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3.5 Application guidelines
The Mux2700 HSM is not normally intended to be designed by the user in a specific application, but to be used as
part of the Elcon HART Interface Solutions (H.I.S.) offering, which provides appropriate interconnection packages
for a wide range of applications, as well as a set of compatible Maintenance Workstation solutions. This makes the
Mux2700 application straightforward, because most “tricky” interface and compatibility issues have already been
solved, and a ready to use, plug-and-play solution can be provided.
As an example, when “HART filters” are required to ensure the compatibility with a specific PLC/DCS, these are
usually directly integrated at the H.I.S. level.
However, there can be situations, as with “general-purpose”, screw-terminals H.I.S. boards, in which some care in
wiring and installation is required to set-up a working system. An overview of some H.I.S. solutions is presented in
section 4., please also refer to the relevant H.I.S. documentation for more information.
As a general guideline, it is worth noting that the Mux2700 input channels must be connected across the 4/20 mA
loop in such a way as to see parallel resistance not lower than 200
Ω
(250
Ω
recommended). If this is not the case,
the available HART signal could be too week to guarantee a reliable communication. A problem of this type can be
found with an analogue-input card with a low internal current-sense resistor, or with any loop-connected device
that, in spite of having an high DC resistance, presents too low an impedance on the 4/20 mA loop at HART fre-
quencies (1200 / 2200 Hz).
In addition, it is always recommended to connect the “+” input terminal at higher DC voltage level with respect to
the “–“ input terminal (that will be in many cases at a GND level).
3.6 Hardware architecture
The heart of the Mux2700 HSM module is the microprocessor (CPU). One CPU serial port – together with the
HART modem and the related analogue circuits – implements the physical layer of the HART protocol. A second
CPU serial port is used to drive the opto-isolated RS-485 interface.
The internal architecture of the Mux2700 Multiplexer is depicted in the following block diagram of Fig. 2. As you can
see, the HART modem is multiplexed on the 32 available field channels to get a cost-effective and compact solu-
tion.
IM-ENG-116/GB
POWER
SUPPLY
UNIT
+
24V
RS-485
INTERFACE
OPTO
ISOL.
CPU
MUX
A
HART
MODEM
+
+
A1
A3
SCREEN
A4,A5,A6
A31
A
A30
B
A32
2.2µF
0.68µF
2.2µF
0.68µF
C32
B32
C1
B1
CH1
CH32
F
IG
. 2
