IAS Blog

Ethernet Trouble, Same MAC ID!

Posted on 02/20/2011 at 02:00 pm by IAS
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I was asked to help with Ethernet communications problems between a Phoenix Contact ILC170 controller and a vision camera. The camera speaks Ethernet TCP and uses ASCII commands such as 'do trigger' to scan an image. This can be done with nothing more than the Windows HyperTerminal program. In our case we were using TCP messaging from the Phoenix Contact controller. After some time the customer had the camera working as desired. This machine was to have a total of 4 cameras so the customer started adding more cameras. This is where things starting going bad.

Once the second camera was added communications became intermittent at best but why? The customer, who will be called Peter to protect the innocent, started troubleshooting. After many tests with the same results each time it was time to call in help. Peter was immediately faced with the finger pointing that is typical when interfacing devices from different manufacturers. This is when Peter asked for help from the involved distributors.

Once on site myself and the other distributor confirmed Peters observations. The camera manufacturer insisted the issue was with the implementation on the PLC. To eliminate this as a possibility we started using HyperTerminal for testing and got the same results. We then collected WireShark data captures and sent them to the camera manufacturer. Some time later the camera manufacturer called and asked us to verify the mac id's of the camera's.

Turns out the camera's all have the same MAC ID(11:22:33:44:55:66 if memory is correct)The Ethernet switch doing what it was designed to do was learning what devices were at which ports and directing traffic. With multiple devices claiming the same identity the switch was doing its best. A temporary fix to get the system working was to replace the switch with a hub. Hubs being less intelligent just echo the traffic to all devices and so this worked. The hub should not be considered a permanent fix because of the poor performance when using a hub over a switch and routing issues when network changes are made.

Devices are supposed to have different mac id's. It's a devices fingerprint so it can be identified by the Data-Link layer of the OSI model. Most people I know have never seen devices with the same mac id so its not something we look for. This would be like every house on a street having the same number. Its a bad idea as it confuses the mail man. Another lesson learned..