User manual

Chapter 3 Installation
Page 35 ©February 2009
3.5.2 RS485 Serial Port
RS485 should not be used with the DF1 Protocol. The RS485 port provides for communication
between the 105G unit and its host device using a multi-drop cable.
Up to 32 devices may be connected in each multi-drop network. Note that the RS485 port is
shared internally with the RS232 port - make sure that the RS232 port is disconnected before
using the RS485 port.
RS485 is a balanced, differential standard but it is recommended that shielded, twisted pair cable
be used to interconnect modules to reduce potential RFI. An RS485 network should be wired as
indicated in the diagram below and terminated at each end of the network with a 120-ohm
resistor. On-board 120 ohm resistors are provided and may be engaged by operating the single
DIP switch in the end plate next to the RS485 terminals. The DIP switch should be in the “1” or
“on” position to connect the resistor. If the module is not at one end of the RS485 cable, the
switch should be off.
It is important to maintain the polarity of the two RS485 wires. On the 105G, terminal A (the
terminal on the right) is positive and terminal B is negative.
105G
INTERNAL
EXTERNAL RESISTOR
REQUIRED
120Ω
RS232 9 PIN
CONNECTOR
TERMINATING
RESISTOR SWITCH
ON = TERMINATED
HOST
HOST
120Ω
RS485 DUAL
TERMINAL
CONNECTOR