ADLINK Technology CM1-86DX2, Technical Manual

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Module Description 35
CM1-86DX2
RS485/422 Termination
Typically the RS485 cabling is done as a bus system with two or more devices on the bus. The
cabling can be done in 4-wire mode (RX+, RX-, TX+, TX- using 4 independent wires) or 2-wire
mode (RX+, TX+ and TX-, RX- are using 2 wires). In RS485 Mode the differential pairs should
be terminated with 120
Ω if the device is located at the beginning or end of the bus.
The termination is done by switching the appropriate sliders on the DIP-Switch from the off posi-
tions to on positions. So COM1 and COM3 use positions 1 & 2 each, and COM2 and COM4 use
positions 3 & 4 each of the DIP-Switch (SW2 and SW1).
When the termination is set, the differential pairs are terminated with 120 Ω between them (e.g.
RX+ and RX-, in the following illustration.)
Additionally, positive/negative receive lines are pulled up/down with 10 k Ω to 5V/GND in order to
protect the transceivers of the CM1-86DX2 from overvoltage.
It is recommended to protect the ports of the external device in the same way.
The maximum current on all supply pins is 0.5A.
Termination resistors
should not
be used in RS232 Mode. Otherwise, the serial ports will
not work.