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Cooper Bussmann 945U-E Wireless Ethernet Modem & Device Server User Manual
17Rev Version 2.14
Because the RS485 communication medium is shared, only one of the units on the RS485 cable may send data at
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 cables may not require the termination resistors to be enabled.
Figure 14 Multidrop Serial
Figure 15 End Plate
2.4 Discrete (Digital) Input/Output
The 945U-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
NPNtransistor devices (such as electronic proximity switches). PNP transistor devices are not suitable. Contact
wetting current of approximately 5 mA is provided to maintain reliable operation of driving relays.
The digital input is connected between the DIO terminal and common COM. The I/O circuit includes a LED indicator
which is green when the digital input is active, that is, when the input circuit is closed. Provided the resistance of
the switching device is less than 200 ohms, the device will be able to activate the digital input.