Instruction manual
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INM4840-5 Nov 2007
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4.4 HCU16AO HART connection unit
Many systems have 4-20mA analogue outputs that are not compatible
with HART data because their impedance is normally too low for the
HART to operate correctly. In addition, the noise generated by the
analogue output can sometimes interfere with the HART data, or
sometimes the HART signal can even affect the stability of the current
signal.
The HCU16AO HART connection unit - see Figure 4.6 & 4.7 - enables
HART signals to be used in current output loops, in non-hazardous
areas. It has 16 isolated channels, each with a low-pass filter in series
with the current signal from the system. This has virtually no effect on
the 4-20mA signals but presents a suitable high impedance (> 240Ω)
to the HART signals for their detection and measurement.
As with the HCU16, the HART data is transferred to the BPHM64
HART multiplexer interface via a 20-way ribbon cable.
Field and system connections are made via pluggable 8-way, screw-
clamp terminals, in 4 groups of 4 channels.
The HCU16AO is supplied in a carrier for DIN rail mounting.
Overall footprint of carrier: 275 x 125mm.
Height from top of DIN rail to top of screw connectors: 58mm
4.5 MTL customised backplanes
MTL has a range of customised backplanes that accept MTL4000
Series isolators. These have been produced for various types of
DCS and PLC equipment and have system connectors that suit the
equipment type. Many are fitted with a DIN41651 20-way ribbon
cable connector to enable the backplane to be connected to a
HART maintenance system. See Appendix A or check with MTL for
availability.
Other types of backplane are available that have provision for the
inclusion of an MTL4841 and one or two MTL4842 modules on the
backplane itself. These are dealt with in Section 4.7.
4.6 HMU HART Backplanes
The HMU range of connection units is designed for applications
where a smaller footprint is required or where only a small number
of HART signals need to be connected. They are based on a HART
control board combined with one or two of the HCU16 or HCU16AO
boards. Consult Figures 4.5 and 4.6 for field and system connections
to these backplanes.
An HMU backplane uses one MTL4841 module and one or two
MTL4842 modules to suit the number of signal channels supported.
The type and number of HART signals handled depends on the HMU
model chosen, as shown below.
HMU16 provides connection for up to 16 AI signals
HMU32 provides connection for up to 32 AI signals
HMU16AO provides connection for up to 16 AO signals
HMU32AO provides connection for up to 32 AOsignals
HMUs are supplied on a DIN-rail mounting carrier - see Figure 4.8
- and may be mounted anywhere in the cabinet in any orientation.
4.6.1 DC power connections
Each HMU backplane has DC POWER terminals for the connection
of a (nominal) 24V dc supply.
Power requirements: Voltage: 21–35V dc
Current: 72mA @ 24V dc
Connect the dc power to the board as shown in Figure 4.8. Observe
polarity.
4.6.2 Links to other HMU backplanes
HMU backplanes may be linked together to provide a total of sixteen
MTL4842 modules under one MTL4841 communications module.
(Consequently, other HMUs do not need an MTL4841 module to be
fitted.) 10-way connectors P1 and P2 - see Figure 4.8 - are used to
link the additional HMU backplanes together.
See Section 4.12 for cabling recommendations and limits. MTL can
provide 10-way cables in a range of different lengths on request.
Up to 16 may be connected in this way but the sum of all the
interconnecting cable lengths between the first and last unit must
not exceed four (4) metres.
4.6.3 MTL48xx modules fitting and configuration
See section 5.1 for details.
IMPORTANT
Please note the position used for the MTL4842 module for the
HMU16 and the HMU16AO in Figure 4.8.
4.6.4 RS485 communication connections
Two RS485 ports are provided on the board - see Figure 4.8. These
are used to connect the HMU backplane to the PC running the
maintenance software and, possibly, to onward link to MTL4841
communications modules on other backplanes.
Full details of this communications link are provided in Section
4.8.x.
TB1 TB5
FIELD SYSTEM
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TB7
HCU16AO
125 mm
275 mm
Figure 4.7 - HCU16AO connection unit