I make a converter for some Fanuc encoder models. The ones I support send data at 1.024 mbit/second, and the data block is 77 bits long, with a CRC at the end. it has a start bit, a couple bits of encoder status (like low battery, battery failed, encoder overtemperature) and then a binary count of encoder angle, then a binary count of number of full turns, then if it is an ABS encoder, it gives a 10-bit count of angular position that repeats in 4 quadrants.
The NOn-ABS encoders just give a position count that starts at zero from where they were at power-on, and then this count abruptly jumps to zero when the index position ir reached. (A status but flips at that time.) So, these encoders MUST have battery backup as there is no way to provide motor commutation without proper alignment after the battery power is lost.
The ABS encoders can give commutation info immediately on power-on, so we can run these without the battery.
The older units in this series take a REQ differential signal on one pair, and reply with the serial string on the SD differential pair.
Some of the newer units use the bi-directional differential schem on one pair, but will also respond to the REQ on the other pair.
There is a new series of these encoders which I'm told send data at 2.73 MBits/second, and use just one differential pair for the command
and resopnse. Sounds like you might have this type.
Fanuc definitely has their own proprietary format, but a few encoder makers make linear encoders that follow the Fanuc protocol.
I do think they have a start bit/stop bit scheme, but they seem to have 16-bit charaters between start and stop bits. I just used the
first start bit and then assumed the two crystals were close enough that I could stay in sync.
Jon