Datasheet

EFFECTIVEPULSEWIDTH
REFRESHTIME
ACTIVEDRIVERSTATE
BUSDIFFERENTIAL VOLTAGE
ISO1176 RECEIVEROUTPUT
RECEIVERPOSITIVETHRESHOLD
RECEIVERNEGATIVETHRESHOLD
DISABLED
EFFECTIVEPULSEWIDTH
REFRESHTIME
ACTIVEDRIVERSTATE
BUSDIFFERENTIAL VOLTAGE
ISO1176 RECEIVEROUTPUT
RECEIVERPOSITIVETHRESHOLD
RECEIVERNEGATIVETHRESHOLD
ISO1176
www.ti.com SLLS897D –MARCH 2008–REVISED MARCH 2010
ISO1176 “Sticky Bit” Issue (Under Certain Conditions)
Summary: In applications with sufficient differential noise on the bus, the output of the ISO1176 receiver may
“stick” at an incorrect state for up to 30 µs.
Description: The ISO1176 isolated Profibus (RS-485) transceiver is rated for signaling up to 40 Mbps on
twisted-pair bus lines. The receiver thresholds comply with RS-485 and Profibus specifications; an input
differential voltage V
ID
= V
A
- V
B
> 200 mV causes a logic High on the R output, and V
ID
< -200 mV causes a
logic Low on the R output. To assure a known receiver output when the bus is shorted or idle, the upper
threshold is set below zero, such that V
ID
= 0 mV causes a logic High on the R output. The data sheet specifies a
typical upper threshold (V
IT+
) of -80 mV and a typical lower threshold (V
IT-
) of -120 mV.
At a signaling rate of 40 Mbps, each valid data bit has a duration of 25 ns. At typical Profibus signaling rates of
12 Mbps or lower, each valid data bit has a duration of 83 ns or more. The ISO1176 correctly sets the R output
for each of these valid data bits.
In applications with a high degree of differential noise on the bus lines, it is possible to get short periods when an
invalid bus voltage triggers a change in state of the internal receiver circuits. An issue with the digital isolation
channel in the ISO1176 may cause the invalid receiver state to “stick” rather than immediately transition back to
the correct state. The receiver output will always transition to the correct state, but may stick in the incorrect state
for up to 30 µs. This can cause a temporary loss of data.
Figure 30 shows two cases which could result in temporary loss of data.
Figure 30.
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