Mesa 5i25+7i85S+7i66

Did you verify these three things:

1. A proper connection between the 7I66 and the 7I85S
2. Correct firmware on the 5I25 (5I25_7i85sx2.bit)
3. LinuxCNC (or at least the driver and read/write thread) running

To which I would add

4. 5V power to the 7I85S (yellow light on)

There is no separate driver for sserial, its built into hostmot2
There is no need to update anything on the 7I66

Does the serial port pin numbering on the 7i66 start at the top or bottom?

That would be pin 8 (this is a standard not something 7I66 specific)

If you use a standard CAT5/CAT6 cable with EIA-568B colors (and it looks like you are)
you can use the table in the 7I85S manual to terminate the wires in the screw terminal plug