3
INM MTL83xC PC Modbus Rev 4
3 BACKGROUND TO THE MTL838C
3.1 The analog-input multiplexer system
The MTL838C is an analog multiplexer receiver that is used with the MTL831C
hazardous area millivolt input multiplexer transmitter. The status of up to 32 analog
inputs may be communicated from the hazardous area to the safe area via a data
highway, comprised of a simple twisted pair - over distances up to 2000m.
Each data highway must be protected by an MTL5553 digital isolator when the
inputs are located in a Zone 0,1 hazardous area. The MTL831C is typically used
with thermocouple and RTD inputs and is intrinsically safe. It can be mounted in a
Zone 0,1 hazardous area and will accept 16 inputs. For systems that do not require
Zone 0 or Zone 1 installation, the MTL5553 can be eliminated.
Up to two MTL831C transmitters can be combined on a single MTL838C receiver
input - up to a total of 32 analog inputs - as shown in Figure 1.
Figure 1 - MTL838C/MTL831C System Diagram
The MTL838C acts as a Modbus slave. It may be connected into any standard
Modbus network, with up to 31 MTL838C slaves on each network. If each unit
has its full complement of 32 analog inputs, the status of a total of 992 analog
inputs may be passed to a Modbus master using a single RS485 network. For the
MTL838C-MBT the single Ethernet connection allows for up to 32 analog inputs.
3.2 Configuring the MTL838C
The MTL838C must first be configured using software on a PC and the USB
connection. This configures things such as the slave address and communication
parameters. After the initial configuration, the MTL838C is ready to communicate
with the Modbus host. At this point, the remaining configuration may be done in
one of two ways:
• on-line via the Modbus link, direct from the host
• off-line using the PC software and USB connection
Using the PC software is required for initial configuration and recommended for
first time configuration of the measuring channels.
MTL831C
transmitter 1
MTL831C
transmitter 2
mV
Sensors can be in Zone 0
MTL5553
isolator/PSU
mV
MTL838C/
MTL838C-MBT
receiver
PLC
RS485
RS485
Modbus TCP*
or
*only available on MTL838C-MBT
19-30Vdc
Power
PC
(MTL 83xC Configurator Software)
USB
2x SPST
relay
contacts
DATA HIGHWAY (BUS)
Up to 16 THC, RTD,
and mV inputs
potentiometer
Up to 16 THC, RTD,
and mV inputs
potentiometer
Zone 0, 1 or 2
Hazardous Area
Safe Area
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