Dendrobium Goldschmidtianum cane turns purple and shriveled
 

Is this normal, or is this a sign of rot? If so , what should I do?

These are suppose to flower around December. Did it? I found some cultural info on the AOS board, looks like old canes may shrivel and can be removed. Do you have a picture?

"Do not give plants any water unless canes begin to look excessively shriveled. When you feel them they should be firm. Give plants water only if the canes begin to really shrivel. DO NOT give plants any fertilizer at this time. In mid winter buds should appear. After they are halfway developed you may resume watering but just enough to keep the media barely damp. In other words, plants are not in growth in the winter so their water needs are less. Just give them enough water to mitigate the stress of flower production. Resume normal watering in the spring when tepmeratures begin to warm and days lengthen."

Purple usually means the plant is getting its maximum limit of light and or it has purple or red flowers. I wouldn't remove any canes unless they turn completely brown and are completely dead. The plant uses the old bulbs to store food and moisture.