Vector and ST506 (was ST506 and-or ST412 hard drives - test-drb@ccmp.vtda.org

1 Jan 2008

...
  Date: Tue, 1 Jan 2008 09:24:54 -0500
 From: "Andrew Lynch" <lynchaj at yahoo.com> 
...
  Yes you are correct.  I spoke with the VG engineer to
get more details on
 "constant index" and you described it exactly.

 Apparently there is a cut and jumper modification which can be made to the
 ST506 circuit board to bring the /INDEX signal to the interface constantly
 even when the drive is not selected. 
I grabbed my ST506 off the shelf--it looks to be a very simple 
change.  The control outputs are driven by 7438s and the cut and 
jumper on IC 6D looks to be easy to do.  If you were loathe to modify 
the PCB on the 506, the index signal is also available on TP4

FWIW, there's a schematic of the ST506 on Bitsavers also.

Cheers,
Chuck