Automating Manufacturing Systems - Process Control and ...

plc advanced functions - 16.11
program file 2
A
JSR (Jump subroutine)
Program File 3
Input par N7:0
Input par 123
Return par N7:1
A separate ladder logic program is stored in program file 3. This feature
allows users to create their own functions. In this case if A is
true, then the program below will be executed and then when done
the ladder scan will continue after the subroutine instruction. The
number of data values passed and returned is variable.
SBR (subroutine arguments)
Input par N10:0
Input par N10:1
program file 3
If B is true the subroutine will return and the values listed will be
returned to the return par. For this example the value that is in
N10:1 will eventually end up in N7:1
B
RET
Return par N10:1
Figure 16.11
Subroutines
The for-next loop in Figure 16.12 will repeat a section of a ladder logic program 5
times (from 0 to 9 in steps of 2) when A is true. The loop starts at the FOR and ends at the
NXT function. In this example there is an ADD function that will add 1 to the value of
N7:1. So when this for-next statement is complete the value of N7:1 will 5 larger. Notice
that the label number is the same in the FOR and NXT, this allows them to be matched.
For-next loops can be put inside other for-next loops, this is called nesting. If A was false
the program would skip to the NXT statement. All 5 loops will be completed in a single
program scan, so a control word is not required. If B is true the NXT statement will no
longer return the program scan to the FOR instruction, even if the loop is not complete.
Care must be used for this instruction so that the ladder logic does not get caught in an
infinite, or long loop - if this happens the PLC will experience a fault and halt.