pulses are no longer sent. The result is the side that is still using the Auto-negotiation feature doesn’t see the capabilities of the connected device and defaults to 10 megabits per second and Half-Duplex. Think of Half-Duplex like a 2-way walkie talkie. You can talk, but then you must wait for the other side to finish talking to be able to respond. Full-duplex is like a phone conversation, both sides can talk and listen at the same time. When you have a Duplex Mismatch, one side is listening waiting for the other side to finish, while the other continues talking without regard to what the other side is doing. This results in dropped packets, Cyclic Redundancy Check failures, and other errors. PROFINET best practice is both sides set to Auto-Negotiation, but if one side is fixed BOTH sides MUST be fixed to the same speed/duplex setting.
2. Incorrect Cable Type
Since PROFINET is based on ethernet standards, many different types of cabling can be used. It’s often assumed that any ethernet cable is fine to use PROFINET, however this is not the case. Industrial environments are very different from some of the office environments for which standard ethernet cabling is designed. The various Electromagnetic Interference sources, climate, vibration, and other factors make the industrial environment much more challenging. PROFINET International provides a good guideline for choosing the right cable for the application. For example, a cable that is used on a machine that is subjected to frequent flexing and moving should be a stranded cable designed for that purpose, whereas a solid conductor cable is more suited for an application that will be stationary once installed. The use of the wrong cable may work just fine at first, but over time it will fail. This will lead to costly network down time. Proper planning and selection of the cabling up-front ensure a reliable network for years to come.
3. Uncertified Devices
PROFINET International has a program that certifies devices to be used on a PROFINET network. This certification process is quite rigorous and ensures that each device is capable of meeting the requirements set forth by the PROFINET standards. Each device is subjected to several tests including network loading tests, speed tests, auto-negotiation ability, etc. This certification should be a must for any device on your network. Without it you are putting the critical network in danger of failure. Insist that your vendors or manufacturers provide you with the certification of each device used on your network. This certification will include the firmware version that was tested as well. It’s worth noting that a FW revision could effectively change how the device performs on the network. If possible, use the firmware that was certified.
4. Cable installed incorrectly
Improperly installed cable is quite common on all networks not just PROFINET. There are several factors to consider when installing PROFINET cabling including but not limited to: separation from power cabling or other EMI sources, protection for physical damage, strain relief and proper support of the cable, and proper identification. One of the most common problems of improper cable installation is the separation from EMI sources. Although PROFINET cable is shielded, this doesn’t make the cable immune to EMI. Oftentimes the PROFINET cable is run directly next to Variable Frequency Drive (VFD) cables and other EMI sources, sometimes it’s even zip-tied to them. This EMI can wreak havoc on the network causing errors and network failure. PI has a very specific cable installation guideline that, if followed properly, will provide you with a very robust network.
Don’t Just Test, Certify!
While it should be standard practice to test cables prior to use, simple testing alone is not enough. As discussed in this article, there are different levels of testing. In order to ensure that your network is tested to the highest possible standard resulting in greater reliability and ultimately more uptime, Certify your network. Certification of the cables and network ensures that the cable performs exactly as expected. If you have any questions or would like to talk with an engineer about your PROFINET installation, call Team Tech today at 812-773-8326, where we are proud to offer 24/7 emergency support.