Chapter Two
Installation
man_105_2.9.doc
Page
23
The RS485 Communications format is 9600 baud, 8 data bits, one stop bit, no parity. Note that
the RS485 port is shared internally with the RS232 port - disconnect the RS232 cable after
configuration is complete.
RS485 is a balanced, differential standard but it is recommended that shielded, twisted pair cable be
used to interconnect modules to reduce potential Radio Frequency Interference (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.
The 105U modules include a terminating resistor on-board. If the 105U module is the first or last
module in the RS485 chain, then the terminating resistor may be connected by operating the single
DIP switch in the end-plate next to the RS485 terminals. “On” or “down” means that the resistor is
connected. The 105S Modules also have a switchable resistor.
TE R M IN ATIN G
R E S IS TO R S W IT C H
+
-
2.4.12
Connecting 105S Modules to 105U Modules
105S modules connect to a 105U via the RS485 port on each module - refer to section 2.4.11. Up
to 31 x 105S modules can be connected to a 105U module. This number is reduced for 105S-3 and
–4 modules, as these modules use two unit addresses (refer to chapter 4 of this manual).
The 105S modules can be mounted next to the 105U module, or they can be remote from the 105U.
The reliable distance for a RS485 multi-drop line depends on the shielding of the wire and how
close it is installed to electrical noise sources - distances of more than 1 km can be achieved by
good installation methods. External RS485 isolators are recommended if the earth potential
difference between modules is greater than 7V.
105U
105S
RS485
A B
105U
105S
RS485
A B
105U
105S
RS485
A B
105U
105S
RS485
A B
Activate resistor-connection
switch at both end
modules