User`s manual

10-4 Motion Mate™ DSM314 for Series 90™-30 PLCs User's Manual – January 2001 GFK-1742A
10
When to Use Local Logic Versus PLC Logic
The local logic programming language contains basic mathematical and logical constructs. The
capabilities have not been designed to replace the PLC’s logic capabilities. Instead, local logic is
designed to complement the PLC logic and mathematical abilities. Specifically, local logic is
designed to solve a small logic and mathematical set that requires tight synchronization with the
controlled motion. The local logic program must run to completion each sample period. Thus,
local logic programs are limited in size. The default local logic program size limit is 150 lines.
The Local Logic build process will generate an error message when the 150 line limit is exceeded.
A warning message is generated when 100 lines are exceeded. If the program is very large and
computationally intensive it may exceed the allowed execution time and result in a watchdog timer
warning/error (refer to Appendix E). In contrast, the PLC program size is limited only by available
memory. However, as PLC program sizes increase, PLC sweep times increase. (Please consult the
Series 90-30/20/Micro PLC CPU Instruction Set Reference Manual
, GFK-0467, for additional
information concerning PLC sweep times.) This is not true with local logic programs. Local Logic
programs always execute to completion every position loop sample period. When using PLC logic,
the added latency associated with the PLC sweep times for time-critical logic operations that are
tightly coupled to motion can be unacceptable or limit process performance. These tightly coupled
and time-critical processes are potential Local Logic applications. Each process will have to be
evaluated on an individual basis to determine which sections to write in PLC logic and which
sections to write in Local Logic.