User manual

Connect Tech Blue Heat/Net User Manual
104 Revision 0.17
Bias and Termination Resistors in RS-485 networks:
Termination Resistors in RS-485 networks:
RS-485 networks often benefit from the installation of termination resistors. Termination is
rarely required for lower baud rates, for example 9600 baud or less. However, RS-485 networks
are transmission lines, and can suffer from the electrical effects of ringing, or undershoot and
overshoot, all of which can cause data errors, especially at higher baud rates, like 115.2 Kbps.
Termination resistors should always be installed at the extreme ends of the network, as close to
the RS-485 transceiver circuits as possible, as outlined in the diagram below.
TIPS:
RS-485 networks using low baud rates, 9600 baud or
less, will not require a termination resistor, unless the
cable length is very long.
Usually the addition of a termination resistor at the
receiver will be adequate.
If DB-9 connectors are used, the termination resistor can
be soldered into the solder cups along with the wires.
Termination resistors should be 120 ohms in value.
Never install more than two termination resistors in a
RS-485 network.