AN1690 Datasheet, PDF (6/15 Pages) STMicroelectronics – Fail-safe biasing for ST485EB

Internal fail-safe and bus termination
4
Internal fail-safe and bus termination
AN1690
Note:
Transceiver manufacturers avoid external biasing resistors by providing internal pull-up
resistors at the receiver inputs, which is effective for detecting open circuits or for those
applications where termination resistors are not needed. The line termination resistors
(typically 54~120 Ω for a twisted pair cable) load the line avoiding the need for internal pull-
up resistors to define the receiver output. Figure 4, Figure 5, and Figure 6 show differential
voltage levels for different line conditions for the ST485EB receiver interface.
There is no driver leading the line.
Figure 4. Terminated line (on both sides)
Figure 5. Open terminated line (end side only)
Figure 6. Unterminated or open line
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