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Interface (Data Transmission)
Texas Instruments Incorporated
Detection of RS-485 signal loss
By Kevin Gingerich (Email: k-gingerich@ti.com)
High-Performance Linear/Interface
Introduction
Fault isolation and safety shutdown protocols are critical
in many industrial, telecommunication, automotive, and
data processing systems. While built-in test routines may
provide fault isolation when the system is offline, real-time
fault detection requires continuous monitoring of signals.
These systems often use RS-485 to share data between
sensors, actuators, single-board computers, or communi-
cation processors.
RS-485 signals are differential, using two signal wires to
transmit data, and detection of valid signal levels requires
a differential window comparator. Designing this circuit
function is complicated by the wide common-mode range
of RS-485 signals and, in many cases, the availability of
only positive supply rails.
This article shows how a differential window comparator
can be constructed with the passive-failsafe feature* of
two SN65HVD3088E RS-485 transceivers and an AND
gate. It also provides theory of operation, the basic circuit
schematic, test results, and other design considerations.
Theory of operation
The differential input threshold is the voltage between the
non-inverting and inverting RS-485 signals above which
the bus state is high and below which the bus state is
low. The differential input voltage threshold of standard
receivers is between –200 mV and 200 mV. The differential
input voltage threshold of the SN65HVD3088E is between
–200 mV and –10 mV. This gives a known (high-level)
receiver output state with zero volts (no input signal) and
is called passive failsafe . It does not distinguish between
a valid high input and no signal.
A single SN65HVD3088E can determine if the differen-
tial input voltage is less than –200 mV or above –10 mV.
Reversing the input polarity of a second SN65HVD3088E
can determine if the differential input voltage is below
10 mV or above 200 mV and is the basis for constructing
the differential window comparator shown in Figure 1.
*See Reference 1 for more on this feature.
Figure 1. RS-485 transceiver with loss-of-signal indicator
Primary RS-485 Transceiver
SN65HVD3088E
A
120
Ω
ptional
Transmit Data
O
B
DE
RE
Received Data
SN65HVD3088E
Added Circuit
V>– 0mV
AB
F
–10 mV < V AB <10mV
V AB <10mV
SN74AHCT1G08
or Equivalent
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Analog Applications Journal
High-Performance Analog Products
4Q 2006
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Texas Instruments Incorporated
Interface (Data Transmission)
The upper-receiver output in Figure 1 is true (high) if
the differential input voltage, V AB , is greater than –10 mV.
Since the inputs of the lower receiver are reversed, the
output is true (high) if –V AB > –10 mV or, dividing both
sides of the inequality by negative one, V AB < 10 mV. If
both receiver outputs are true, then the differential bus
voltage is between –10 mV and 10 mV and is not a valid
input. This fault is indicated by the AND gate F output
using inputs of the two receiver outputs.
Test results
Figures 2 and 3 show the F and V AB low-to-no and high-to-
no signal transitions and the desired fault indication.
Other design considerations
While this example circuit uses the SN65HVD3088E, any
RS-485 receiver with the passive failsafe feature may be
used (Texas Instruments offers over thirty such products).
A similar approach may be applied to unidirectional
(simplex) connections. The parallel connection of the
two transceivers will halve the unit loading and double the
stray capacitance presented to the bus. This may limit the
number and spacing of devices on a bus segment (see
References 2 and 3).
If the system timing budget allows, filtering of F may
prevent false fault indications from differential noise or
from very slowly changing input signals. Filtering may be
done by adding gating or by choosing a very slow AND gate.
Conclusion
A differential window comparator can be constructed by
adding a passive-failsafe RS-485 receiver and one AND
gate to another passive-failsafe receiver. The circuit then
provides a loss-of-signal indication from an RS-485 data
bus. This fault flag may then be used for system fault
isolation or safety shutdown protocols.
References
For more information related to this article, you can down-
load an Acrobat Reader file at www-s.ti.com/sc/techlit/
litnumber and replace “litnumber” with the TI Lit. # for
the materials listed below.
Figure 2. Loss of valid low-level signal
F
V AB
Document Title
TI Lit. #
1. Kevin Gingerich, “Failsafe in RS-485 data
buses,” Analog Applications Journal
(3Q 2004) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .slyt080
2. Kevin Gingerich, “The RS-485 unit load and
maximum number of bus connections,”
Analog Applications Journal (1Q 2004) . . . . .slyt086
3. Kevin Gingerich, “Device spacing on RS-485
buses,” Analog Applications Journal
(2Q 2006) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .slyt241
Related Web sites
Time (100 ns/div)
Figure 3. Loss of valid high-level signal
V AB
F
Time (200 ns/div)
19
Analog Applications Journal
4Q 2006
High-Performance Analog Products
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SLYT257
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