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_______________General Description
The MAX3480A/MAX3480B are electrically isolated
RS-485/RS-422 data-communications interfaces.
Transceivers, optocouplers, and a transformer are all
included in one low-cost, 28-pin DIP package. A single
+3.3V supply on the logic side powers both sides of the
interface.
The MAX3480B features reduced-slew-rate drivers that
minimize EMI and reduce reflections caused by
improperly terminated cables, allowing error-free data
transmission at data rates up to 250kbps. The
MAX3480A’s driver slew rate is not limited, allowing
transmission rates up to 2.5Mbps.
These devices typically draw 180mA of quiescent sup-
ply current. The MAX3480B provides a low-power shut-
down mode in which it consumes only 0.2µA.
Drivers are short-circuit current limited and are protect-
ed against excessive power dissipation by thermal
shutdown circuitry that places the driver outputs into a
high-impedance state. The receiver input has a fail-safe
feature that guarantees a logic-high output if the input
is open circuit.
The MAX3480A/MAX3480B typically withstand 1600VRMS
(1 minute) or 2000VRMS (1 second). Their isolated inputs
and outputs meet RS-485/RS-422 specifications.
________________________Applications
Isolated RS-485/RS-422 Data Interface
Transceivers for EMI-Sensitive Applications
Industrial-Control Local Area Networks
Automatic Test Equipment
HVAC/Building Control Networks
____________________________Features
♦Isolated Data Interface to 1600VRMS (1 minute)
♦Slew-Rate-Limited Data Transmission (MAX3480B)
♦High-Speed, Isolated, 2.5Mbps RS-485 Interface
(MAX3480A)
♦-7V to +12V Common-Mode Input Voltage Range
with Respect to Isolated Ground
♦Single +3.3V Supply
♦Current Limiting and Thermal Shutdown for
Driver Overload Protection
♦Standard 28-Pin DIP Package
MAX3480A/MAX3480B
Complete, Isolated, 3.3V
RS-485/RS-422 Data Interface
________________________________________________________________
Maxim Integrated Products
1
28
27
26
25
24
23
22
21
1
2
3
4
5
6
7
8
AC1
AC2
ISO VCC1
B
D2
D1
VCC2
VCC1
ISO RO DRV
A
ISO DI IN
ISO DE INVCC3
SD
FS
GND1
20
19
18
17
9
10
11
12
ISO COM1
ISO DI DRV
ISO VCC2
ISO DE DRV
GND2
DE
VCC4
DI
16
15
13
14
ISO COM2
ISO RO LED
VCC5
RO
MAX485
MAX487
MAX845
ISOLATION BARRIER
TOP VIEW
MAX3480A/B
DIP
__________________Pin Configuration
19-0453; Rev 0; 1/96
PART
MAX3480ACPI
MAX3480AEPI
MAX3480BCPI 0°C to +70°C
-40°C to +85°C
0°C to +70°C
TEMP. RANGE PIN-PACKAGE
28 Plastic DIP
28 Plastic DIP
28 Plastic DIP
MAX3480BEPI -40°C to +85°C 28 Plastic DIP
DATA
RATE
(kbps)
2500
2500
250
250
______________Ordering Information
MAX3480A/MAX3480B
Complete, Isolated, 3.3V
RS-485/RS-422 Data Interface
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VCC = VCC1 = VCC2 = VCC4 = VCC5 = 3.0V to 3.6V, FS = 0V, TA= TMIN to TMAX, unless otherwise noted. Typical values are at
VCC = VCC1 = VCC2 = VCC4 = VCC5 = 3.3V and TA= +25°C.) (Notes 1, 2, 3)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
With Respect to GND:
Supply Voltage (VCC1, VCC2, VCC4, VCC5).........-0.3V to +3.8V
Supply Voltage (VCC3) ..........................................-0.3V to +7V
Control Input Voltage (SD, FS)................-0.3V to (VCC + 0.3V)
Receiver Output Voltage (RO).................-0.3V to (VCC + 0.3V)
With Respect to ISO COM:
Control Input Voltage (ISO DE _) .........-0.3V to (ISO VCC + 0.3V)
Driver Input Voltage (ISO DI _).........-0.3V to (ISO VCC + 0.3V)
Receiver Output Voltage (ISO RO _)......-0.3V to (ISO VCC + 0.3V)
Driver Output Voltage (A, B)................................-8V to +12.5V
Receiver Input Voltage (A, B)..............................-8V to +12.5V
LED Forward Current (DI, DE, ISO RO LED) ......................50mA
Continuous Power Dissipation (TA= +70°C)
Plastic DIP (derate 9.09mW/°C above +70°C)...............727mW
Operating Temperature Ranges
MAX3480_CPI......................................................0°C to +70°C
MAX3480_EPI...................................................-40°C to +85°C
Storage Temperature Range.............................-65°C to +160°C
Lead Temperature (soldering, 10sec).............................+300°C
R = 50Ω(RS-422)
TA= +25°C, VISO = 50VDC
TA= +25°C, VISO = 50VDC
FS = 0V
DE´, DI´(Figure 1)
DE´, DI´(Figure 1)
FS high
FS low
SD = VCC3
Low
High
RL= 54Ω
MAX3480A,
DE´= VCC or open RL= ∞
CONDITIONS
V
0.3
VOD2
Differential Driver Output (no load) V
2
VOD1 8pF10CISO
Isolation Capacitance MΩ100 10,000RISO
Isolation Resistance V0.4VIL
Input Low Voltage VVCC - 0.4VIH
Input High Voltage pA10IFSM
FS Input Leakage Current µA50IFSL
FS Input Pull-Up Current
V
0.8VFSL
FS Input Threshold 2.4VFSH
180 260
V
600
VS3.0 3.3 3.6Supply Voltage
µAShutdown Supply Current (Note 3) 0.2
mA
240
ICC
Operating Supply Current 280
120 200
UNITSMIN TYP MAXSYMBOLPARAMETER
ISHDN
R = 27Ω(RS-485), Figure 3 1.5 5
Switch Frequency fSWL kHz
RL= 54Ω
R = 27Ωor 50Ω,
Figure 3
R = 27Ωor 50Ω, Figure 4
V
4
∆VOD
Change in Magnitude of Driver
Output Voltage for Complementary
Output States
fSWH 900
RL= ∞
FS = VCC or open
MAX3480B,
DE´= VCC or open
Differential Driver Output (with load)
Driver Common-Mode Output VOC V
Differential
0.3Common Mode
0.8
VIN = 12V
MAX3480A/MAX3480B
Complete, Isolated, 3.3V
RS-485/RS-422 Data Interface
_______________________________________________________________________________________ 3
DI´= VCC
CONDITIONS 1.0
ISO IIN
VCM = 0V
V0.4VOL
Receiver Output/Receiver Output
Low Voltage
mA
70∆VTH
Input Current (A, B)
UNITSMIN TYP MAXSYMBOLPARAMETER
ELECTRICAL CHARACTERISTICS (continued)
(VCC = VCC1 = VCC2 = VCC4 = VCC5 = 3.0V to 3.6V, FS = 0V, TA= TMIN to TMAX, unless otherwise noted. Typical values are at
VCC = VCC1 = VCC2 = VCC4 = VCC5 = 3.3V and TA= +25°C.) (Notes 1, 2, 3)
Receiver Input Hysteresis
VOUT = 3.6V, DI´= 0V µA250IOH
Receiver Output/Receiver Output
High Current
-7V ≤VO≤12V (Note 4) mA100ISO IOSD
Driver Short-Circuit Current
DE´= 0V,
VCC = 0V or 3.6V
mV
SWITCHING CHARACTERISTICS—MAX3480A
(VCC = VCC1 = VCC2 = VCC4 =V
CC5 = 3.0V to 3.6V, FS = 0V, TA= TMIN to TMAX, unless otherwise noted. Typical values are at
VCC = VCC1 = VCC2 = VCC4 = VCC5 = 3.3V and TA= +25°C.)
ns
PARAMETER SYMBOL MIN TYP MAX UNITS
tPLH - tPHLDifferential
Receiver Skew
Driver Input to Output
Propagation Delay tPLH 100 275 ns
Driver Output Skew (Note 5) tSKEW 25 90 ns
Driver Rise or Fall Time tR, tF15 40 ns
Driver Enable to Output High tZH 0.2 1.5 µs
Driver Enable to Output Low tZL 0.2 1.5 µs
Driver Disable Time from High tHZ 0.3 1.5 µs
CONDITIONS
Figures 4, 6; RDIFF = 54Ω,
CL1 = CL2 = 100pF
Figures 5, 7; CL= 100pF, S2 closed
Figures 5, 7; CL= 100pF, S1 closed
Figures 4, 6; RDIFF = 54Ω,
CL1 = CL2 = 100pF
Figures 4, 6; RDIFF = 54Ω,
CL1 = CL2 = 100pF
Figures 5, 7; CL= 15pF, S2 closed
Driver Disable Time from Low tLZ 0.3 1.5 µsFigures 5, 7; CL= 15pF, S1 closed
tSKD
100 225
Receiver Input to Output
Propagation Delay tPLH ns
Figures 4, 8; RDIFF = 54Ω,
CL1 = CL2 = 100pF
20
Figures 4, 8; RDIFF = 54Ω,
CL1 = CL2 = 100pF
Maximum Data Rate fMAX 2.5 Mbps
tPLH, tPHL < 50% of data period
tPHL
tPHL 100 275
100 225
-7V ≤VCM ≤12V
0.25
RIN kΩReceiver Input Resistance
-7V ≤VCM ≤12V -0.2 0.2VTH VReceiver Differential Threshold MAX3480B 48
12 -0.2
-0.8
VIN = 12V
VIN = -7V
VIN = 12V
VIN = -7V
MAX3480B
MAX3480A
MAX3480A
MAX3480A/MAX3480B
Complete, Isolated, 3.3V
RS-485/RS-422 Data Interface
4 _______________________________________________________________________________________
Note 1: All currents into device pins are positive; all currents out of device pins are negative. All voltages are referenced to
logic-side ground (GND1, GND2), unless otherwise specified.
Note 2: For DE´and DI´pin descriptions, see
Detailed Block Diagram
and
Typical Application Circuit
(Figure 1 for
MAX3480A/MAX3480B).
Note 3: Shutdown supply current is the current at VCC1 when shutdown is enabled.
Note 4: Applies to peak current. See
Typical Operating Characteristics
and
Applications Information
.
SWITCHING CHARACTERISTICS—MAX3480B
(VCC = VCC1 = VCC2 = VCC4 =V
CC5 = 3.0V to 3.6V, FS = 0V, TA= TMIN to TMAX, unless otherwise noted. Typical values are at
VCC = VCC1 = VCC2 = VCC4 = VCC5 = 3.3V and TA= +25°C.)
ns
PARAMETER SYMBOL MIN TYP MAX UNITS
tPLH - tPHLDifferential
Receiver Skew
tPLH 1.0 2.0
Driver Input to Output
Propagation Delay µs
Driver Output Skew tSKEW 100 800 ns
Driver Rise or Fall Time tR, tF1.0 2.0 µs
Driver Enable to Output High tZH 50 100 µs
Driver Enable to Output Low tZL 50 100 µs
Driver Disable Time from Low tLZ 13 50 µs
CONDITIONS
Figures 4, 6; RDIFF = 54Ω,
CL1 = CL2 = 100pF
Figures 5, 7; CL= 100pF, S2 closed
Figures 5, 7; CL= 100pF, S1 closed
Figures 4, 6; RDIFF = 54Ω,
CL1 = CL2 = 100pF
Figures 4, 6; RDIFF = 54Ω,
CL1 = CL2 = 100pF
Figures 5, 7; CL= 15pF, S1 closed
Driver Disable Time from High tHZ 13 50 µsFigures 5, 7; CL= 15pF, S2 closed
tSKD
tPLH 0.8 2.0
Receiver Input to Output
Propagation Delay tPHL 0.8 2.0 µs
Figures 4, 8; RDIFF = 54Ω,
CL1 = CL2 = 100pF
50
Figures 4, 8; RDIFF = 54Ω,
CL1 = CL2 = 100pF
Maximum Data Rate fMAX 0.25 MbpstPLH, tPHL < 50% of data period
tPHL 1.0 2.0
MAX3480A/MAX3480B
Complete, Isolated, 3.3V
RS-485/RS-422 Data Interface
_______________________________________________________________________________________
5
2.0-40 20
DRIVER DIFFERENTIAL OUTPUT VOLTAGE
vs. TEMPERATURE
MAX3480A/B-08
TEMPERATURE (°C)
DIFFERENTIAL OUTPUT VOLTAGE (V)
0-20 6040 80
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0 DI´ = HIGH OR OPEN
RL = 54Ω
0
OUTPUT CURRENT
vs. DRIVER OUTPUT HIGH VOLTAGE
MAX3480 A/B-06
OUTPUT HIGH VOLTAGE (V)
OUTPUT CURRENT (mA)
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-6 -4 -2 2 4 6
OUTPUT CURRENT
vs. DRIVER OUTPUT LOW VOLTAGE
MAX3480 A/B-05
OUTPUT LOW VOLTAGE (V)
OUTPUT CURRENT (mA)
0
160
180
140
100
120
80
40
60
0
20
24681012
0
-40 20
RECEIVER OUTPUT LOW VOLTAGE
vs. TEMPERATURE
MAX3480 A/B-04
TEMPERATURE (°C)
OUTPUT LOW VOLTAGE (V)
0-20 6040 80
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8 MEASURED AT ISO RO DRV
IRO = 8mA
3.00-40 20
RECEIVER OUTPUT HIGH VOLTAGE
vs. TEMPERATURE
MAX3480 A/B-03
TEMPERATURE (°C)
OUTPUT HIGH VOLTAGE (V)
0-20 6040 80
3.25
3.50
3.75
4.00
4.25
4.50
4.75
5.00 MEASURED AT ISO RO DRV
IRO = 8mA
__________________________________________Typical Operating Characteristics
(VCC_ = 3.3V, TA= +25°C, unless otherwise noted.)
0
OUTPUT CURRENT
vs. RECEIVER OUTPUT HIGH VOLTAGE
MAX3480 A/B-02
OUTPUT HIGH VOLTAGE (V)
OUTPUT CURRENT (mA)
-5
MEASURED AT ISO RO DRV
1.00 3.02.0 5.04.0
-10
-15
-20
-25
-30
0
OUTPUT CURRENT
vs. RECEIVER OUTPUT LOW VOLTAGE
MAX3480 A/B-01
OUTPUT LOW VOLTAGE (V)
OUTPUT CURRENT (mA)
10
20
30
40
50
60
70
80 MEASURED AT ISO RO DRV
1.00 3.02.0 5.04.0
MAX3480A/MAX3480B
Complete, Isolated, 3.3V
RS-485/RS-422 Data Interface
6 _______________________________________________________________________________________
____________________________Typical Operating Characteristics (continued)
(VCC_ = 3.3V, TA= +25°C, unless otherwise noted.)
0.1
10
1
100
DRIVER ENABLE TIME
vs. TEMPERATURE
MAX3480A/B-10
TEMPERATURE (°C)
DRIVER ENABLE TIME (µs)
-40 -20 0 20 40 60 80
MAX3480A
MAX3480B
RL = 54Ω, DI´ = 0V MEASURED FROM
DE´ TO VALID OUTPUT
350
0
MAX3480A 
SUPPLY CURRENT vs. SUPPLY VOLTAGE
300
250
MAX3480A-11
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (mA)
100
150
50
3.0 3.2 3.4 3.6 3.8
200
DE´ HIGH, 50Ω LOAD
DE´ HIGH, 100Ω LOAD
DE´ LOW, DI´ LOW
DE´ LOW, DI´ HIGH
300
0
MAX3480B 
SUPPLY CURRENT vs. SUPPLY VOLTAGE
200
250
MAX3480B-12
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (mA)
100
50
3.0 3.2 3.63.4 3.8
150
DE´ HIGH,
100Ω LOAD
DE´ HIGH, 50Ω LOAD
DE´ LOW, DI´ LOW
DE´ LOW, DI´ HIGH
MAX3480B DRIVER INPUT (AB)
AND RECEIVER OUTPUT (RO)
CIRCUIT OF FIGURE 2, TERMINATION: 100Ω
DE´ INPUT,
2V/div
A
B
RO, 
74HC240, 
2V/div
1µs/div
DI,
2V/div
MAX3480A DRIVER INPUT (AB)
AND RECEIVER OUTPUT (RO)
CIRCUIT OF FIGURE 2, TERMINATION: 100Ω
100ns/div
DE´ INPUT,
2V/div
A
B
RO, 
74HC240, 
DI, 
2V/div
2V/div
MAX3480A DRIVER ENABLE (AB) 
AND RECEIVER OUTPUT (RO)
CIRCUIT OF FIGURE 2, TERMINATION: 100Ω
DE´ INPUT,
1V/div
A, 1V/div
B, 2V/div
100ns/div
MAX3480B DRIVER ENABLE (AB) 
AND RECEIVER OUTPUT (RO)
CIRCUIT OF FIGURE 2, TERMINATION: 100Ω
DE´ INPUT,
1V/div
A, 1V/div
B, 2V/div
25µs/div
0.1
10
1
100
DRIVER DISABLE TIME
vs. TEMPERATURE
MAX3480A/B-9
TEMPERATURE (°C)
DRIVER DISABLE TIME (µs)
-40 -20 0 20 40 60 80
MAX3480A
MAX3480B
RL = 54Ω, DI´ = 0V MEASURED FROM
DE´ TO VALID OUTPUT
MAX3480A/MAX3480B
Complete, Isolated, 3.3V
RS-485/RS-422 Data Interface
_______________________________________________________________________________________ 7
______________________________________________________________Pin Description
Driver Input. With DE´high, a low on DI´forces output A low and output B high. Similarly, a high on DI´
forces output A high and output B low. Drives internal LED cathode through R1 (Table 1 of Figure 2).
DI9
Logic-Side (non-isolated side) +3.3V Supply Voltage Input. Connect to pins 1, 2, and 14.VCC4
10
Boosted V+ Voltage Input. Must be connected as shown in Figures 1 and 2.VCC3
8
Frequency Switch Input. If FS = VCC, switch frequency is high; if FS = 0V, switch frequency is low
(normal connection).
FS6
Boost-Voltage Generator Outputs. See Figures 1 and 2.D1, D2
Logic-Side Ground Inputs. Must be connected; not internally connected.
GND1,
GND2
2 Logic-Side (non-isolated side) +3.3V Supply Voltage Input. Connect to pins 1, 10, and 14.VCC2
5, 12
PIN
Logic-Side (non-isolated side) +3.3V Supply Voltage Input. Connect to pins 2, 10, and 14.VCC1
1
3, 4
FUNCTIONNAME
Driver-Enable Input. The driver outputs, A and B, are enabled by bringing DE´high. The driver outputs
are high impedance when DE´is low. If the driver outputs are enabled, the device functions as a line
driver. While the driver outputs are high impedance, the device functions as a line receiver. Drives
internal LED cathode through R2 (Table 1 of Figure 2).
DE11
Receiver Output. If A > B by 200mV, RO will be low; if A < B by 200mV, RO will be high. Open collector;
must have pull-up (R3) to VCC (Table 1 of Figure 2).
RO13
Logic-Side (non-isolated side) +3.3V Supply Voltage Input. Connect to pins 1, 2, and 10.VCC5
14
Power-Supply Shutdown Input. Must be connected to logic ground.SD7
PINS ON THE NON-ISOLATED SIDE
MAX3480A/MAX3480B
Complete, Isolated, 3.3V
RS-485/RS-422 Data Interface
8 _______________________________________________________________________________________
Note: For DE´and DI´pin descriptions, see
Detailed Block Diagram
and
Typical Application Circuit
.
_________________________________________________Pin Description (continued)
Isolated Driver-Enable Drive Input. The driver outputs, A and B, are enabled by bringing DE´high.
The driver outputs are high impedance when DE´is low. If the driver outputs are enabled, the device
functions as a line driver. While the driver outputs are high impedance, the device functions as a line
receiver. Open collector output; must have pull-up (R4) to ISO VCC and be connected to ISO DE IN for
normal operation (Table 1 of Figure 2).
ISO DE
DRV
17
ISO RO
LED
15
Isolated-Supply Common Input. Connect to ISO COM1.ISO COM216
PIN FUNCTIONNAME
Isolated-Supply Positive Input Voltage. Connect to ISO VCC1.ISO VCC2
18
Isolated Driver-Input Drive. With DE´high, a low on DI´forces output A low and output B high. Similarly,
a high on DI´forces output A high and output B low. Open-collector output; must have pull-up (R5) to
ISO VCC and be connected to ISO DI IN for normal operation (Table 1 of Figure 2).
ISO DI DRV19
Isolated-Supply Common Output. Connect to ISO COM2. If RS-485 wires have a shield, connect ISO
COM1 to shield via 100Ωresistor.
ISO COM120
Isolated Receiver-Output LED Anode (input). If A > B by 200mV, ISO RO LED will be high; if A < B by
200mV, ISO RO LED will be low.
Isolated Driver-Enable Input. Connect to ISO DE DRV for normal operation.ISO DE IN21
Noninverting Driver Output and Noninverting Receiver Input.A23
Inverting Driver Output and Inverting Receiver InputB25
Isolated Driver Input. Connect to ISO DI DRV for normal operation.ISO DI IN22
Isolated Receiver-Output Drive. Connect to ISO RO LED through R6 (Table 1 of Figure 2).
ISO RO
DRV
24
Isolated Supply Positive Output Voltage. Connect to ISO VCC2.
ISO VCC1
26 Internal Connections. Leave these pins unconnected.AC2, AC127, 28
PINS ON THE ISOLATED RS-485/RS-422 SIDE
MAX3480A/MAX3480B
Complete, Isolated, 3.3V
RS-485/RS-422 Data Interface
_______________________________________________________________________________________ 9
_______________Detailed Description
The MAX3480A/MAX3480B are electrically isolated,
RS-485/RS-422 data-communications interface solu-
tions. Transceivers, optocouplers, a power driver, and a
transformer are in one standard 28-pin DIP package.
Signals and power are internally transported across the
isolation barrier (Figure 1). Power is transferred from the
logic side (non-isolated side) to the isolated side of the
barrier through a center-tapped transformer. Signals
cross the barrier through high-speed optocouplers. A
single +3.3V supply on the logic side powers both
sides of the interface.
The MAX3480B features reduced-slew-rate drivers that
minimize EMI and reduce reflections caused by
improperly terminated cables, allowing error-free trans-
mission at data rates up to 250kbps. The MAX3480A’s
driver slew rates are not limited, allowing transmission
rates up to 2.5Mbps.
The frequency-select FS is connected to GND_ in normal
operation, which selects a switching frequency of
approximately 600kHz. Connect to high for a higher
900kHz switching frequency.
Drivers are short-circuit current limited and are protect-
ed against excessive power dissipation by thermal
Figure 1. Detailed Block Diagram
AC1 (MAKE NO CONNECTION)
AC2 (MAKE NO CONNECTION)
SHIELD (OPTIONAL)
EXTERNAL RS-485/RS-422 WIRING
ISO VCC1
B
D2
D1
D1, D2
1N914 VCC2
VCC1
ISO RO DRV
A
B
A
SH
ISO DI IN
ISO DE IN
VCC3
SD
FS
GND1
ISO COM1
ISO DI DRV
ISO VCC2
ISO DE DRV
GND2
DE
VCC4
DI
ISO COM2
ISO RO LED
VCC5
RO
RL
RL
R4
R5
R8
100Ω
BOOSTED V+
C3
0.01uF
R6 
200Ω
R7
R1
R2
DI´
DE´
R3
RO
LOGIC
I/O
VIN
3.0V TO 3.6V
22µF 0.1µF
C1 C2
MAX3480A/B
LOGIC GROUND
ISOLATION BARRIER ISOLATION COMMON
MAX485
MAX487
MAX845
1
2
28
27
3
4
5
6
7
8
9
10
11
12
13
14
26
25
24
23
22
21
20
19
18
17
16
15
MAX845
N
MAX3480A: MAX485
MAX3480B: MAX487
RE
ISO RO DRV
ISO DE IN
ISO DI IN
ISO COM1
ISO VCC1
A
B
R
D
N
Q
Q
TF/F
V
CC3
FS OSC
1.1MHz/
1.6MHz
SD GND1
D2
D1
SHIELD (OPTIONAL)
NOTE: RESISTOR R8 PROTECTS THE
MAX3480 FROM TRANSIENT
CURRENTS BETWEEN SHIELD AND
“A” & “B”.
TWISTED PAIR
TO OTHER TRANSCEIVERS
TERMINATING RESISTOR
(ONE RESISTOR ON EACH END)
TWISTED PAIR
TO OTHER TRANSCEIVERS
MAX3480A/MAX3480B
Complete, Isolated, 3.3V
RS-485/RS-422 Data Interface
10 ______________________________________________________________________________________
shutdown circuitry that puts the driver outputs into a
high-impedance state. The receiver input has a fail-safe
feature that guarantees a logic-high output if the input
is open circuit.
The driver outputs are enabled by bringing DE´high.
Driver-enable times are typically 200ns for the
MAX3480A and 50µs for the MAX3480B. Allow time for
the devices to be enabled before sending data. When
enabled, driver outputs function as line drivers. Driver
outputs are high impedance when DE´is low. While
outputs are high impedance, they function as line
receivers.
The MAX3480A/MAX3480B typically withstand
1600VRMS (1 minute) or 2000VRMS (1 second). The iso-
lated outputs of these devices meet all RS-485/RS-422
specifications. The logic inputs can be driven from any
TTL/CMOS-logic family with a series resistor, and the
received data output can directly drive any of the
TTL/CMOS-logic families with only a resistive pull-up.
Boost Voltage
The MAX3480 requires external diodes on the primary
of the transformer to develop the boost voltage for the
power oscillator. In normal operation, whenever one of
the oscillator outputs (D1 and D2) goes low, the other
Figure 2. Typical Application Circuit
AC1 (MAKE NO CONNECTION)
AC2 (MAKE NO CONNECTION)
SHIELD (OPTIONAL)
EXTERNAL RS-485/RS-422 WIRING
ISO VCC1
B
D2
D1
D1, D2
1N914 VCC2
VCC1
ISO RO DRV
A
B
A
SH
ISO DI IN
ISO DE IN
VCC3
SD
FS
GND1
ISO COM1
ISO DI DRV
ISO VCC2
ISO DE DRV
GND2
DE
VCC4
DI
ISO COM2
ISO RO LED
VCC5
RO
RL
RL
R4
R5
R1
R2
16
14
12
5
7
13
15
18
3
8 2
DI
DE
RO
17
20
9
10
11 RECEIVER OUTPUT
6
4
R8
100
C3
0.01µF
R6 
200Ω
R7
R3
BOOSTED V+
74HC240
DRIVER INPUT
DRIVER ENABLE
VIN
3.0V TO 3.6V
22µF
6V 0.1µF
C1 C2
MAX3480A/B
LOGIC GROUND ISOLATION BARRIER ISOLATION COMMON
MAX485
MAX487
MAX845
1
2
28
27
3
4
5
6
7
8
9
10
11
12
13
14
26
25
24
23
22
21
20
19
18
17
16
15
SHIELD (OPTIONAL)
NOTE: RESISTOR R8 PROTECTS
THE MAX3480 FROM TRANSIENT
CURRENTS BETWEEN SHIELD AND
“A” & “B”.
TWISTED PAIR
TO OTHER TRANSCEIVERS
TERMINATING RESISTOR
(ONE RESISTOR ON EACH END)
TWISTED PAIR
TO OTHER TRANSCEIVERS
Table 1. Pull-Up and LED Drive Resistors
PART R1 (Ω) R2 (Ω) R3 (Ω) R4 (Ω) R5 (Ω) R6 (Ω)
MAX3480A 150 130 680 3000 1000 200
MAX3480B 130 330 2000 2200 3000 200
R7 (Ω)
Open
470
MAX3480A/MAX3480B
Complete, Isolated, 3.3V
RS-485/RS-422 Data Interface
______________________________________________________________________________________ 11
_________________________________________________________________Test Circuits
Figure 3. Driver DC Test Load Figure 4. Driver/Receiver Timing Test Circuit
Figure 5. Driver Timing Test Load
R
R
A
B
VOD
VOC
DI´
DE´
3.3V
A
B
CL1
CL2
A
BRO
R3
3.3V
RDIFF
VID
ISOLATION BARRIER ISOLATION BARRIER
RD
OUTPUT
UNDER TEST
500ΩS1
S2
ISO VCC1,
ISO VCC2
CL
_______________________________________________________Switching Waveforms
Figure 6. Driver Propagation Delays and Transition Times Figure 7. Driver Enable and Disable Times
DI´
VCC_ - 0.4V VCC_ - 0.4V
2VCC_ - 0.4V
2
0V
B
A
VO
0V
-VO
VO
tPLH
1/2 VO
10%
tR
90% 90%
tPHL 1/2 VO
10%
tF
VDIFF = VA - VB
VDIFF
tSKEW = | tPLH - tPHL |
OUTPUT NORMALLY LOW
OUTPUT NORMALLY HIGH
0V
A, B
VOL
A, B
0V
VOL + 0.5V
VOH - 0.5V
2.3V
2.3V
tZL tLZ
tZH tHZ
DE´
VCC_ - 0.4V VCC_ - 0.4V
2VCC_ - 0.4V
2
MAX3480A/MAX3480B
Complete, Isolated, 3.3V
RS-485/RS-422 Data Interface
12 ______________________________________________________________________________________
Figure 8. Receiver Propagation Delays
VOH
VOL
VID
-VID
1.5V
0V
1.5V
OUTPUT
INPUT 0V
RO
VB - VAtPLH
tPHL
__Switching W aveforms (continued)
____________________Function Tables
Table 2. Transmitting
INPUTS
DE´DI´B A
1 1 0 1
1 0 1 0
0 X High-Z High-Z
INPUTS OUTPUT
DE´A-B –
R
—
O
–
0≥+0.2V 0
0≤-0.2V 1
0 Inputs open 0
OUTPUTS
Table 3. Receiving
X = Don't care
High-Z = High impedance
goes to approximately double the supply voltage.
Since the circuit is symmetrical, the two outputs can be
combined with diodes, filtered, and used to power the
oscillator itself.
The diodes on the primary side may be any fast-switch-
ing, small-signal diodes, such as the 1N914, 1N4148,
or CMPD2838. The nominal value of the primary filter
capacitor C3 is 0.01µF.
Driver Output Protection
There are two mechanisms to prevent excessive output
current and power dissipation caused by faults or by
bus contention. A foldback current limit on the output
stage provides immediate protection against short cir-
cuits over the whole common-mode voltage range (see
Typical Operating Characteristics
). In addition, a ther-
mal shutdown circuit forces the driver outputs into a
high-impedance state if the die temperature rises
excessively.
Resistor R8 provides additional protection by current
limiting between the shield and the two signal wires. In
the event that shielded cable is used and an external
voltage or transient is inadvertently applied between
the shield and the signal wires, the MAX3480 can be
damaged. Although unlikely, this condition can occur
during installation.
The MAX3480 provides electrical isolation between
logic ground and signal paths; it does not provide
isolation from external shields and the signal paths.
When in doubt, do not connect the shield. The
MAX3480 can be damaged if resistor R8 is shorted
out.
MAX3480A/MAX3480B
Complete, Isolated, 3.3V
RS-485/RS-422 Data Interface
______________________________________________________________________________________ 13
__________Applications Information
The MAX3480A/MAX3480B are designed for bidirection-
al data communications on multipoint bus-transmission
lines. Figure 9 shows a typical network application cir-
cuit. To minimize reflections, terminate the line at both
ends with its characteristic impedance, and keep stub
lengths off the main line as short as possible. The slew-
rate-limited MAX3480B is more tolerant of imperfect ter-
mination and stubs off the main line.
The MAX3480A/MAX3480B are specified and charac-
terized using the resistor values shown in Table 1 of
Figure 2. Altering the recommended values can
degrade performance.
The DI and DE inputs are the cathodes of LEDs whose
anodes are connected to VCC. These points are best
driven by a 3.3V CMOS-logic gate with a series resis-
tor to limit the current. The resistor values shown in
Table 1 are recommended when the 74HC240 gate or
equivalent is used. These values may need to be
adjusted if a driving gate with dissimilar series resis-
tance is used. DI and DE are intended to be driven
through a series current-limiting resistor. Directly
grounding these pins destroys the device.
MAX3480A/MAX3480B
Complete, Isolated, 3.3V
RS-485/RS-422 Data Interface
14 ______________________________________________________________________________________
Figure 9. Typical RS-485/RS-422 Network
DI
DE
RO
BA
R
D
RE
RO
DE
DI
R
120ΩD
A
B
RERE
DI
DE
RO
BA
R
D
AC1 (MAKE NO CONNECTION)
AC2 (MAKE NO CONNECTION)
SHIELD
(OPTIONAL)
ISO VCC1
B
D2
D1
VCC2
VCC1
ISO RO DRV
A
B
A
SH
ISO DI IN
ISO DE IN
VCC3
SD
FS
GND1
ISO COM1
ISO DI DRV
ISO VCC2
ISO DE DRV
DE
GND2
VCC4
DI
ISO COM2
ISO RO LED
VCC5
RO
R4
R7
R5
R8
100Ω
120Ω
R6, 200Ω
MAX3480A/B
LOGIC GROUND ISOLATION BARRIER ISOLATED COMMON
TERMINATING RESISTOR
(ONE RESISTOR ON EACH END)
MAX485
MAX487
MAX845
1
2
28
27
3
4
5
6
7
8
9
10
11
12
13
14
26
25
24
23
22
21
20
19
18
17
16
15
TERMINATING RESISTOR
(ONE RESISTOR ON EACH END)
D1, D2
1N914
R1
130Ω
R2
R3
BOOSTED V+
DRIVER INPUT
DRIVER ENABLE
VIN
3.0V to 3.6V C1
22µF
6V
C2
0.1µF
C3
0.01µF
16
14
12
5
7
13
15
8 2
DI
DE
RO
17
20
18
3
9
10
11 RECEIVER OUTPUT
6
4
74HC240
NOTE: RESISTOR R8
PROTECTS THE MAX3480 
FROM TRANSIENT
CURRENTS BETWEEN 
SHIELD AND “A” & “B”.
MAX3480A/MAX3480B
Complete, Isolated, 3.3V
RS-485/RS-422 Data Interface
______________________________________________________________________________________ 15
________________________________________________________Package Information
DIM
A
A1
A2
A3
B
B1
C
D1
E
E1
e
eA
eB
L
MIN
–
0.015
0.125
0.055
0.016
0.045
0.008
0.050
0.600
0.525
0.100
0.600
–
0.120
MAX
0.200
–
0.175
0.080
0.020
0.065
0.012
0.090
0.625
0.575
–
–
0.700
0.150
MIN
–
0.38
3.18
1.40
0.41
1.14
0.20
1.27
15.24
13.34
2.54
15.24
–
3.05
MAX
5.08
–
4.45
2.03
0.51
1.65
0.30
2.29
15.88
14.61
–
–
17.78
3.81
INCHES MILLIMETERS
Plastic DIP
PLASTIC
DUAL-IN-LINE
PACKAGE
(0.600 in.)
DIM
D
D
D
PKG.
P
P
P
MIN
1.230
1.430
2.025
MAX
1.270
1.470
2.075
MIN
31.24
36.32
51.44
MAX
32.26
37.34
52.71
INCHES MILLIMETERS
PINS
24
28
40
C
A
A2 E1
D
E
eA
eB
A3
B1
0°-15°
B
A1
L
D1
e
21-0044A
PART
MAX1480A 2.50
GUARANTEED
DATA RATE
(Mbps)
SLEW-RATE
LIMITED
No
SUPPLY
VOLTAGE
(V)
5.0
No. OF
Tx/Rx FULL/HALF
DUPLEX No. OF Tx/Rx
ON BUS
1/1 Half 32
MAX1480B 0.25 Yes1/1 Half 5.032
MAX1490A 2.50 No1/1 Full 5.032
MAX1490B 0.25 Yes1/1 Full 5.032
MAX3480A 2.50 No1/1 Half 3.332
MAX3480B 0.25 Yes1/1 Half 3.3128
Table 4. Maxim’s Isolated RS-485 Product Family
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
16
__________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600
© 1996 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
MAX3480A/MAX3480B
Complete, Isolated, 3.3V
RS-485/RS-422 Data Interface