Rockwell Automation 1775-S5_SR5,D17756.5.5 User Manual PLC-3 FAMILY I/0 - Manual (Page 112)

Programming DH and DH+
Message Procedures
Chapter 6
6-21
The 300-second timer in the example procedure given in Figure 6.7 is not
an accurate, real-time clock. This is because the time between successive
executions of the bit/timer check depends on activity on the DH or DH+
and on the activity of the local PLC-3 controller.
For example, if the 300-second timer times out immediately after its done
bit is checked, the scanner does not detect this condition until its next pass
through LOOP2. If the link is busy with other activity, it takes a while for
LOOP2 to check the remote bit.
The example procedure also assumes that the referenced memory areas
have been created. Specifically:
1. Status file S9 must be big enough to hold ten errors of 28 words each.
2. Timer T1 is a one-second timebase timer. Bit B0:0/0 controls the
ladder rung that activates the timer. Figure 6.7 refers to this bit as
TIM_START/T_ON_BIT.
3. Bit S4:3/4 and S4:3/5 activate message instructions that execute
report-generation procedures. In this way, the scanner can indirectly
cause execution of a report-generation procedure that displays a
message on the operator’s terminal.