Converting fahrenheit to celsius

I am currently running into an issue where I need to convert my fahrenheit set point to celsius. I am storing the values in BCD and need accuracy to the tenth. If I have a value of 682 this is actually 68.2 degrees F. It should be converted to 20.0 degrres C.

Can anyone please help?

C = (F - 32)/1.8

If you are using a factor of ten to get an assumed decimal place, just mulitply the whole conversion by ten:

Cx10 = (Fx10 - 320)/1.8

Fx10 = 682
Cx10 = 201

If you can't handle the decimal in your PLC with a floating point, use another factor of 10:

Cx10 = ((Fx10 - 320)*10)/18

Fx10 = 682
Cx10 = 201

Fx10 = 2120
Cx10 = 1000

You don't identify the PLC model you are using, so the details will vary, but this can be done with the simple four function math available in most PLCs.