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• Line termination is required and is typically done with a 120 Ohm 1/4 W resistor. For
RS-485 operation the bus must be terminated at both ends. For RS-422 operation a
termination resistor must be inserted between the RX+/RX- signals.
• Maximum number of RS-485 nodes without repeater is 32.
• Stub connections off the main line should be avoided if possible or at least be kept as
short as possible. Stub connections must not have terminating resistors.
• Maximum cable length to 1200 m (4000 ft).
• To assure a high degree of electromagnetic compatibility and surge protection the
cable should be twisted pairs and shielded. An additional cable conductor or pair may
be used for the GND reference.
Do not connect the cable shield to the GND pins! Use an external chassis ground
connection to terminate the shield.
Wiring the RS-232 interface
The RS-232 port can alternativly be used for serial communication to a Modbus Slave
device instead of RS-485. The use of the RS-232 interface must be configured using the
web interface (See the section called "Configuring Modbus network"). The MODG-100
is a Modbus Master device on this interface.
The Modbus RS-232 connector is a male 9-pin D-sub type located at the bottom side of
the mounted unit (refer to Figure 2.1, "Location of connectors" ). It has industry standard
EIA-574 data terminal equipment (DTE) pinout as shown in the following table and
picture:
Pin
1
1
6
2
3
4
5
6
7
8
9
Table 3.3: Modbus RS-232 connector pinout
• Maximum cable length is 15 m (50 ft) or a length equal to a line capacitance of 2500
pF, both at the maximum standard bit rate of 20 kbps. If operating at higher bit rates
the maximum cable length drops to 3 m (10 ft) at a bit rate of 57.6 kbps.
• To assure a high degree of electromagnetic compatibility and surge protection the
RS-232 cable should shielded. The shield shall be connected to an external chassis
ground at the either or both ends, depending on the application.
8
Signal
Function
DCD
(unused)
RXD
Receive data
TXD
Transmit data
DTR
(unused)
GND
Signal ground
DSR
(unused)
RTS
(unused)
CTS
(unused)
RI
(unused)
Direction
in
in
out
out
in
out
in
in
UMMODG100-2201
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