Large Point I/O rack communication issue

I was recently called into a “consumer goods” plant to look at a communication problem with Point I/O. The night before I was told all the Point I/O, which had been successfully tested overseas, was no longer working. However, when I arrived on-site the problem was limited to just four of the many racks.

The on-site engineer described the problem as a lack of communications between the Point I/O and ControlLogix. The symptoms included flashing red network indicators on the Point I/O adaptors, and error messages displayed in the RSLogix 5000 I/O tree. Initially they suspected the Point I/O Ethernet adaptors had failed, but when they were swapped with known good units the problem persisted. They then called Rockwell Tech Support and were told (among other things) that they needed additional 1734-EP24DC power modules based on the vertical installation of the Point I/O, but even with the additional modules installed the problem remained.

It was at this time they began to consider the large number of I/O modules in the Point I/O racks not communicating. Each of these racks had forty or more Point I/O modules, several more than the many working racks. To test the theory that the problem had to do with the number of I/O modules, they re-sized one of the racks down to 34 modules, changed the RSLogix 5000 program to match, and when they put the system back into run mode the new smaller configuration worked.

Below is a picture of the Point I/O rack mentioned above with several modules removed to reduce it’s size to 34 modules (the picture is rotated horizontally to better fit on this page:)

Point I/O

Next, we headed to the engineering office to join a pre-scheduled conference call with Tech Support. After introductions were made and the history of the situation was repeated, Tech Support began going down their list of likely causes. When the question of firmware arose, the customer said they didn’t upgrade any firmware because the Rockwell Compatibility Website clearly showed which firmware was compatible with which versions, and that the newest firmware for the Point I/O adaptor module and the 1756-EN2T Ethernet Bridge Module (acting as the Ethernet I/O Scanner) were not compatible with the version of RSLogix5000 they were using on this project.

It was at this point when a Tech Support lead, who was new to this situation, spoke up. He informed the customer and techs that they had been misreading the compatibility site and that the versions listed are not the “compatible version,” but the “lowest compatible version” (we later went back and confirmed that the compatibility website does not state anything of the sort.) He also went on to say that they must upgrade the firmware, especially on the 1756-EN2T, because it is a known issue that it will not communicate to racks of Point I/O with forty or more I/O modules without the latest firmware.

The old Dual Port Ethernet Device issue with Single Port Ethernet Devices.

When Rockwell released it’s new products with dual Ethernet ports, I began to receive calls from customers using these new products to communicate with the older single port devices. What they would experience, and I confirmed in the lab, was the single port remote Ethernet I/O would repeatedly connect and disconnect from the parent PLC if a new dual port Ethernet device was being used as the master/scanner. The fix for this condition was to simply flash the firmware on the new dual port Ethernet device to the latest version, which also turns out to be the same fix for the above problem with large remote racks of Ethernet Point I/O.

If you’ve run into similar Ethernet I/O issues, or would like to add your own insights to the above, please use the below “leave a reply” field to add your comments to this article.


Shawn Tierney
Automation Consultant
Insights In Automation

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