Introduction
Reduce the cost of installing
hazardous-area cabling
Save installation time, space and
weight
Highlight problems quickly with
status reporting systems
Protect the process with sensor
failure detection and safety drives
Connect directly to host systems
through serial-data outputs
Analogue inputs to controllers with
Modbus® communications
Redundant data highways
MTL830 Series multiplexers with Modbus®
outputs provide a cost-effective alternative to singleloop
isolation. The cost of installed wiring is reduced
by up to 50% by communicating the input of multiple
hazardous-area sensors over dual-redundant data
highways. Further savings are achieved by reducing
the number of inputs to the host, cabinet space and
weight.
A multiplexer transmitter, mounted in the
hazardous area, caters for thermocouple, RTD and
mV analogue inputs. A compatible safe-area receiver
provides serial Modbus® outputs for feeding to host
PLC, PC or DCS controllers.
Dual-redundant data highways between the
transmitter and the receiver allow continuous normal
operation with only one highway connected. The
highway cable, a simple twisted pair or a pair of
wires within an IS multi-core cable, carries both
power and data, over distances up to 2km. If the
multiplexer transmitter is located in the hazardous
area, each data highway must be protected by an
MTL3052 digital isolator.
Multiplexer systems can communicate the status
of up to 32 inputs, reducing the number of hazardous
area wiring pairs from 32 to two.
Reduce costs by eliminating long runs of expensive
thermocouple compensation cable from the hazardous
to the safe area. AIso, if 3- or 4-wire RTDs are used,
costs are reduced by terminating each RTD at the
transmitter or its enclosure.
Dual redundant highways increase system
reliability. Failure detectors and safety drives protect
against the consequences of sensor failure, while
built-in systems report failures to the host controller.
Accessories include steel and stainless-steel
enclosures, earth-leakage detectors and a PC-based
configuration software package.
For more information see AN9010, ‘A user’s guide
to multiplexers’ and TP1098, ‘Cut the cost of intrinsic
safety – by multiplexing!’
MTL831B
MULTIPLEXER
TRANSMITTER
MTL838B-MBF
RECEIVERS
MTL831B multiplexer transmitters are normally sited in the hazardous
area. They are connected to sensors in the field and communicate
these inputs to the safe area via single-pair data highways. The data
highways support communication between the safe and hazardous
areas,and also provide power to the transmitters – no additional field
power is required.
The MTL831B would typically be mounted close to the field instruments
in an enclosure such as the ENC8 or ENC83 (see 'MTL800 Series
Enclosures').
MTL831B ANALOGUE INPUT TRANSMITTER
Monitors inputs from up to 16 THC or millivolt sources (max ±60mV)
or up to 15 2-, 3- or 4-wire RTDs within a hazardous area.
Intrinsically safe; Zone 0 location
Communication and power pass over twin data highways
Powered by the data highway
Thermocouples and RTDs may be mixed on one transmitter
MULTI-DROPPING TWO MTL831BS
Two MTL831B multiplexer transmitters can share the same singlepair
highway to a single safe-area isolator/receiver combination,
providing up to 32 multiplexed inputs.
DATA HIGHWAY CABLING
Cabling with low capacitance and resistance is recommended to
achieve greater distances between transmitters and receivers. See
cable parameters in 'Basic Specifications' for specific requirements,
and contact MTL for latest cable recommendations. The system is
designed to use both data highways, although it can be used with
only one data highway in place. However, use of a single highway
will increase response time.
Data highway lengths of up to 2km in hazardous areas and 3km in
safe areas have been achieved.
ENCLOSURES
A range of steel and stainless steel enclosures is available for
mounting MTL831B units in the field (see 'MTL800 Series Enclosures').
ENC8 and ENC8SS (stainless steel) enclosures provide protection for
a single MTL831B. The enclosures are dust-tight and waterproof to
IP67.
When using 4-wire RTDs, ENC83 and ENC83SS (stainless steel)
enclosures provide additional terminals to accommodate the third
and fourth RTD wires, which are not connected to the MTL831B
itself.
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MTL838B-MBF receivers translate the information transmitted from
the MTL831B via the data highway. The MTL838B-MBF provides a
Modbus® serial-data output representation of the inputs together with
status information.
MTL838B-MBF MODBUS® ANALOGUE RECEIVER
Translates data received from the MTL831B in the hazardous
area and makes it available as twin RS485 Modbus® serial data
outputs. The MTL838B-MBF is configured by personal computer for
thermocouple and RTD type, safety drive, high and low alarm and
any other operational parameters.
MTL3052 DATA HIGHWAY ISOLATOR
An MTL3052 isolator is required for each data highway, when the
transmitter is located in a hazardous area. It is not required for safearea
applications.
MULTI-DROPPING MODBUS® RECEIVERS
Up to 31 Modbus receivers can be multi-dropped on a single RS485
link to the host system. Modbus® receivers can be controlled by any
suitable Modbus® master. The receivers may be used with other
Modbus® slaves on the same RS485 link.
