tenthofjune

If you are getting a code 95 error on a 1769-SDN scanner card after extending the network, check your end-of-line resistors.

I recently had a false error code on an Allen Bradley 1769-SDN DeviceNet Scanner card. The card had been operating OK on a very simple network talking to a single device. The network had been extended to add another identical device. Once power had been reapplied, the scanner was showing error code 95. According to the manual, this was:

Flash Update In Progress DO NOT disconnect the module from the network while a flash update is in progress.

Since I hadn’t initiated any flash update, I feared that the module had suffered a spike from some welding that had been taking place nearby. Cycling the power did not help. Removing the connector from the front of the scanner correctly showed code 92 – No DeviceNet power. I could not even connect to the network using a PC and the 1770-KFD adapter. I checked and double checked the wiring, looking for loose connections and even belled out the network.

Everything looked OK, however I had noticed that the network LED on the front of the scanner went red as soon as I plugged the connector back in, suggesting that the scanner was detecting some electrical fault. I removed the new section of wiring, returning the system to its original state and everything was OK again, suggesting a fault with the new wiring. Returning to check the new wiring, I noticed that the end-of-line resistor didn’t look like the one on the other end of the network. The contractor who had wired the network extension had used a different resistor to the correct 120 Ohm one.

Rectifying this cleared the error and the network now functions correctly.

However, the red-herring of the ‘Flash Update In Progress’ error had wasted two hours of my time. So the question is, why does the scanner show this false error under this condition?