We will use the S95 standard (Figure 1) as a framework for our discussion of system integration. S95 divides control architecture
into five distinct layers that perform separate functions before sending data to higher levels of the hierarchy. S95's five
Figure 1. ISA’s S95 Architecture
- Level 0: discrete manufacturing and control (instrumentation)
- Level 1: discrete process devices (field bus controllers)
- Level 2: automation network (supervisory controllers)
- Level 3: operations information network (user interfaces and application servers)
- Level 4: business process information network (ERP, business systems, etc.).
This article will discuss the pros and cons of pursuing system integration at each of the five levels defined by S95:
- field bus instrumentation integration
- field bus controller integration
- supervisory level integration
- enterprise level computer protocol integration
- database integration.
As you examine the diagrams representing these strategies, you will see the same set of building blocks, but the main connection
between the two systems moves further to the right, indicating integration at higher levels in the S95 architecture.
S95 was created as a standard for process control, but BMS solution providers also are applying their solutions to this architecture.
Figure 2. Field Bus Instrumentation Integration with Two Sensors
Field Bus Instrumentation Integration (S95 Level 0)
There are two main ways of integrating the BMS and PCS at the field bus level using instrumentation. The first way is to use
dual sensors sending separate signals to the two systems (Figure 2). One sensor will be validated, and the data will be sent
to the PCS for room condition monitoring. The other sensor will typically remain unvalidated and will provide data to the
BMS for control. This approach is widely used in the industry but creates some challenges. First, it can be difficult to ensure
that the two sensors remain consistent with each other and remain identically calibrated. If the validated sensor is gathering
environmental data for the production records, that data must match the data used to control the room.
Table 1. Disadvantages and Advantages of Field Bus Instrumentation Integration with Two Sensors
Figure 3. Field Bus Instrumentation Integration with One Sensor
The second method uses a single sensor (with a transmitter) and splits the signal between the two systems. This approach requires
validating the sensor, the transmitter, and the splitter (Figure 3). One side of the splitter goes to the PCS for record retention,
while the data on the other side of the splitter goes to the BMS for control.
Table 2. Disadvantages and Advantages of Field Bus Instrumentation Integration with One Sensor
Field Bus Controller Integration (S95 Level 1)
Another method of integrating the systems at the field bus level is passing data between controllers. This is not always possible
since the BMS and PCS systems use different communication protocols. At the field bus level (Level 0, 1), BMS systems typically
use one of the following protocols: proprietary RS-485, open RS-485 (N2), LONworks, or BACNet (MSTP). In comparison, a PCS
typically uses the following protocols: ModBus (the most widely used), ProfiBus, DeviceNET, or ControlNet.