5 Big reasons for PROFIBUS or Process Field Bus Failures

PROFIBUS has been around since 1989 and can be a very robust and excellent choice for many process networks if done correctly. I have seen many PROFIBUS networks that have been installed correctly operate for 10+ years with no issues whatsoever. However, just as with any network, if installed incorrectly the network will be prone to failures. Throughout our years of troubleshooting, testing, and repair of PROFIBUS networks we have seen many types of issues that have eventually led to failures. We have documented the reasons for the ultimate cause of these failures. Team Tech’s promise is to quickly find and fix these problems to provide the most reliable network possible. More Uptime GUARANTEED!

1. Improperly Connected Shield

 All PROFIBUS cables have shielding. The exact type of shielding can vary, but typically it consists of an overall foil and braided shielding. It’s important to follow the recommendations of the PROFIBUS International (PI) cable installation guideline. PROFIBUS was designed to work best with both ends of the cable shield properly connected to the equipotential bond. This practice is argued by some to create ground loops. These ground loops are eliminated by creating a properly designed equipotential bonding grid which I will discuss in a future article. Many who are familiar with instrumentation cabling are used to terminating the shield only on one side. For many applications, this technique works fine, but with PROFIBUS it’s important that both ends of the cable shield are properly connected.

2. Terminator in wrong position

 PROFIBUS, as with other RS485 based communications, requires terminations at both ends of the network segment. This termination is accomplished by the end device applying the 5V to the built-in resistors in the connectors. This method of termination is critical in order to prevent the reflection of signals within the network. This reflection can cause multiple network issues and ultimately can cause the network to fail. The severity of the issues depend on the length and speed of the network segment. However, regardless of the network a terminator in the wrong position will present errors. The connectors at each end of the segment must have the terminators in the ON position while all the other connectors must have the terminator in the OFF position. My preferred method for achieving this is to install active termination devices at each end of the segment. These active terminations allow for all the devices on the network to be serviced without affecting the overall network since the terminators on all field devices will be in the OFF position. The minimal cost for these termination devices is easily offset by the additional troubleshooting and robustness that is brought to the network.

3. Incorrectly Set Baud Rate

The speed of the PROFIBUS network (baud rate) is limited by the length of the network segment. The longer the segment, the slower the speeds that are supported. It’s important to adhere to the PI recommendations for the baud rate applicable to your network segment lengths. As a general rule, stick with the slowest speed that is required for your application. The following list is the baud rate limitations as listed by PI.

  • 9.6 kbps = 1200 meter
  • 19.2 kbps = 1200 meter
  • 45.45 kbps = 1200 meter
  • 93.75 kbps = 1200 meter
  • 187.5 kbps = 1000 meter
  • 500 kbps = 400 meter
  • 1.5 Mbps = 200 meter
  • 3 Mbps = 100 meter
  • 6 Mbps = 100 meter
  • 12 Mbps = 100 meter

These lengths can be extended by using repeaters. There are many different types of repeaters; both Procentec and Hirschmann make repeaters that perform well. An important note is that repeaters count towards the 32 total devices allowed per segment.

4. Cable installed incorrectly

 Improperly installed cable is quite common on all networks not just PROFIBUS. There are several factors to consider when installing PROFIBUS 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 PROFIBUS cable is shielded, this doesn’t make the cable immune to EMI. Oftentimes the PROFIBUS 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.

5. Insufficient Grounding/Bonding

The grounding and bonding requirements for PROFIBUS are somewhat unique in regards to the shield connections. PROFIBUS International guidelines recommend that the shields be grounded using the greatest amount of surface area possible. There are several manufacturers that produce easy to use ground clips specifically for PROFIBUS cables. It’s important to make sure that there is properly designed Equipotential Bonding (EB) in place. This EB eliminates the presence of undesired ground loops. PROFIBUS cables must be bonded using the cable clamps as mentioned above at the entrance/exit to every enclosure as well as the entry/exit to each building.

Get It Tested

In order to have the most robust network possible it’s important to follow the guidelines for PROFIBUS cabling installation. Then, prior to putting your network in operation, get it tested. Proper testing and commissioning of the PROFIBUS and other Process Networks ensure that your network will perform as expected for years to come. If you have any questions or would like to have 3rd-party testing scheduled, call Team Tech today at 812-773-8326 where we are proud to offer 24/7 emergency support. To read more about how we’ve help others check out this article.