User manual

Connect Tech Blue Heat/Net User Manual
Revision 0.17 103
Bus contention on RS-485 multi-drop networks:
Bus contention occurs when two or more devices enabled on a bus attempt to run the bus to
opposite logic values. From the diagram above, we can see that there are multiple RS-485
transmitters (TXD) on the bus. To avoid the bus contention problem, the RS-485 transmitter
features a tri-state, or high impedance mode controlled by an input pin (enable). Software and
hardware in the Blue Heat/Net and the RS-485 devices will always place its transmitter into the
high impedance mode when not transmitting. This feature is managed by the Blue Heat/Net and
is fully transparent to your application.
For example, in a multi-drop network, the RS-485 transmitter is enabled prior to the master
initiating transmission. When transmission is complete, the transmitter is placed in high
impedance mode. Each slave will receive that transmission from the master. (A protocol must be
in place to address, or select, the desired slave device, but that discussion is beyond the
intentions of this tutorial, and is entirely application dependent). When the slave device has
received the data, it will respond by enabling its transmitter and transmitting data onto the bus,
then placing its transmitter into high impedance mode, just as the master did.