905U Wireless I/O Module User Manual
105S Serial I/O Module
Page 26 © January 2011
The modules include a terminating resistor on-board. If the 905U 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.
2.4.12 Connecting 105S Modules to 905U Modules
105S modules connect to a 905U via the RS485 port on each module - refer to section 2.4.11. Up to
31 x 105S modules can be connected to a 905U 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 905U module, or they can be remote from the 905U.
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 ½ mile (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.
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