30 November 1998
(6) Some of the control devices necessary to implement the control sequence have an input
impedance of 250 ohms, and their output circuits can accept from 800 ohms to 1000 ohms of impedance.
The amount of allowable impedance in their output circuits is product specific. Any control device whose
modulating control output circuit has greater impedance loading capability than the impedance of its
modulating control input circuit can function as a loop driver, in addition to performing a specific control
sequence function. This output driving capability is found in most modulating control devices.
(7) When the control system requires more than one damper with electric or electronic actuators
to be modulated by a control circuit, the designer will show the signal (on the schematic) connected to one
of the damper actuators. The AAD circuit of that actuator will be shown as connected to drive a separate
actuator on another damper, which, in turn, can drive another actuator on still another damper.
(8) When a modulating control circuit must drive multiple panel mounted control devices, the
designer will show on the schematic:
(a) Not more than 2 devices (such as IPs) connected to that circuit, unless one of the devices
is a control device that accepts a modulating input signal and produces a modulating output signal.
(b) Not more than two panel mounted control devices connected to the modulating output of a
panel mounted control device.
(9) The schematic is not intended to show the physical connections to the devices, but rather to
show the relationship of the necessary control devices in the control loop.
n. Transmitters. Variables such as temperature, pressure, and relative humidity are sensed by means
of elements that are connected to the control loops via transmitters. The output signal of the transmitter is
the standard 4 to 20 milliampere dc signal, which is factory calibrated for zero point and span relative to the
input resistance value of the sensing element. The transmitters are 2-wire, loop-powered (i.e., powered by
the control panel power supply) devices that connect in a series circuit with the controller input. The
impedance limitation of the circuit in which the transmitter can function is product specific. A typical value is
700 ohms at 24 volts dc.
o. Single loop digital controller.
(1) As shown for the standard control systems, single loop controllers are used for essentially all
systems other than simple unitary systems and terminal units that are controlled directly from room or zone
thermostats. In all applications where it is used, the controller will be mounted in a HVAC control panel.
The controller mounting dimensions will conform to a standard panel cutout requirement. The controller
will be used for the following applications:
(a) As a controller for maintaining temperature, relative humidity, static pressure, and/or airflow
(b) As an economizer mode switchover controller that determines whether outside air is
suitable for cooling.
(c) As an outside air temperature controller for scheduling hydronic heating supply
temperature and for starting and stopping pumps.
(2) The controller will be a microprocessor-based device with manually configurable control
features resident in solid state electronic memory components. Manual access to the features of the
controller will be through a keypad and an alphanumeric indicator on the face of the controller. The
controller will have standard features that will allow it to serve all functions prescribed in its application.