I can't find the program, but the ion vacuum gages I was using (which I think is typical) calculates the analog output voltage using:
Vout=(-Vac.Exponent x 1.0 VDC) + (1-(Vac.Mantissa/10))
To convert this back you take the voltage read and subract the fraction (to the right of the decimal) from 1 and multiply by 10. Then you take the whole number part to the left of the decimal and negate it, and it becomes your exponent.
Thus for 1.56 volts you get
10 x (1 - .56) = 4.4 x 10-1 Torr.
For 5.67 volts you get
10 x (1 - .67) = 3.3 x 10-5Torr.
IIRC, I scaled my 0-10V analog input into 0-1000 (voltage in * 100) for an implied 2 decimal places, and then put it into the math register and used a double divide to separate it into fraction and whole number parts. (Assuming you are using Logix 500 SLC and not a ML or Logix 5 or 5K)
MOV SCALED_ANALOG_INPUT S:13
MOV 0 S:14
DDV 100 N7:0 //The value in N7:0 is discarded
MOV S:13 N7:1 //remainder
MOV S:14 N7:2 //unrounded quotient
SUB 100 N7:1 N7:3 //subract from 1 - remember volts is multiplied by 100
DIV N7:3 10.0 F8:0 //multiply by 10 - remember, volts is already multiplied by 100, dividing by 10 produces the desired result.
MUL N7:2 -1 N7:2 //negate the quotient that will become the exponent
XPY 10 N7:2 F8:1 //Raise 10 to the now negative exponent
MUL F8:0 F8:1 F8:2 //put the two parts back together again.
This should give you a real number in F8:2 that is the same as what is displayed on the ION gage controller.
Refer to the specifications of your ion gage to make sure it uses the same formula.
Sorry I couldn't find the actual program, but I hope this helps you.