Calculating The Out-Of-Range Limits - Allen-Bradley SLC 500 1746–FIO4I User Manual

Chapter 5
Writing Ladder Logic
5–6
Calculating the Out–of–Range Limits
Use the following equation to compute low and high out–of–range limits.
Input value = (scaled value – offset) / slope
low limit: (275 – 100) / (0.0977)
high limit: (300 – 100) / (0.0977)
Ladder Logic
We present two examples for programming the processor.
The first example uses standard math instructions available in any SLC
500 processor. This ladder logic prevents a processor fault by unlatching
the mathematical overflow bit S2:5/0 before the end of the scan.
The second example uses the scaling instruction (SCL) available in SLC
5/02 (and later) processors. The rate parameter is calculated by multiplying
the slope by 10,000. If the slope exceeds 3.2767, you cannot use the SCL.
rate = (400/4095) x 10,000 = 977
Standard Math Example
Rung 2:0
LES
Check for below range
LESS THAN
Source A
Source B
Rung 2:1
GRT
Check for above range
GREATER THAN
Source A
Source B
Rung 2:2
Scale the analog input
Multiply by the
scaled range
Clear fault bit
from overflow
Divide result
by input range
Add offset
N7:0 contains
process pressure
Rung 2.3
1750 counts
2750 counts
(The slope is 0.0977 so you can use the SCL instruction.)
Below-range
flag
I:3.1
1750
Above-range
flag
I:3.1
2750
MUL
MULTIPLY
Source A I:3.1
Source 400
Dest N7:0
S2:5/0
DDV
DOUBLE DIVIDE
Source A 4095
Dest N7:0
ADD
ADD
Source A N7:0
Source B 100
Dest N7:0
END
B3/0
(L)
B3/1
(L)
(U)