
GASMAX CX Operation & Maintenance Manual, Revision 1.0
Page 13
+24
GND
A
+24
GND
A
PW
R
to
MO
D
B
U
S
to
+24
GND
“A”
“B”
+24
GND
“A”
“B”
OPTIONAL
MODBUS
JUNCTION
BOX
MODBUS
system
architecture
requires
that
the
devices
in
any
MODBUS
loop
be
connected
in
a
daisy
‐
chain
layout.
This
minimizes
signal
reflections
and
improves
signal
noise
margin.
A
MODBUS
Termination
Jumper
installs
a
load
resistor
across
the
MODBUS
signal
lines
and
should
only
be
set
to
“A”
(ON)
at
the
last
device
in
the
string.
Cable
selection
for
MODBUS
systems
is
important
for
both
signal
integrity
and
power
distribution.
MODBUS
/
RS
‐
485
transmissions
use
low
‐
voltage
differential
signaling
to
achieve
reasonable
data
rates
over
very
long
distances,
up
to
4000
feet
without
a
repeater.
For
MODBUS
data
signals,
GDS
Corp
recommends
20GA
to
24GA
twisted
shielded
cable.
Daisy
‐
chain
power
distribution
may
require
larger
gauge
wire
since
it
is
critical
that
the
supply
voltage
for
the
GASMAX
CX
at
the
far
end
of
the
string
not
fall
below
12VDC
during
power
‐
up.
Note
that
while
the
GASMAX
CX
has
two
sets
of
wiring
terminals
for
MODBUS
“A”
and
“B”
signals,
daisy
‐
chain
power
wiring
requires
that
two
wires
be
installed
in
the
“+24”
and
“GND”
terminals
on
the
GASMAX
CX
I/O
Power
Supply
board.
This
can
be
difficult
if
wire
sizes
are
larger
than
#18GA.
For
these
reasons,
if
MODBUS
is
required
GDS
Corp
recommends
the
addition
of
the
MODBUS
Wiring
Junction
Box
(see
Fig.
5
‐
7).
This
option
minimizes
the
need
to
access
wiring
inside
the
GASMAX
CX,
provides
individual
wire
landing
points
for
incoming
and
outgoing
MODBUS
and
power
wiring
and
shields,
and
makes
it
easy
to
temporarily
disconnect
the
GASMAX
CX
power
or
MODBUS
connections
without
affecting
any
other
MODBUS
device.
Figure 4-4: MODBUS Wiring
Junction Box
TO
GASMAX
CX
ENCLOSURE