Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 99)

Programming Examples
Chapter 8
8Ć5
Some of the statements in the sample procedure are not necessary to
accomplish the bit monitoring. However, they were included to illustrate
more of the functions and programming techniques available with the
1775–KA module.
Note that the 300 second timer used in this example is not an accurate,
real– time clock. This is because the time between successive executions
of the bit/timer check depends on Data Highway activity and on the
activity of the local PLC–3 processor. For example, if the 300 second
timer times out immediately after its done bit is checked, the 1775–KA
module will not detect this condition until its next pass through LOOP2.
If the Data Highway is busy with other activity, it will take a while for
LOOP2 to check the remote bit. PLC–3 ladder diagram programming
provides better timer updates and responses.
The example procedure also assumes that the referenced memory areas
have been created. Specifically:
1. Status file S5 must be big enough to hold a reasonable number of
timeout errors (error #37).
2. Timer T1 is a one–second timebase timer. Bit B0:0/0 controls the
ladder diagram rung that activates the timer. Figure 8.4 refers to this
bit as TIM_START/T_ON–BIT.
3. Bit S4:3/4 activates a message instruction that executes a report
generation procedure. In this way, the 1775–KA module can
indirectly cause execution of a report generation procedure to display
a message on the operator’s terminal.