Hi Tom,
I do not know anything about EQ8.
I had an EQ6 mount.
The cables I used for EQ6 are the cables I have for sale on IIS and they are TTL232R cables, from FTDI.
There is some info here that may help you.
http://eqmod.sourceforge.net/eqdirect2.htm
You will see that under the heading "EQDIRECT-USB" it states,
"The AZ-EQ6GT and EQ8 mounts have a 5V tolerant 3.3V signaling interface and so TTL232R3V3 or TTL232R can be used".
You will need to read more on the FTDI driver to load for use with EQ8. You also need to check if EQ8 has a D9 serial port wired the same as EQ6.
Sorry I cannot help you more. Maybe you could ask questions on IIS forum.
Kind regards,
John