I guess you need this to do the calibration steps mentioned in the manual? I heard (from a person within Anritsu) that these units somtimes loose lock, and when you do these adjustment steps over serial, these errors go away. Any experience re. this?
Anyway, mine is working, but I wanted to be prepred, and was looking for the same data as you, no success. So all I found out was the following:
The RJ connector on the back supports 2 serial lines. The T1678 seems to do nothing else than splitting these up into two separate connectors.
I have done some measurements on the connector, and my conclusion was that, when you look at the connector with the locking nut in upward direction, so pins on the lower side, the two left pins are RxD, the two right pins are TxD. This is given without a guarantee, but it should alow to build up a test cable and figure out the rest. So in other words, which signal relates to serial 1 and which to serial 2, I have not figured out, I stopped there.
It would be nice if you could publish your results here, for teh sake of the community.