HVAC Control in the New Millennium.pdf - HVAC.Amickracing

HVAC Control in the New Millenniumto create this interface is using the network socket interface providedwith most TCP/IP implementations. This provides the data stream fromthe client to the server.Sockets are used to talk to Ethernet-enabled PLCs such asModicon’s MODBUS TCP which provides a widely supported socketinterface to Modicon and other PLCs.Distributed ObjectsOne flexible technique used to communicate across a network isthe distributed object. This is a higher-level protocol that is more standardized.Distributed objects allow software running on multiple machinesto interact almost as if it were on a single machine.Distributed object technology allows the processing and data to bedistributed to various devices where it may be most needed or mosteffectively handled, including distribution to the Web applications.Distributed object standards include the common object requestbroker architecture (CORBA), guided by the Object Management Groupand the distributed component object model (DCOM) which supersedesanother earlier Microsoft standard (COM) component object model.CORBA and DCOM provide similar capabilities for large-scaleand facility integration (See Table 9-1). For the users of Internet technologies,the object request broker that enables CORBA, has the advantageof being available on almost every computer platform andoperating system. This includes the embedded real-time operatingsystems used to control machines. DCOM is available only on MicrosoftWindows products.Java provides the networking support for socket programming. Adistributed object technique called remote method invocation allowsclients using browsers to easily interface to servers supporting thesestandards (Table 9-2).Some industries have their own protocols, such as the semiconductorequipment communication protocol, SECS/GEM, or the pharmaceuticalequipment communication protocol, LECIS.Most embedded and real-time computer platforms support Java inthe form of Java Virtual Machine (JVM). This is an application or librarythat converts the portable Java byte code into platform-specific code asit is executed.©2001 by The Fairmont Press, Inc. All rights reserved.