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245U-E Wireless Ethernet Modem User Manual
Rev Version 2.24
RS485 Serial Port
The RS485 port provides for communication between the 245U-E unit and its host device using a
multi-drop cable. Up to 32 devices may be connected in each multi-drop network. Because the RS485
communication medium is shared, only one of the units on the RS485 cable may send data at any a time.
Therefore, communication protocols based on the RS-485 standard require some type of arbitration.
RS485 is a balanced differential standard, but it is recommended that shielded twisted pair cable be used
to interconnect modules to reduce potential RFI. It is important to maintain the polarity of the two RS485
wires. 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” (on) position to connect the resistor. If the module is not at one end of the RS485 cable, the switch
should be off.
NOTE Shorter runs of 485 cable may not require the termination resistors to be enabled.
Figure 15 Multidrop Serial
Figure 16
End Plate
2.4 Discrete (Digital) Input/Output
The 245U-E has one on-board discrete/digital I/O channel. This channel can act as either a discrete input or
discrete output. It can be monitored, set remotely, or alternatively used to output a communications alarm
status.
If used as an input, the I/O channel is suitable for voltage-free contacts (such as mechanical switches) or