Datasheet

SN65HVD1785, SN65HVD1786
SN65HVD1787, SN65HVD1791
SN65HVD1792, SN65HVD1793
SLLS872H –JANUARY 2008–REVISED FEBRUARY 2010 www.ti.com
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 outputs a failsafe logic High state, so that the output of the
receiver is not indeterminate.
In the HVD17xx family of RS-485 devices, 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 HVD17xx receiver
parameters which determine the failsafe performance are V
IT+
and V
IT-
and V
HYS
. In the Electrical Characteristics
table, V
IT-
has a typical value of -150 mV and a minimum (most negative) value of -200 mV, so 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, because the typical value of V
IT+
is -100mV, and V
IT+
is
never more positive than -10 mV under any conditions of temperature, supply voltage, or common-mode offset.
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+
.
For the HVD17xx devices, the typical noise immunity is typically about 150 mV, which is the negative noise level
needed to exceed the V
IT-
threshold (V
IT-
TYP = -150 mV). In the worst case, the failsafe noise immunity is never
less than 40 mV, which is set by the maximum positive threshold (V
IT+
MAX = -10mV) plus the minimum
hysteresis voltage (V
HYS
MIN = 30 mV).
70-V Fault-Protection
The SN65HVD17xx family of RS-485 devices is designed to survive bus pin faults up to ±70V. The devices
designed for fast signaling rate (10 Mbps) will not survive a bus pin fault with a direct short to voltages above
30V when:
1. the device is powered on AND
2a. the driver is enabled (DE=HIGH) AND D=HIGH AND the bus fault is applied to the A pin OR
2b. the driver is enabled (DE=HIGH) AND D=LOW AND the bus fault is applied to the B pin
Under other conditions, the device will survive shorts to bus pin faults up to 70V. Table 1 summarizes the
conditions under which the device may be damaged, and the conditions under which the device will not be
damaged.
Table 1. Device Conditions
POWER DE D A B RESULTS
OFF X X -70V < V
A
< 70V -70V < V
B
< 70V Device survives
ON LO X -70V < V
A
< 70V -70V < V
B
< 70V Device survives
ON HI L -70V < V
A
< 70V -70V < V
B
< 30V Device survives
ON HI L -70V < V
A
< 70V 30V < V
B
Damage may occur
ON HI H -70V < V
A
< 30V -70V < V
B
< 30V Device survives
ON HI H 30V < V
A
-70V < V
B
< 30V Damage may occur
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Product Folder Link(s): SN65HVD1785, SN65HVD1786 SN65HVD1787, SN65HVD1791 SN65HVD1792, SN65HVD1793