Programming instructions

13.18 Program Flow Control

By default the PLC program processes top to bottom and left to right. At the

beginning of the program, the actual inputs on the system are read into an image

memory. The program is then processed, with modifications being made to the

internal image memory. Once the END instruction is reached, the outputs are

updated based on the image memory. Then the process begins again.

Sometimes this program flow needs to be controlled. There are several options

for controlling the flow of the program. These methods include jumps and

subroutine calls.

These commands use the pointer addresses mentioned earlier. To set a pointer,

double click in the left margin of the ladder before the power rail and enter an

address beginning with P. This is the pointer. When referencing this location in

a jump or subroutine, call it by its Pxx address.

NOTE: Address P63 is reserved. It is a jump to the END instruction. Do NOT

code a P63 in your program, or an error will be generated.

CJ – Conditional Jump

The conditional jump command allows a section of logic to be skipped. The

command is activated when the conditions leading up to the CJ instruction are

true. When the jump is executed, all code between the CJ command and the

pointer is not executed.