Sigma - Programmable Object: Plant Override Using Momentary Switches

[Product_Support]

Issue

Need programming assistance to allow HVAC plant override using momentary switches for Plant Start and Plant Stop, bypassing the standard time schedule control.

Product Line

Satchwell BAS & Sigma

Environment

Sigma Programming

Cause

The HVAC plant is typically governed by a time schedule. However, operational flexibility is required to manually override this schedule using physical momentary switches.

Resolution

The solution involves creating a programmable object that responds to two momentary switch inputs:

• Point 1: Override ON (Plant Start)
• Point 2: Override OFF (Plant Stop)
• Point 33: Time Schedule Status

The logic ensures:

• When the time schedule is ON, the plant can be stopped or returned to schedule control.
• When the time schedule is OFF, the plant can be started for a fixed duration or returned to schedule control.
• If overridden OFF, the plant will automatically revert to time schedule control the next day.

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Sample Code Logic

10  IF POINT 0|1 ON THEN XInt = 1
20  IF POINT 0|2 ON THEN XInt = 2
30  IF POINT 0|33 OFF THEN GOTO 90
40  IF XInt = 2 THEN YInt = 1
50  IF XInt = 1 THEN YInt = 0
60  XInt = 0
70  IF YInt = 0 THEN RETURN TRUE   'Plant On'
80  RETURN FALSE                   'Plant Off'
90  YInt = 0
100 IF XInt = 1 THEN SET TIMER 3600   'Start for 1 hour'
110 IF XInt = 2 THEN SET TIMER 0      'Stop immediately'
120 XInt = 0
130 IF TIME LEFT THEN RETURN TRUE     'Plant On'
140 RETURN FALSE                      'Plant Off'

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Notes

• Point 1: Momentary switch for starting the plant.
• Point 2: Momentary switch for stopping the plant.
• Point 33: Indicates whether the time schedule is active.