E-Zine June 2006
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The initial ANSI/ISA 50.02 standard called for Fieldbus to have two-level
architecture. The lower level (H1) provided the highest practical speed for an
intrinsically safe environment while supplying power to the field devices. The
upper level (H2) was intended to interconnect the H1 segments and aggregate the
data for distribution to and from higher level systems.
Implementation of H2 as specified in ANSI/ISA 50.02 and the IEC 61158-2
were found require custom chips that were more expensive than desired. In early 1998, the
FOUNDATIONT Fieldbus HSE project was funded to specify low cost, high speed switched Ethernet
appropriate for use in H2 service. Specifications for HSE were included in the IEC 61158
fieldbus standard as Type 5.
HSE products are widely available and are being used to build process
control systems based on FOUNDATIONT Fieldbus. As a result, many large and relatively
complex systems are being installed at lower cost as compared to distributed control
systems or FOUNDATIONT Fieldbus H1 systems. The cost savings for HSE is achieved by
reducing the number of long wires (homerun cable) needed to connect the field H1
networks to the control room.
The two-level FOUNDATIONT Fieldbus architecture utilizes Linking
Devices (LD) to terminate H1 segments in the field. HSE cabling then runs from the
LDs to the control room.
One of the primary motivations for the use of HSE is to better enable
Field Control. Consider the case when HSE is not used, where each Fieldbus H1 segment
terminates in the I/O unit of a multifunction controller. If a measurement value or
cascade controller input is obtained from another H1 segment, there is no direct H1
path for the Fieldbus communications system to provide the link, so each controller
must provide a software bridge to provide such links. However, the software bridge
is not necessarily a real-time function, and may vary between different controller
suppliers and designs, or may not be provided. Most control systems either do not
allow or caution against making connections through a controller in this manner.
Excerpted from The Consumer Guide to Fieldbus Network Equipment for Process Control