Specifications

Chapter 1 - Overview RS-422 And RS-485 Applications Ebook
Page 16 of 137 Manual Documentation Number: <DocNumber>
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Tri-State Control
One important feature of RS-485 systems is the ability to multidrop
network nodes. To do so the outputs of all RS-485 drivers not currently
transmitting must be effectively disconnected from the transmission
line. Without this feature, drivers (which are typically in the idle state
when not transmitting), would hold a negative voltage (logic 1 or mark)
on the transmission line. If another driver connected to the same
transmission line started sending data, it would not be able to pull the
line into the logic 0 state. This is called bus contention.
Figure 9: Tri-State Control
To facilitate multiple drivers on one transmission line, RS-485 balanced
line drivers have an enable input. By controlling the enable input the
driver can be effectively disconnected from the transmission line. In the
disconnected state the output becomes a high impedance to the line
and will not affect the state of the line at all. This effectively adds a