Your cable isn't technically wrong, but its not enough to do what it
needed. You have a standard mac serial cable, but not the "high speed"
version that is needed for OpenFirmware IO.

It looks like your problem is that you don't have RTS wired to DTR on
the DB-25 side, and you have GPi floating instead of connected to CD. See:

http://www.biology.utah.edu/goldenberg/software/serial.html


According to Apple, the GeoPort capable serial ports are supposed to be
able to handle 2Mbps+.

http://pinouts.ru/data/apple_geoport_pinout.shtml
http://pinouts.ru/data/MacRs422_pinout.shtml

More than you wanted to know about mac serial pinouts:

http://www.dogparksoftware.com/MacLoggerDX/Manual/Pages/cables.html

If you have a mini9 cable instead of mini8, make sure you don't connect
pin 9 to anything.