To allow a second PG to send a job to the CPU, the status function is
interrupted between two requests and then continued on completion of
the inserted job. Since the interrupting function requires CPU
facilities, the whole CPU system facilities must be divided between
the two functions, e.g. the updating of the data output by the "status
variables" function takes somewhat longer.
With both PGs working simultaneously, the sequence shown in figure
11.8 results.
This also applies when cyclic functions are active on both PGs; the
two PGs then access the PLC alternately.
Parallel Operation of Two Serial PG Interfaces
CPU 948 Programming Guide
C79000-G8576-C848-04
11 - 23
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