Applications Guide Tracer Graphical Programming - Trane

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Writing the condenser water pump module
Figure 100: Pump start/stop module
a
e
b
c
d
Table 9: Pump start/stop module state table
4. See Table 9 for an analysis of this logic.
Run a b c d e Comments
1 F F F F F Initially, Request is off, and Status is off.
2 T F F F F The chiller plant calls for condenser water, so Request is on. Flow is not
confirmed yet. The Feedback Alarm delay countdown begins.
3 T F F T T The Feedback Alarm delay expires and its output transitions to on, controlling
the pump on and indicating a pump failure.
4 T T T T T Flow status is confirmed. The Feedback Alarm remains on because it has
not been reset.
What is wrong with the sequence presented in Table 9? The pump did
not start until the Feedback Alarm delay expired. Thus, flow is never
confirmed during the delay period of 30 seconds. But the sequence of
operation calls for the pump to start, allowing 30 seconds for flow confirmation.
If after the 30-second delay period, flow status is not confirmed,
a pump failure results.
Only one block is required to fix this problem, but which one? The
answer is the Not block. The Not block changes the logic of the data to
the opposite state. For example, if the Feedback Alarm block sends a
true message, the Not block changes the message to false.
5. Delete the connection between the Feedback Alarm block and the And
block.
6. From the Blocks menu, choose Logic. From the Logic menu, choose
Not.
7. Click in the design space to place a Not block.
8. Connect the Feedback Alarm block to the Not block.
9. Connect the Not block to the And block (Figure 101 on page 100).
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