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6
After changing the state of the command XX setpoint indications on the map for the Secondary B system bit, the controlling PLC logic should
monitor the corresponding status indication bit and verify it corresponds to the command bit before monitoring the corresponding setpoint indi-
cation from the Therm-O-Flow.
7
The PLC or controlling logic needs to generate a dynamic signal at approximately a 0.333 Hz (1.5 seconds high, 1.5 seconds low) oscillation
rate. The signal is present to enable detection of a communication failure between the Therm-O-Flow system and the controlling logic. The
TOF system also generates the same rate dynamic signal indication (heart beat indication for PLC from TOF) signal for controlling logic to the
monitor.
8
The PLC Control Command bit needs to be set, and the controlling logic needs to generate a valid heart beat output (described in note 7) to
the Therm-O-Flo system before the TOF will grant the PLC requests or commands. If the heart beat output to TOF becomes a static signal
(always low or high) after an extended period of time while the TOF is in PLC control, the TOF reverts to a safe state, and generates a corre-
sponding alarm. This situation would be similar to an operator pressing the
3A5186B
Appendix A - I/O Signal Descriptions
key
15
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