mesa 7i96s analog spindle

I have a vfd that is controlled by +/-10V signal without a fwd or rev pin. My question is if it still works with the 7i96s because in my understaning it support negative voltage but i needs a fwd and rev pin. so if i use +/-10V then 0V would be 50% max speed and -10V would be 0rpm. Am i right or is it possible so -10V i max rpm ccw and 10v is max rpm cw. 

Its possible but a bit awkward as it requires +10 and -10V power supplies
Is there a separate spindle enable input? This would be necessary as
the power up state would be with the output at -10V.

It might be easier to use an isolated 10V/12V supply so you get a 0-+10V output
and reverse the output with a DPDT relay.
 

I have a +/-15v power supply. is it possible to use that? yes it has a spindle enable input.

No, the maximum voltage from SPIN- to SPIN+ is 20V
You could use +15 and a DPDT relay

so when the relay is activated it acts like a reverse pin and it switches the polarity to the vfd? the vfd is only +/-10V so can i put ends stops in the mesa card so it only puts out 10v maximum or will i do a calibration?

Yes, you would do this with scaling

Note that the external power supply must be isolated.

A small 12V wall adapter would work also