GASMAX II + GDS-IR Operation & Maintenance Manual, Revision 1.1
Page 13
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 24GA shielded cable for best results. Power distribution, however may require larger gauge
wire since it is critical that the supply voltage for the GASMAX at the far end of the string
not fall below
18VDC during power-up and normal operation.
Figure 3-3: MODBUS Wiring (Typical)
ote that while the G M has two sets of wiring terminals for MO B “ ” and “B” signals, daisy
-
chain power wiring requires that two wires be installed in the “+ ” and “G ” terminals on the G M
I/O Power Supply board. This can be difficult if wire sizes are larger than #18ga. Further, the addition of
multiple MODBUS and power wires inside the GASMAX enclosure can create problems and possibly
damage components if the display has to be forced into place. Finally, if it is desired to disconnect the
GASMAX from the MODBUS loop for troubleshooting, disconnecting the power wire removes power from
all devices further down the loop and leaves a live male pin with +24V inside the enclosure.
For these reasons, GDS Corp recommends the addition of the MODBUS Wiring Junction Box. This option
minimizes the need to access wiring inside the GASMAX II, provides individual wire landing points for
incoming and outgoing MODBUS and power wiring and shields, and makes it easy to temporarily
3
6
3
B
B
B
K
B
B3
C
C
O
C
C
O
C
C
O
K
K3
MO B / elay
-
3
ibbon to isplay
IO/Power upply
Bridge ensor
E
C
O
M
C
P
W
O
O
G
B
B
+
G
“ ”
“B”
+
G
“ ”
“B”
