DS485
Low Power RS-485/RS-422 Multipoint Transceiver
General Description
The DS485 is a low-power transceiver for RS-485 and
RS-422 communication. The device contains one driver and
one receiver. The drivers slew rate allows for operation up to
2.5 Mbps (see Applications Information section).
The transceiver draws 200 µA of supply current when un-
loaded or fully loaded with the driver disabled and operates
from a single +5V supply.
The driver is short-circuit current limited and is protected
against excessive power dissipation by thermal shutdown
circuitry that places the driver outputs into TRI-STATE®
(High Impedance state) under fault conditions. The driver
guarantees a minimum of 1.5V differential output voltage
with maximum loading across the common mode range
(V
OD3
).
The receiver has a failsafe feature that guarantees a
logic-high output if the input is open circuit.
The DS485 is available in surface mount and DIP packages
and is characterized for Industrial and Commercial tempera-
ture range operation.
Features
nMeets TIA/EIA RS-485 multipoint standard
nGuaranteed full load output voltage (V
OD3
)
nLow quiescent current: 200 µA typ
n−7V to +12V common-mode input voltage range
nTRI-STATE outputs on driver and receiver
nAC performance:
— Driver transition time: 25 ns typ
— Driver propagation delay: 40 ns typ
— Driver skew: 1 ns typ
— Receiver propagation delay: 200 ns typ
— Receiver skew: 20 ns typ
nHalf-duplex flow through pinout
nOperates from a single 5V supply
nAllows up to 32 transceivers on the bus
nCurrent-limiting and thermal shutdown for driver
overload protection
nIndustrial temperature range operation
nPin and functional compatible with MAX485 and LTC485
Connection and Logic Diagram
Order Number Temp. Range Package/###
DS485N 0˚C to +70˚C DIP/N08E
DS485M 0˚C to +70˚C SOP/M08A
DS485TN −40˚C to +85˚C DIP/N08E
DS485TM −40˚C to +85˚C SOP/M08A
Truth Table
DRIVER SECTION
RE*DE DI A B
XHHHL
XHLLH
XLXZZ
RECEIVER SECTION
RE*DE A-B RO
LL ≥+0.2V H
LL ≤−0.2V L
HX X Z
L L OPEN*H
*Note: Non Terminated, Open Input only
X=indeterminate
Z=TRI-STATE
TRI-STATE®is a registered trademark of National Semiconductor Corporation.
DIP and SOIC
DS012880-1
July 1998
DS485 Low Power RS-485/RS-422 Multipoint Transceiver
© 1999 National Semiconductor Corporation DS012880 www.national.com
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage (V
CC
) +12V
Enable Input Voltage (RE*, DE) −0.5V to (V
CC
+ 0.5V)
Driver Input Voltage (DI) −0.5V to (V
CC
+ 0.5V)
Driver Output Voltage (A, B) −14V to +14V
Receiver Input Voltage (A, B) −14V to +14V
Receiver Output Voltage (RO) −0.5V to (V
CC
+ 0.5V)
Maximum Package Power Dissipation @+25˚C
M Package 1.19W
N Package 0.74W
Derate M Package 9.5 mW/˚C above +25˚C
Derate N Package 6.0 mW/˚C above +25˚C
Maximum Package Power Dissipation @+70˚C
M Package 0.76W
N Package 0.47W
Storage Temperature Range −65˚C to +150˚C
Lead Temperature Range
(Soldering, 4 sec.) +260˚C
ESD (HBM) ≥2kV
Recommended Operating
Conditions
Min Typ Max Units
Supply Voltage (V
CC
) +4.75 +5.0 +5.25 V
Operating Free Air
Temperature (T
A
)
DS485 0 +25 +70 ˚C
DS485T −40 +25 +85 ˚C
Bus Common Mode Voltage −7 +12 V
Electrical Characteristics
Over Supply Voltage and Operating Temperature Ranges, unless otherwise specified (Notes 2, 3)
Symbol Parameter Conditions Pin Min Typ Max Units
V
OD1
Differential Driver Output Voltage (No Load) A, B 5 V
V
OD2
Differential Driver Output Voltage R
L
=50Ω, (RS422),
Figure 1
2 2.8 V
with Load R
L
=27Ω, (RS485),
Figure 1
1.5 2.3 5 V
∆V
OD
Change in Magnitude of Output R
L
=27Ωor 50Ω(Note 4) 0.2 |V|
Differential Voltage
V
OD3
Differential Driver Output Voltage— R1 =54Ω,R2=375Ω1.5 2.0 5 V
Full Load with Max V
CM
V
TEST
=−7V to +12V,
Figure 5
V
OC
Driver Common-Mode Output Voltage R
L
=27Ωor 50Ω,
Figure 1
3V
∆V
OC
Change in Magnitude of Common-Mode R
L
=27Ωor 50Ω,
Figure 1
(Note 4) 0.2 |V|
Output Voltage
V
IH
Input High Voltage DI,
DE,
RE*
2.0 V
V
IL
Input Low Voltage 0.8 V
I
IN1
Input Current V
IN
=0V or V
CC
±2µA
I
IN2
Input Current (Note 5) V
IN
=+12V A, B 1.0 mA
DE =0V, V
CC
=0V or 5.25V V
IN
=−7V −0.8 mA
V
TH
Receiver Differential Threshold Voltage −7V ≤V
CM
≤+12V −0.2 0.2 V
∆V
TH
Receiver Input Hysteresis V
CM
=0V 70 mV
V
OH
Receiver Output High Voltage I
O
=−4 mA, V
ID
=0.2V RO 3.5 V
V
OL
Receiver Output Low Voltage I
O
=4 mA, V
ID
=−0.2V 0.4 V
I
OZR
TRI-STATE Output Current at Receiver 0.4V ≤V
O
≤2.4V ±1µA
R
IN
Receiver Input Resistance −7V ≤V
IN
≤+12V A, B 12 kΩ
I
CC
No-Load Supply Current (Note 6) DE =V
CC
,RE
*=0V or V
CC
V
CC
200 900 µA
DE =0V, RE*=0V or V
CC
200 500 µA
I
OSD1
Driver Short Circuit Current, V
O
=HIGH −7V ≤V
O
≤+12V A, B 35 250 mA
I
OSD2
Driver Short Circuit Current, V
O
=LOW −7V ≤V
O
≤+12V 35 250 mA
I
OSR
Receiver Short Circuit Current 0V ≤V
O
≤V
CC
RO 7 85 mA
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Switching Characteristics
Over Supply Voltage and Operating Temperature Ranges, unless otherwise specified (Notes 3, 7, 8)
Symbol Parameter Conditions Min Typ Max Units
t
PLHD
Driver Differential Propagation Delay— Low to High R
L
=54Ω,C
L
=100 pF 10 40 65 ns
t
PHLD
Driver Differential Propagation Delay— High to Low 10 39 65 ns
t
SKEW
Differential Skew |t
PHLD
−t
PLHD
| 1 10 ns
t
r
Driver Rise Time 32540 ns
t
f
Driver Fall Time 32540 ns
t
ZH
Driver Enable to Output High C
L
=100 pF 170 ns
t
ZL
Driver Enable to Output Low C
L
=100 pF 170 ns
t
LZ
Driver Disable from Output Low C
L
=15 pF 170 ns
t
HZ
Driver Disable from Output High C
L
=15 pF 170 ns
t
PLHD
Receiver Differential Propagation Delay— Low to High C
L
=15 pF (RO) 70 190 320 ns
t
PHLD
Receiver Differential Propagation Delay— High to Low 70 210 320 ns
t
SKEW
Differential Skew |t
PHLD
−t
PLHD
|2050ns
t
ZH
Receiver Enable to Output High C
L
=15 pF 110 ns
t
ZL
Receiver Enable to Output Low 110 ns
t
LZ
Receiver Disable from Output Low 110 ns
t
HZ
Receiver Disable from Output High 110 ns
f
max
Maximum Data Rate (Note 9) 2.5 Mbps
Note 1: “Absolute Maximum Ratings” are those values beyond which the safety of the device cannot be guaranteed. They are not meant to imply that the devices
should be operated at these limits. The table of “Electrical Characteristics” specifies conditions of device operation.
Note 2: Current into device pins is defined as positive. Current out of device pins is defined as negative. All voltages are referenced to ground except VOD1/2/3 and
VID.
Note 3: All typicals are given for: VCC =+5.0V, TA=+25˚C.
Note 4: ∆|VOD| and ∆|VOC| are changes in magnitude of V OD and VOCrespectively, that occur when the input changes state.
Note 5: IIN2 includes the receiver input current and driver TRI-STATE leakage current.
Note 6: Supply current specification is valid for loaded transmitters when DE =0V or enabled (DE =H) with no load.
Note 7: f=1 MHz, trand tf≤6 ns, ZO=50Ω.
Note 8: CLincludes jig and probe capacitance.
Note 9: fmax is the guaranteed data rate for 50 ft of twisted pair cable. f max may be conservatively determined from the ratio of driver transition time (tr) to the data
rate unit interval (1/fmax). Using a 10%ratio yields fmax =(0.1)/40 ns =2.5 Mb/s. Higher data rates may be supported by allowing larger ratios.
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Parameter Measurement Information
DS012880-2
FIGURE 1. V
OD
DS012880-4
FIGURE 2.
DS012880-6
FIGURE 3.
DS012880-8
FIGURE 4.
DS012880-3
FIGURE 5. V
OD3
DS012880-5
FIGURE 6.
DS012880-7
FIGURE 7.
DS012880-9
FIGURE 8.
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Parameter Measurement Information (Continued)
DS012880-10
FIGURE 9.
DS012880-11
FIGURE 10.
DS012880-12
FIGURE 11.
DS012880-13
FIGURE 12.
DS012880-14
FIGURE 13.
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Parameter Measurement Information (Continued)
Pin Descriptions
Pin #I/O Name Function
1 O RO Receiver Output: If A >B by 200 mV, RO will be high; If A <B by 200 mV, RO will be low. RO will
be high also if the inputs (A and B) are open (non-terminated).
2IRE
*
Receiver Output Enable: RO is enabled when RE*is low; RO is in TRI-STATE when RE*is high.
3 I DE Driver Output Enable: The driver outputs (A and B) are enabled when DE is high; they are in
TRI-STATE when DE is low. Pins A and B also function as the receiver input pins (see below).
4 I DI Driver Input: A low on DI forces A low and B high while a high on DI forces A high and B low when
the driver is enabled.
5 NA GND Ground
6 I/O A Non-inverting Driver Output and Receiver Input pin. Driver output levels conform to RS-485 signaling
levels.
7 I/O B Inverting Driver Output and Receiver Input pin. Driver output levels conform to RS-485 signaling
levels.
8NAV
CC
Power Supply: 4.75V ≤V
CC
≤5.25V
Related National Low Power RS-485 Transceivers
Part Number Temperature Number of Comments
Range XCVRs on Bus
DS36C278 0˚C to +70˚C 128 Ultra Low Power Transceiver
DS36C278T −40˚C to +85˚C 64 Ultra Low Power Transceiver
DS36C279 0˚C to +70˚C 128 Auto-Sleep Mode
DS36C279T −40˚C to +85˚C 64 Auto-Sleep Mode
DS36C280 0˚C to +70˚C 128 Adjustable Slew Rate Control
DS36C280T −40˚C to +85˚C 64 Adjustable Slew Rate Control
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Applications Information
The DS485 is a low power transceiver designed for use in
RS-485 multipoint applications. The DS485 can transmit
data up to 2.5 Mbps based on a ratio of driver transition time
to the unit interval (bit time) of 10%. This maximum data rate
may be further limited by the interconnecting media. The
DS485 provides a standard unit load to the RS-485 bus
across the common mode range of −7V to +12V. This allows
up to 32 transceivers (standard unit load) to be connected to
the bus. More transceivers may be connected to the bus if
they support a reduced unit load (see DS36C278). The
DS485 also guarantees the driver’s output differential volt-
age into a worst case load that models standard termination
loads and 32 unit loads referenced to the maximum common
mode voltage extremes. With a minimum of 1.5V swing into
this load, a 1.3V differential noise margin is supported along
with the standard common mode rejection range of the re-
ceivers.
Due to the multipoint nature of the bus, contention between
drivers may occur. This will not cause damage to the drivers
since they feature short-circuit protection and also thermal
shutdown protection. Thermal shutdown senses die tem-
perature and puts the driver outputs into TRI-STATE if a fault
condition occurs that causes excessive power dissipation
which can elevate the junction temperature to +150˚C.
A typical multipoint application is shown in the following fig-
ure. Note that termination is typically required but is only lo-
cated at the two ends of the cable (not on every node). Com-
monly pull up and pull down resistors may be required at one
end of the bus to provide a failsafe bias. These resistors pro-
vide a bias to the line when all drivers are in TRI-STATE. See
National Application Note 847 for a complete discussion of
failsafe biasing of differention buses.
Multipoint RS-485 Application
DS012880-16
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8
Physical Dimensions inches (millimeters) unless otherwise noted
Order Number DS485M or DS485TM
NS Package Number M08A
Order Number DS485N or DS485TN
NS Package Number N08E
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DS485 Low Power RS-485/RS-422 Multipoint Transceiver
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