8.3.1 Receiver Failsafe

The differential receiver is failsafe to invalid bus states caused by:

• open bus conditions such as a disconnected connector
• shorted bus conditions such as cable damage shorting the twisted-pair together, or
• idle bus conditions that occur when no driver on the bus is actively driving

In any of these cases, the differential receiver will output a failsafe logic High state so that the output of the receiver is not indeterminate.

Receiver failsafe is accomplished by offsetting the receiver thresholds so that the “input indeterminate” range does not include zero volts differential.  In order to comply with the RS-422 and RS-485 standards, the receiver output must output a High when the differential input V_ID is more positive than 200 mV, and must output a Low when the V_ID is more negative than –200 mV.  The receiver parameters which determine the failsafe performance are V_IT+ and V_IT– and V_HYS.  As seen in the Electrical Characteristics table, differential signals more negative than
–200 mV will always cause a Low receiver output.  Similarly, differential signals more positive than 200 mV will always cause a High receiver output.

When the differential input signal is close to zero, it will still be above the V_IT+ threshold, and the receiver output will be High.  Only when the differential input is more negative than V_IT– will the receiver output transition to a Low state.  So the noise immunity of the receiver inputs during a bus fault condition includes the receiver hysteresis value V_HYS (the separation between V_IT+ and V_IT– ) as well as the value of V_IT+.

Signals which transition from positive to negative (or from negative to positive) will transition only once, ensuring no spurious bits.

8.3.2 Low-Power Standby Mode

When both the driver and receiver are disabled (DE transitions to a low state and RE transitions to a high state) the device enters standby mode. If the enable inputs are in this state for a brief time (e.g. less than 100 ns), the device does not enter standby mode. This prevents inadvertently entering standby mode during driver/receiver enabling. Only when the enable inputs are held in this state a sufficient duration (e.g. for 300 ns or more), the device is assured to be in standby mode. In this low-power standby mode, most internal circuitry is powered down, and the steady-state supply current is typically less than 400 nA. When either the driver or the receiver is re-enabled, the internal circuitry becomes active.

Table 1. Driver Function Table

INPUT	ENABLE	OUTPUTS
D	DE	A	B
H	H	H	L	Actively drive bus High
L	H	L	H	Actively drive bus Low
X	L	Z	Z	Driver disabled
X	OPEN	Z	Z	Driver disabled by default
OPEN	H	H	L	Actively drive bus High by default

Table 2. Receiver Function Table

DIFFERENTIAL INPUT	ENABLE	OUTPUT
VID = VA – VB	RE	R
VIT+ < VID	L	H	Receive valid bus High
VIT– < VID < VIT+	L	?	Indeterminate bus state
VID < VIT–	L	L	Receive valid bus Low
X	H	Z	Receiver disabled
X	OPEN	Z	Receiver disabled by default
Open-circuit bus	L	H	Fail-safe high output
Short-circuit bus	L	H	Fail-safe high output
Idle (terminated) bus	L	H	Fail-safe high output