Internal fail-safe and bus termination

AN1690

6/15

4

Internal fail-safe and bus termination

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.

Note:

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