TI505 - SF "Scale" ?

I inherited a TI505 control system when my employer bought out another company. Now I'm trying to learn about the system so I can maintain and modify it. I've managed to learn a lot about it from the manuals alone, but I've hit a snag understanding the "Scale" command in a SF program. The TISoft manual simply says "Converts binary inputs into engineering units." One SF program in the PLC converts a 4-20ma signal to 1/2 feet using:

SCALE BIN IN: WX14 SCALED RESULT: V21
LOW LIMIT: 0.0 HIGH LIMIT: 100
20% OFFSET: YES BIPOLAR: NO

I understand what this does, takes the input reading from WX14 (from an 8 channel analog i/o board) and gives me the 1/2 feet in V21, but how does it know how to convert the current reading to the correct 1/2 feet?

The ultimate problem I'm trying to solve here is this reading is the fluid level in a storage tank, the previous company was satisfied to have the reading to the nearest 1/2 foot, but we want to see it to the nearest inch.

If anyone can shed any light on this for me I'd really appreciate it.

Dan

The range of the analog input is 0 to 32000.

If you declare the input to be 20% Offset, you are declaring the input to be 4-20mA. 20% of 32,000 is 6400. The range, in this case, is 6400 to 32000.

The low limit is 0.0 that is, 6400 = 0.0 "1/2-foot's".
The high limit is 100.0 that is, 32000 = 100 "1/2-foot's".

6400 and 32,000 are "raw numbers". They are simply numbers, developed by the analog card, indicating the amount of current that is being read by the analog card.

If the analog card sees 4 mA, the card places 6400 into WX14.
If the analog card sees 20 mA, the card places 32000 into WX14.

The scaling routine looks at the value in WX14, and generates the corresponding "Engineering Unit" and places it in V21.

             LOW LIMIT     WX14     HIGH LIMIT

Raw Number     6400        xxxx       32000

Limits          0.0        ????       100.0



Now... play the "ratio game"...



The ratio of (WX14 - 6400) to (32000 - 6400)



is equal to...



The ratio of (???? - 0.0) to (100.0 - 0.0)



Which is to say...



   WX14 - 6400          ????

   -----------   =   ---------

      25600             100.0



Now, solve for ???? (V21)



                                WX14 -6400

   V21  =  ????  =   100.0  x  ------------

                                  25600

.
.

Thanks

Thanks Terry! That makes perfect sense and with that single bit of information it was a simple matter to change the level reading from 1/2 feet to inches.

Dan