Flowserve MX FOUNDATION Fieldbus Field Unit - Manual (Page 13)

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MX/QX FOUNdaTION Fieldbus Field Unit FCd LMENIM2330-01 – 08/11
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Unscheduled Communication
All devices on the fieldbus are given a chance to send “unscheduled” messages between transmis-
sions of scheduled messages. The LAS grants permission to a device to use fieldbus by sending
a pass token (PT) message to the device. Upon receipt of the PT, the device is allowed to send
messages until the token hold time has expired.
Live List Maintenance
The list of all devices that are properly responding to the pass token is called the “Live List.” New
devices may be added to the fieldbus at any time. The LAS periodically sends Probe Node (PN)
messages to the addresses not in the Live List. If a device is present at the address and receives the
PN, it immediately returns a Probe Response (PR) message, and the LAS adds the device to the Live
List.
2.2.4 Network Cable
Network cabling should be in accordance with FOUNDATION fieldbus guidelines. Table 2.3 details
various types of cable that can be used for a network cable. For additional guidelines see the
following publications:
• FOUNDATION Fieldbus Wiring and Installation. 31.25 kbits/s, Voltage Mode, Wire Medium AG-140
• Relcom Inc. Fieldbus Wiring Design and Installation Guide
• ANSI/ISA-S50.02, Part 2-1992, Fieldbus Standard for Use in Industrial Control Systems Part 2:
Physical Layer Specification and Service Definition.
Table 2.3 – Recommended Cable Types
Cable Type Gauge No. Resistance,
Ohms/km
Atten,
dB/km
Maximum
Length
A Shield twisted-pair #18 AWG 22 3 1900 m
B Shielded multi-twisted pair #22 AWG 56 5 1200 m
C Twisted-pair #22 AWG 132 8 400 m
D Shielded multi-core #16 AWG 20 8 200 m
NOTE: The maximum lengths are estimates and depend on the condition of the actual cable.
For example, Belden 3076F Fieldbus Cable, or equivalent, could be used.
There are several possible topologies for fieldbus networks:
• Point-to-point
• Bus with Spurs
• Daisy Chain
• Tree topology
• Combination of the above