30 November 1998
(a) The EMCS can be used to perform basic monitoring and supervisory control functions.
(b) The 4-20 mA I/O between the control panel controllers and the EMCS remains non-
proprietary and, in principle, any vendors EMCS can be interfaced with the controllers.
(6) The disadvantages of this method of interface are:
(a) In the absence of an industry standard communications protocol, the EMCS interface
device (located in the field) and the EMCS central station are likely to be proprietary.
(b) In the absence of a control panel mounted time clock, the stand-alone capability of the
panel is compromised. For example, with EMCS performing the time clock function, failure of the EMCS
may result in loss of the time clock function.
(c) The electrical connection between the EMCS and the controller CPA port and/or PVR
terminals may present ground loop problems thus requiring the use of loop drivers to provide for
(d) The cost of the interface can be prohibitive. Preliminary results, based on one installed
system, indicates a cost of about k per panel. Subsequent technological developments may lead to a
b. EMCS interface using digital communication signals. Controllers are available with optional
digital communications ports, such as Electronics Industries Association (EIA)-485. Using the
communications port and vendor developed protocol, the control panel controllers can exchange data
with an EMCS. Contact closure (binary) I/O signals are used to interface the EMCS with control panel
shutdown and status devices and to override the control panel.
(1) Process variable retransmission (PVR) is not required. The functional equivalent is
performed using digital communications. Delete the wiring between the control panel PVR terminal block
connections and the controller PVR terminals.
(2) Control point adjustment (CPA) is not required. The functional equivalent is performed using
digital communications. Delete the wiring between the control panel CPA terminal block connections and
the controller CPA input terminals.
(3) Binary outputs to EMCS. Freezestats and smoke detectors operate relays, located inside the
control panel, as part of the HVAC control system shutdown circuits. Contacts of these relays are wired
to terminal blocks in the HVAC system control panel for EMCS use. The economizer controller operates
a relay located inside the control panel. A contact on this relay will be wired to terminal blocks in the
HVAC system control panel for EMCS use. Differential pressure switches across the air handling system
filters will have a contact in the device reserved for EMCS use.
(4) Binary inputs from EMCS. The control system ladder diagrams and HVAC control panel
details will show provisions for override of HVAC control panels by:
(a) Replacing HVAC control panel time clocks with EMCS start-stop contacts.
(b) Installing EMCS override of the HVAC system's economizer controller signal.