After reading about protocol to talk with a PLC (from the Communication with Unitronics PLC" on the Technical Library page), I tried sniffing the packets sent out by the download tool to the PLC. To my surprise, the payload had some extra information at the start of the messages.
In looking around at the Unitronics site, I've seen mention of an "Ethernet header" in several postings on this forum as well as on the archived forums. There's also mention of a document describing it, but the title mentioned in http://www.unitronics.com/forum/topic.asp?TOPIC_ID=2982, VisionCommunication, is nowhere to be found on the Unitronics technical library page.
I was able to use google to locate a document with a file mentioning "Ethernet header" at http://www.unitronics.com/data/uploads/%20utilities/VisionCommunication.pdf Is this document no longer current? If it is, why isn't it on the Technical Library page? If it's not current, what's the best place to find a definition for the "Ethernet header"?