Appendix A: Basic Integration Guidelines 27
RTM II FlexNet User Guide DUG-10009-01
Pull Up/Pull Down Resistors
If you are using RS-485 for multidrop communications, or if it is in an electrically noisy environment, you
might need to use pull up and pull down resistors. To do this, use two (2) 4.7K Ohm resistors to jumper
Rx+ to Term and Rx- to Gnd.
Terminating Resistors
For long runs or high Baud rates, it may be necessary to use a terminating resistor. This is generally not
needed.
Half-Duplex
Install a 120 Ohm resistor between the + and - signals.
Full-Duplex
On the master, install a 120 Ohm resistor between the Rx+ and Rx- signals. On the farthest slave
device, install a 120 Ohm resistor between the Rx+ and Rx- signals.
Data Line Polarity
For communication to work, the polarity of the two RS-485 lines must be correct. Always connect +
signals to + signals and - signals to - signals. If your equipment uses the naming convention of RDA/
RDB/TDA/TDB, the As will either be + or - and no convention is strictly followed. Try A with the + signal
first but if that does not work, try switching to A with the - signals.
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