DN203 - RS485 Transceivers Sustain ±60V Faults

RS485 Transceivers Sustain ±60V Faults – Design Note 203
Gary Maulding
Introduction
The LT®1785 and LT1791 RS485/RS422 transceivers
with ±60V fault tolerance solve a real-world problem
of field failures in typical RS485 interface circuits.
Modems and other computer peripherals use point-topoint RS422 connections to support higher communication speeds with better noise immunity over greater
distances than is possible with RS232 connections.
Multipoint RS485 networks are used for LANs and
industrial control networks. All of these applications are
vulnerable to the unknown, sometimes hostile environment outside of the controlled, shielded environment
of a typical electrical equipment chassis. Because the
RS485 transceivers are directly in the line of fire, the
transceiver chips are often socketed PDIP packages to
allow easy field servicing of equipment. Field failures
in standard transceiver circuits are caused by data-line
voltages exceeding the absolute maximum ratings of
the transceiver chips. Installation wiring faults, ground
voltage faults and lightning-induced surge voltages are
all common causes of overvoltage conditions.
5mA
0mA
1mA/DIV
–60V
–5mA
–60
0V
VA, VB
RE
DE
DI
1
RX
B 7
The LT1785 and LT1791, with ±60V absolute maximum ratings on the driver output and receiver input
pins are inherently safe in most environments that
will destroy other interface circuits. Standard pinouts
in either PDIP or SO packages allow easy upgrades
to existing RS422/RS485 networks. Whether the
circuit is transmitting, receiving, in standby or powered off, any voltage within ±60V will be tolerated
by the chip without damage. Data communication
will be interrupted during the fault condition, but the
circuit will live to talk another day. Figure 1 shows the
I-V characteristics at the RS485 input/output pins.
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of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
DN203 F01
Figure 1. LT1785 Input Current vs VIN
RO
Up to ±60V Faults
The electrical standards for RS422 and RS485 signaling reflect the need for tolerance of ground voltage
drops in an extended network by requiring receivers to
operate with input common mode voltages from –7V
to 12V. The RS485 and RS422 transceivers commonly
available from various vendors are all vulnerable to
damage from fault voltages only slightly outside of the
operating envelope. One vendor’s RS485 transceivers
have absolute maximum voltage ratings of –8V to 12.5V
on the data I/O pins. Such narrow margins beyond
the required –7V to 12V operating conditions makes
such circuits very fragile in a real-world environment.
In addition, external protection circuitry is ineffective
at protecting these circuits without corrupting normal
operating signal levels.
RT
120Ω
RT
120Ω
7 B
1
RX
2
2
4
6 A
A 6
3
6
A
TX
6
A
7
B
7
B
LT1785
3
TX
LT1785
4
DI
1
3 2
DE RE RO
4
DI
1
3 2
DE RE RO
DN203 F02
Figure 2. Half-Duplex RS485 Network Operation ±60V DC, ±15kV ESD Protected
04/99/203_conv
4
RO
RE
DE
DI
128-Node Networks at 250kbaud
In addition to their unique fault tolerance capabilities,
these transceivers feature high input impedance to
support extended RS485 networks of up to 128 nodes
(Figure 2). Controlled slew-rate outputs minimize EMI
problems while supporting data rates up to 250kbaud
(see Figure 3). Driver outputs are capable of working
with inexpensive telephone cable with characteristic
impedance as low as 72Ω with no loss of signal amplitude. “A” grade devices are available that ensure “fail
safe” receiver outputs when inputs are open, shorted
or no signal is present.
RX OUT
TX OUT
TX IN
DN203 F03
Figure 3. Normal Operation Waveforms at 250kbaud
Extending Protection Beyond ±60V
While ±60V fault tolerance forgives a great number of
sins, higher voltage demons may still be lurking. ESD is
one such demon, with voltage spikes into the thousands
of volts. The LT1785 and LT1791 have on-chip protection
to ±15kV air gap ESD transients for other high voltage
faults, such as lightning-induced surge voltages or AC
line shorts. For such high energy faults, external protection must be used to protect the circuits. Typical protection networks use voltage clamping and current limiting
networks. In concept, such networks could be used with
normal RS485 circuits to afford extended protection, but
in practice, the addition of protection networks would
8
RO
RE
DE
DI
1
RX
VCC
±60V
B 7 MAX
interfere with normal operation of the data network.
The voltage clamping Zeners or TransZorbs are not
available in tight voltage tolerances, and in addition,
their internal impedances cause several volts of additional potential above their nominal breakdown voltage
to appear at the protected device’s pins. To protect a
circuit with a –8V to 12.5V absolute maximum voltage
rating would require the use of protection devices with
voltage ratings much below the required common mode
range of RS485 networks interfering with normal data
transmission.
Figure 4 gives an example of the use of external clamping and limiting components to extend the LT1785’s
±60V tolerance to the peak 120V AC line voltage. 36V
TransZorbs are used to clamp the transceiver’s line pins
below the 60V capability of the transceiver. During a
120V AC line fault, peak surge currents of nearly 3A
will flow through the 47Ω limiting resistors and the
PolySwitch limiters. The peak current rating and series
resistance of the TransZorbs must be considered when
selecting the clamp device to ensure that the clamp
limiter can withstand the surge and that the peak voltage
will remain below the ±60V limitations of the LT1785.
At 3A, even high current TransZorbs will exceed their
nominal breakdown voltage by several volts, making this
protection method ineffective with transceiver circuits
with only 1V to 4V margin above their operating ranges.
The PolySwitch limiters are thermally activated and
increase in resistance by many orders of magnitude
in about 10ms. After the PolySwitch transition, fault
currents are only a few milliamperes. Carbon composite
resistors must be used for limiting the initial surge current before the PolySwitch transition point. Metal film
resistors do not have effective surge overload ratings
and will fail before the PolySwitch transition drops the
currents to sustainable levels.
RAYCHEM
POLYSWITCH
TR600-150
w 2 47Ω
120V AC
LINE FAULT
2
3
4
RT,120Ω
LT1785
TX
A 6
47Ω
CARBON
COMPOSITE
5W
5
1.5KE36CA
0.1μF
300V
LIMITER NETWORK
DN203 F04
Figure 4. Limiter Network Clamps 120V AC Fault Voltage to Less Than ±60V
Data Sheet Download
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