Instruction manual

3
INM4840-5 Nov 2007
3
Various solutions are available to suit individual DCS types and a listing
of the currently available connection units and backplanes is given in
Appendix A.
2.3 Large or small scale application?
The MTL HART maintenance solution is capable of addressing
several thousand field devices, but most real world applications
are significantly smaller than this. MTL has also devised a simple,
integrated solution (HMUxx) that is proving popular for smaller
installations of up to 32 devices.
2.3.1 Up to 32 field loops
The HMU range of connection units (see Section 4.6) is designed for
applications where a smaller footprint is required and there is only a
small number of HART signals to be connected.
The type of HART signal defines the type of HMU that will be used:
HMU16 Provides connections for up to 16 AI signals
HMU32 Provides connections for up to 32 AI signals
HMU16AO Provides connections for up to 16 AO signals
HMU32AO Provides connections for up to 32 AO signals
Each HMU accommodates one MTL4841 communications module
and one, or two - as appropriate, MTL4842 multiplexer modules,
each of which can handle 16 signals.
This method can be expanded to accommodate more signals but,
depending on the amount of expansion required, it would probably
be better to use the method described next for more than 32 loops.
2.3.2 More than 32 field loops
For larger installations, the system should probably be based
on the BPHM64 HART interface - see Section 4.2. This unit will
accommodate one MTL4841 communications module and up to
four MTL4842 multiplexer modules to accommodate a total of 64
signals. However, the MTL4841 can manage a total of 16 MTL4842
modules, i.e. 256 signals, so three more BPHM64 carriers can be
linked to the first, each fitted with four MTL4842 modules.
Further expansion is possible by repeating this structure up to 30
times. The overall method is illustrated in Figure 2.1. See Section
5.3.1 for configuration details.
2.4 Connection methods
There are three basic methods of interfacing to the HART signals:
a) via a connection unit
b) via an I/O module backplane
c) via a HART + I/O module backplane
A connection unit - see Figure 2.2 - has one set of screw terminals for
the field devices and another set of screw terminals for connection to
the main system.
The HART signals, for routing to the maintenance system, are derived
from those passing through the connection unit. The connection unit
method is used for field devices in non-hazardous areas.
The second method of getting the HART signals for the maintenance
system is to pick them off the backplane (Figure 2.3) used to
accommodate intrinsic safety isolators, e.g. MTL4000 Series units.
The backplane can be customised to provide a host/system connector
that best suits the user’s choice of DCS or PLC.
In these two cases the derived HART signals are then multiplexed
together for transmission to the maintenance system - see Section 5
for configuration information.
Finally, backplanes are available that not only have the I/O modules,
similar to the one above, but can also accommodate the required
MTL4842 and MTL4841 modules (for up to 32 channels) and
provide RS485 ports to link to the management system. See Section
4.7 for further details.
3 SAFETY INFORMATION
Before beginning the installation of any of this equipment it is
IMPORTANT that the information in this section is read
and understood.
3.1 Precautions - General
Units MUST NOT be installed in a hazardous
area unless certified and marked for this
purpose or unless protected by a locally
accepted explosion-proof technique.
Make sure all installation work is carried out in accordance with
local standards, codes of practice, and site regulations.
Check that the hazardous-area equipment complies with the
descriptive system document.
If in doubt, refer to the certificate/catalogue for clarification
of any aspects of intrinsic safety or contact MTL, or your local
representative, for assistance.
Check that the interface unit(s) functions(s) are correct for the
application.
3.2 Precautions - BPHM64
The BPHM64 is FM-Approved as non-incendive for use in Class I,
Division 2, Groups A,B,C and D hazardous locations.
WARNING - EXPLOSION HAZARD. DO NOT
REMOVE OR REPLACE FUSES UNLESS POWER
HAS BEEN DISCONNECTED OR THE AREA IS
KNOW TO BE NON-HAZARDOUS.
•
•
•
•
Field connections
Host/system connections
HART signals
4/20mA input
(or 4/20mA output)
signals
HART maintenance
connections
+
Figure 2.2 - Generic HCU connection unit
Field connections
Host/system connector
HART signals
4/20mA signals
HART maintenance
connections
+
MTL4000 Isolators
Figure 2.3 - Generic backplane connection unit