Output Override and I/O Update Instructions
Chapter 7
716
The 1772-SD2 scans remote I/O racks and stores the information in its
buffer. The processor, during the I/O scan, updates any local I/O racks and
then gets the information from the 1772-SD2 buffer. This information in
the buffer may be combination of new and old data depending on where
the 1772-SD2 was in its scan when the processor requested the
information. To get the information from the 1772-SD2 takes 0.5 ms per
remote rack.
NOTE: A remote rack is defined as 128 I/O.
The scan time determined by using the program Section 5.2.2 is the
summation of program scan time + local I/O update time + time to update
remote I/O from the 1772-SD2. Therefore:
Program scan time = total scan time (from program of Section 5.2.2) +
local I/O time + (1772-SD2) time.
The timer is used to monitor logic circuits controlling the processor.
It is set at 115 milliseconds and if it times out a processor fault occurs
and the system shuts down. If the time for complete scan exceeds 115
milliseconds, the watchdog timer will time out and the processor will fault.
The watchdog is reset every I/O scan.
Some instructions (Tables 5.E, 5.F, 7.B) can require many times the scan
time of the simple instructions of Table 5.D or cause the program to loop.
Therefore, to avoid exceeding the 115 millisecond watchdog timer limit,
the watchdog is automatically reset when the processor responds to certain
instructions. Table 7.D lists the instructions that reset the watchdog timer.
WARNING: The lower limit of input device cycle time should
not be less than the scan time of the processor. If so, incorrect
input data could be used during program execution and damage
to equipment and/or personal injury could result. Critical inputs
can be monitored and critical outputs can be controlled in an
accelerated manner using the I/O update instructions described
in Section 7.2.
7.6
Watchdog Timer
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