Find and Fix I/O Problems

Diagnostic digital modules detect faults and report back.


System designers and end users want to minimise the time it takes to find and fix field device and power supply problems. Using Rockwell Automation FLEX I/O eliminates long wiring runs and reduces terminations per point. But monitoring the condition of I/O and identifying the source of any system failure can be a challenge. Detecting open wire, short circuit and reverse polarity of external power can be tough if you're not right next to the device.

New FLEX I/O diagnostic digital modules (1794-IB16D and 1794-OB16D) detect such faults and report them back to the controller. Once there, they can be monitored via operator panels or a human machine interface (HMI) located remotely.


The FLEX diagnostic modules detect faults such as open/missing input or output field devices or wiring, shorted output field devices, shorted input or output wiring and reverse polarity of user power supply wiring. While they do not check for the presence of the user-supply voltage, this is most easily done by directly wiring the supply voltage to an input channel.

Troubleshooting an Ethernet/IP System

When troubleshooting any EtherNet/IP™ system, you must have a logical order to troubleshooting. The order for each troubleshooting issue is dependent on the details for that issue. This TechTip will list and detail, in order of priority, the troubleshooting steps for EtherNet/IP systems.

When troubleshooting Ethernet/IP systems, there are potentially many possible troubleshooting scenarios. In general, there are three types of problems:

• It does not work at all
  Examples: an I/O node is not connected to a switch (missing cable), cannot ping a node, all MSG instruction to a specific Allen-Bradley® 1756-ENBT ControlLogix® EtherNet/IP Module fails.
• It works but is too slow
  Example: A resource (PC, controller, 1756-ENBT) in the system is overloaded.
• It works but fails intermittently
  Examples: The ControlLogix controller outgoing unconnected message buffer is being exceeded, Noise is causing an I/O connection to be lost.

Resolving the Problem
To resolve any of the above problems, you need to know where to look and what to examine. Check all of the following carefully as possible sources of the problem:

• slow PC or slow application running on the PC
• node configuration (IP address, etc.)
• congested network (lots of traffic such as broadcast)
• slow network (satellite or frame relay)
• misconfigured switch or router
• Logix controller resources
  - controller processing capability (5550, 5555, 5563)
  - timeslice for communications
  - cached message queue (32 max)
  - unconnected outgoing buffers (40 max)
• insufficient processing capability in an ENBT module
• duplicate IP addresses
• defective Ethernet network hardware (e.g., cable, switch port, or ENBT module)
• web server diagnostics or RSLinx® diagnostics