30 November 1998
(5) The advantages of this method of interface are:
(a) With this interface, the EMCS can be used to perform monitoring and supervisory control
(b) This method provides greater functionality than does the analog/binary interface method.
All controller configuration parameters can be viewed and adjusted from the EMCS central station.
(c) This method of interface, in a demonstration project, was shown to be slightly less
expensive than the analog/binary interface method primarily due to the need for less wiring.
(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 an industry standard communications protocol, the control panel
controllers are most likely to be proprietary. In addition, a later version of the same vendors controller
may not be communications compatible, on a replacement basis, with the original panel controller(s).
(c) This method does not eliminate the need for a binary (contact closure) interface between
the control panel and EMCS to accommodate control panel status devices (freezestats and smoke
detectors) and override (time clock, economizer, remote safety shutdown, and remote safety override).
(d) 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.
(e) 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
c. EMCS interface using frequency modulation (FM) switches. Locally mounted FM switches are
controlled by one-way transmission from a centrally located FM transmitter. Individual switches may be
used to override HVAC control panel operating modes. If this option is chosen, the designer should
modify the control system ladder diagrams and HVAC control panel details to show provision for:
(1) Replacing HVAC control panel time clocks with FM switch start-stop contacts.
(2) Installing FM switch override of the HVAC system's economizer controller signal.
(3) The advantages of using this method of interface are:
The cost is relatively inexpensive.
(4) The disadvantages of using this method of interface are:
(a) Funcionality is limited to binary outputs (contact closures).