30 November 1998
(4) A signal selector is a device with multiple modulating inputs and a modulating output. This
device is used to select the highest or the lowest of its input signals as its output.
(5) A sequencer is a device with a modulating input and one or more contact outputs which
operate in sequence and is used in applications that require the operation of stages in refrigeration control
from a modulating control signal. A deadband surrounding each of the sequencer setpoints prevents all
stages of refrigeration from starting simultaneously when the HVAC system starts. The contact opens
instantaneously on power failure (or on signal failure) to the sequencer.
(6) A loop-driver module is used where required to allow modulating control circuits to be designed
within a 600 ohm impedance limitation. A loop-driver module can be any modulating input or modulating
output device used for this purpose alone or while performing an additional control loop function. Signal
selectors and signal invertors strategically located in control loops often perform such dual service.
q. Relays, including time-delay relays. All relays, including time-delay relays, will be 2-pole, double-
throw devices; they are used for control system interlocking functions and will be located in the system's
HVAC control panel.
r. Time clocks.
(1) A time clock will be used to control the timing of the modes of operation of an HVAC control
system when the control system is not interfaced with EMCS. When a time clock is used, it will be located
in a HVAC control panel. The modes of operation are occupied, unoccupied and ventilation delay.
(2) The time clock will be a device that accepts a time schedule by manual input through a keypad
and an alphanumeric display. The time clock features will be:
(a) Four independent time-controlled contacts.
(b) A program of 4 "on" events and 4 "off" events for each contact.
(c) 365 day schedule.
(d) Twelve selectable holidays.
(e) Standard-time and daylight-time adjustments.
(f) Timed override of programs.
(g) Battery backup of memory.
(3) When used to time the modes of operation of air handling systems, one contact of the clock
will be used for occupied and unoccupied timing; the second contact will be used for ventilation delay mode
timing. For other applications, the contacts may be used as convenient to the design.
4. CONTROL LOOPS. A control loop performs three distinct functions: sensing of a variable as the input
to a controller; decision making or control based on the value of the input; and output or actuation as a
result of control. Figure 2-7 illustrates a simple control loop. The input signal is a continuous analog of the
process, and the controller either continuously sees the input or continually scans it. The controller
changes its output as required by changes in its input.
Figure 2-7. Control loop.