External Time Clock; Modbus Scheduling; Bacnet Scheduling; Optimal Start - McQuay MicroTech II Operation Manual

External Time Clock

If desired, an external clock can be used to schedule chiller system operation. The clock must be connected to the CSM's
external start/stop input (UI-13 of the CSM's onboard I/O panel). If the switch or relay contact connected to it is closed, the
CSM is occupied. If nothing else is enabling the CSM (for example, an occupied schedule), the operating state returns to
Off: Unoccupied when the switch or relay opens. If you don't want the CSM's internal schedule to influence scheduling
(likely), remove the occupied periods from the CSM's internal weekly, holiday and special-event schedules.
An external clock does not actually schedule the CSM; it works by overriding the Off: Unoccupied state. Therefore, when
the external clock is in the occupied mode, the CSM's system status is "On: Input" instead of "On: Schedule." The effect is
the same—except that the CSM's optimal start feature cannot work with an external clock.
Note: The external start/stop contact can be wired to a manual switch, external time clock, or both (wired in parallel).

Modbus Scheduling

If the CSM is integrated into a Modbus network, a Modbus master may schedule the CSM by writing to Coil Index 00081.
Writing a "1" to this variable sets the CSM to occupied. Writing a "0" to this variable sets this input to AUTO which
allows internal schedules or external time clock scheduling to command the CSM to occupied. If you don't want the
CSM's internal schedule to influence scheduling (likely), remove the occupied periods from the CSM's internal weekly,
holiday and special-event schedules. When the Modbus scheduling coil is set to Occupied, the CSM's system status is
"On: Modbus Network". The CSM's optimal start feature cannot work with Modbus scheduling.
See the Configuring the CSM for BAS Communication section of this document for information on setting up the CSM to
communicate on a Modbus network.

BACnet Scheduling

If the CSM is integrated into a BACnet network, a BACnet device may schedule the CSM by writing to Binary Output
instance 50. Writing a "1" to this variable sets the CSM to occupied. Writing a "0" to this variable sets this input to AUTO
which allows internal schedules or external time clock scheduling to command the CSM to occupied. If you don't want the
CSM's internal schedule to influence scheduling (likely), remove the occupied periods from the CSM's internal weekly,
holiday and special-event schedules. This BACnet binary output can be written to at any BACnet priority in accordance
with the BACnet standard, but the CSM's internal logic will always display priority 9 as a BACnet priority. When the
BACnet scheduling binary output is set to Occupied, the CSM's system status is "On: BACnet Network". The CSM's
optimal start feature cannot work with BACnet scheduling using this binary output method.
Alternatively, a BACnet device may edit the CSM's internal schedules directly. The CSM's schedule is presented to a
BACnet network as Schedule Object instance 1. The CSM's calendar is presented to a BACnet network as Calendar
Object instance 1. If you are using BACnet to edit the CSM's internal schedules, the CSM's system status is "On:
Schedule" when in the occupied mode. Scheduling in this way allows the CSM's optimal start feature to work with
BACnet scheduling.
See the Configuring the CSM for BAS Communication section of this document for information on setting up the CSM to
communicate on a BACnet network.

Optimal Start

The optimal start feature works with the internal scheduling functions (not with External Start/Stop or BAS scheduling via
BACnet or Modbus inputs) to start the chiller system early during periods of high cooling load. The goal of optimal start is
to drop the chilled water supply temperature to the System Setpoint just as the normal occupied period begins. Optimal
start uses an algorithm that adapts to the characteristics of the chiller system. If Optimal Start is enabled the following
events occur:
1. The cooling load pumps are started and operated just long enough to get a representative return chilled water
temperature.
2. The return chilled water and outdoor air temperatures are sampled. Based on these temperatures, an estimate is made
of the amount of time required to pull the chilled water supply temperature down to the System Setpoint.
3. An optimal start time is calculated by subtracting the estimate from the scheduled start time.
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