Automating Manufacturing Systems - Process Control and ...

plc memory - 14.8
indirect - another memory location can be used in the description of a location.
I:000/[N7:2] -If N7:2 location contains 5 this will become I:000/05
I:[N7:1]/03 -If the integer memory location contains 2 this will become I:002/03
#I:[N7:1] -If the integer memory location contains 2 the file will start at I:002
N[N7:0]:8 - If the number in N7:0 is 10 the data address becomes N10:8
Figure 14.9
Indirect Addressing
Expressions allow addresses and functions to be typed in and interpreted when the
program is run. The example in Figure 14.10 will get a floating point number from file 8,
location 3, perform a sine transformation, and then add 1.3. The text string is not interpreted
until the PLC is running, and if there is an error, it may not occur until the program
is running - so use this function cautiously.
expression - a text string that describes a complex operation.
“sin(F8:3) + 1.3” - a simple calculation
Figure 14.10
Expression Data Values
These data types and addressing modes will be discussed more as applicable functions
are presented later in this chapter and book. Floating point numbers, expressions and
indirect addressing may not be available on older or lower cost PLCs.
Figure 14.11 shows a simple example ladder logic with functions. The basic operation
is such that while input A is true the functions will be performed. The first statement
will move (MOV) the literal value of 130 into integer memory N7:0. The next move function
will copy the value from N7:0 to N7:1. The third statement will add integers value in
N7:0 and N7:1 and store the results in N7:2.