INM 9370-RD Rev 5
22
7
ALARM MODULE - OPERATION & CONFIGURATION
7.1
Introduction
As with all redundant systems, early notification of the failure of a system
component is important so that the fault can be rectified before a second fault
is allowed to occur. A second fault is likely to interrupt the process, so restoring
redundant operation is vital in maintaining high levels of system availability. With
the MTL Fieldbus Barrier, notification of the health of the redundant 9377-FB-R
barrier modules can be made available to the host control system via the fieldbus
network. Host notification is achieved using an Alarm Module mounted inside
the Fieldbus Barrier enclosure. The alarm module is a Foundation Fieldbus Device
that is connected to the fieldbus segment using one of the six available spurs in
the enclosure, and appears on the segment ‘live list’ along with other devices that
are supported on the segment. The device monitors both fieldbus barrier modules
within the field enclosure, and signals the healthy/failed status of each spur via
fieldbus Digital Input (DI) function blocks. Each barrier module is assigned its own DI
function block; the control system must therefore monitor the status of both DIs in
order to detect a spur or a module failure.
The integrated fieldbus device is also able to provide additional diagnostic
information that is not normally available in conventional fieldbus barrier
installations. Importantly, each of the Fieldbus Barrier modules is able to detect a
short-circuit or a low impedance between the plus (+) or minus (–) connections of
any spur and the shield (S) of the spur cable. Undetected “short to shield” faults
jeopardise the reliability of a fieldbus network, because a second fault may cause
the loss of one or more active spurs. With the MTL Redundant Fieldbus Barrier, as
well as being indicated by the module’s status LEDs, a “short-to-shield” fault is also
indicated via status bits contained in the fieldbus alarm module’s Analog Input (AI)
function blocks. This can therefore give early indication of faults via the fieldbus
control system, allowing early rectification.
The fieldbus alarm module also incorporates a temperature sensor, which can,
if required, be used to ensure that the ambient temperature around the Fieldbus
Barriers does not exceed the 75°C value required to promote reliable operation and
compliance with certification requirements.
7.2
Device details
The fieldbus alarm device is an ABB type TTH300. It has a pre-configured device tag
“MTL 9372-FB”. It contains the following FF blocks:
Function block type
Default Tag
name
Use in MTL Alarm application
Resource Block
Resource
(RB2)
Alarm module internal temperature, bus
voltage, firmware & hardware versions
DI (digital input)
Block 1
DI1 (DI)
9377-FB-R module A health status, via PV_D
value
DI (digital input)
Block 2
DI2 (DI)
9377-FB-R module B health status, via PV_D
value
AI (analogue input)
Block 1
AI1 (AI)
Not used
AI (analogue input)
Block 2
AI2 (AI)
Short-to-shield detection, 9377-FB-R module
A, via OUT_D value
AI (analogue input)
Block 3
AI3 (AI)
Short-to-shield detection, 9377-FB-R module
B, via OUT_D value
AI (analogue input)
Block 4
AI4 (AI)
Not used
ePID Block (PID controller
with expanded features)
PID (PID)
Not used
AO (analogue output)
Block
AO (AO)
Not used
