GDS-68XP Operation & Maintenance Manual, Revision 3.4
Page 18
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 (See Fig. 5-3).
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 GDS-68XP at the far end of the string
not fall below 22VDC during power-up.
Note that while the GDS-68XP 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 GDS-68XP 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
GDS-68XP, provides individual wire landing points for
incoming and outgoing MODBUS and power wiring and
shields, and makes it easy to temporarily disconnect the GDS-68XP
power or MODBUS connections without affecting any other MODBUS
device.
+24 GND A
+24 GND A
P
W
R
t
o
M
O
D
B
U
S
t
o
+24
GND
“A”
“B”
+24
GND
“A”
“B”
Figure 5-4: MODBUS Wiring
Junction Box
