Overview

AN1690

4/15

1

Overview

In a point-to-point configuration (such as the RS-422 standard) the driver is normally always

enabled.

The bus can remain only in the HIGH or LOW state (the bus is always biased). In a

multipoint application, when more than one driver is physically connected to the bus and

only one driver at a time is enabled during data transmission, all the drivers can be disabled

when there is no data to send. In this case there is no bus biasing (undefined state). Fail-

safe biasing solves this problem providing the bus with a proper known state. This

application note describes the topic of fail-safe biasing.

2

Bus states

When a bus is driven by an active driver, it can be in one of two states, either high or low. It

can also be kept in one of these states by external pull-up resistors that provide the

necessary voltage to get a known bus state. The undefined state in RS-485 standard buses

occurs every time the differential voltage is less than +/-200 mV. In Figure 2 the bus is driven

from low to high and is then disabled. The bus, however, remains high due to external fail-

safe biasing.

Figure 2.

Differential plot for driver outputs