classicladder index-enable

I'm rebuilding a 2004 shenyang lathe in lismore nsw Australia.  Some parts like the turret encoder didn't make it through 4metres of water. I've replaced it with a 5000line incremental encoder (a b z) I had laying around. I can read in the encoder value as an s32in to classic ladder,  but I'd like to reset index-enable as it clicks over so the count value is easier to decipher for tool positions.  This would be easy if I could link index-enable into classic ladder as an output, but it only seems to be available as an input.(I can't set it from classic ladder).
Any ideas people? Even if it could be done in hal?

Yes I thought that, and index-enable is a bidirectional pin, but it seems I can only net it to classic ladder as an input (read, can't set the value)

You can feed the classic ladder output through the
tristate_bit component to set index enable true

Thanks!