Connect Tech Blue Heat/Net User’s Manual
112 Revision
0.13
Bias Resistors in RS-485 networks:
In multi-drop RS-485 networks, the transmitters are always placed in the high impedance mode
when there is no transmit activity. In this mode, the voltage at the RS-485 receivers tends to float
to 0 Volts which will not maintain a stable value. In addition, the high impedance of RS-485
receivers makes them susceptible to electrical noise. This can cause the reception of garbage
characters and framing errors resulting in problems for your application software.
To avoid this problem, the RS-485 network or bus is held in the inactive or stop bit state using
bias resistors. You should
always
have bias resistors installed when you are using a multi-drop
network, as outlined in the diagram below:
Tip:
In multi-drop and half duplex (2-wire) RS-485 networks,
the Bias and Termination network should always be
installed.
How to implement Bias and Termination Resistors
The Blue Heat/Net is equipped with unique, software selectable bias and termination resistors on
RS-485 models. The user can select the Bias and Termination Option using the
TCM
, the
SCM
or the
WCM
Blue Heat/Net configuration tools. These settings will be stored in the Flash
memory on the Blue Heat/Net. The setting is immediately applied when the setting is changed
and automatically applied after a reset/reboot.
Note that the Bias and Termination network is one network; you can not apply only bias or only
termination. Both are applied at the same time.
