Hi Phil ,
if the board you have is like the one in the first post
you can unplug the EL817 opto-isolator from the socket and connect what was the diodes cathode connector to the transistors collector connector
as in the picture in post 217 on page 19
the wire link shown connects the socket terminals 2 & 4 together
I've only seen the 6N137 used on single axis board
http://media.digikey.com/pdf/Data%20...7%20Series.pdf
the faster 6N137 used for the step and direction signals and the EL817 for the enable input if used
if the 6N137 LED is wired the same way with the anode (pin 2) is connected to +5V
you will need a wire a link from terminal 3 to terminal 6
(some detailed pictures of your board could help to trace the connections)
John