Figure 4. Field Bus Controller Integration
|
Table 3. Disadvantages and Advantages of Field Bus Controller Integration
|
There is almost no commonality between the two sets of protocols. Drivers are available that allow BMS systems to communicate
with one or more of the PCS protocols. However, it is best to be cautious of such solutions, as the drivers may not provide
all the data necessary for seamless integration (Figure 4). ModBus requires a master-slave arrangement, and you must ensure
that two masters do not exist on a single network. However, many of BMS-ModBus integration tools must be set up as masters,
and the PCS system probably includes a ModBus master.
Open and interoperable protocols, such as LONworks and BACnet, are the most desirable solution since they offer maximum flexibility
to the facility owner. However, a complete facility-integration strategy must address systems and devices employing proprietary
protocols (due to incumbency or application specifications). The main concept to keep in mind is that protocols are a means
to an end. They simply exchange data between devices and applications; they do not provide automation or control.
Supervisory Level Integration (S95 Level 2)
Integration possibilities also exist at the supervisory level (Level 2 in the S95 architecture). This level acts as a gateway
to the building IT network, handles global calls, and monitors systems traffic to ensure that the system is being a good
IT citizen. Most supervisory controllers offer functions such as global point passing, global trending and buffering, and
global alarm management.
Figure 5. Supervisory Level Integration
|
Table 4. Disadvantages and Advantages of Supervisory Level Integration
|
Level 2 integration schemes pass data from the BMS supervisory controller directly to the PCS supervisory controller (or vice
versa) over a common protocol or via a gateway. In the case shown in Figure 5, a network communication module (NCM) with a
network port that speaks ABDH (Allen Bradley Data Highway) or DH+ (Data Highway Plus) and can pass data directly to the Control
Logix PLC. If the BMS does not have this capability, a gateway device can table the data, convert it to the appropriate protocol,
and send it to the PLC. Obviously, a direct path is better because it is faster, more reliable, and less costly to set up
and maintain.
ELCPI (S95 Level 2-3)
Enterprise level computer protocol integration (ELCPI) uses computer workstations (PCs) to share data. There are a few options
available to pass data from PC to PC. The most widely used and most robust solution in the industry today is object linking
and embedding (OLE) for process control (OPC). This Microsoft technology has become an industry standard.
OPC took over where dynamic data exchange (DDE) left off and provides a relatively seamless path between two OPC-compliant
software packages. By utilizing component object model (COMM) and distributed COM (DCOM) technologies, OPC (representing info
at the data presentation level on the ISO diagram), can act as a client or server application to present data (DA OPC), alarms
and events (A&E OPC), and, in some cases, to collect trends (historical OPC). Alternatively, the most advanced systems support
web services and can transfer data directly using embedded extensible markup language (XML).
|
