Conclusion

AN1690

14/15

Figure 12 and Figure 13 show how the choice of termination could influence the signal

quality at the end of the transmission line. In particular, the AC termination seems to be

worse than the DC one, when the cable length increases (the output presents jitter and

inter-symbolic interferences).

9

Conclusion

External fail-safe bias resistors can be used to solve the idle line state problem that

commonly occurs in multipoint applications using asynchronous protocols. This hardware

approach is well accepted. In fact many complete interface standards such as SCSI-1 and 2

(Small Computer System Interface) and IPI (Intelligent peripheral Interface) have adopted

this method. This application note provides guidance to select proper fail-safe schematic

and external component values that will provide an adequate bias, while minimizing the

loading effect on the line driver.

10

Revision history

Table 2.

Document revision history

Date

Revision

Changes

21-Jun-2004

1

First release

02-Oct-2007

2

– No content changes, document reformatted.

– ST485 replaced by ST485EB