Maxim MAX3292ESD, MAX3292EPD, MAX3292CSD, MAX3292CPD, MAX3291EPD Datasheet

MAX3291/MAX3292

RS-485/RS-422 Transceivers with Preemphasis

for High-Speed, Long-Distance Communication

________________________________________________________________

Maxim Integrated Products

1

19-1405; Rev 1; 4/99

For free samples & the latest literature: http://www.maxim-ic.com, or phone [phone].
For small orders, phone [phone].

________________General Description

The MAX3291/MAX3292 high-speed RS-485/RS-422
transceivers feature driver preemphasis circuitry, which
extends the distance and increases the data rate of reli­able communication by reducing intersymbol interfer­ence (ISI) caused by long cables. The MAX3291 is
programmable for data rates of 5Mbps to 10Mbps,
while the MAX3292 is programmable for data rates up
to 10Mbps by using a single external resistor.

The MAX3291/MAX3292 are full-duplex devices that
operate from a single +5V supply and offer a low-cur­rent shutdown mode that reduces supply current to
100nA. They feature driver output short-circuit current
limiting and a fail-safe receiver input that guarantees a
logic-high output if the input is open circuit. A 1/4-unit­load receiver input impedance allows up to 128 trans­ceivers on the bus.

________________________Applications

Long-Distance, High-Speed RS-485/RS-422
Communications

Telecommunications
Industrial-Control Local Area Networks

____________________________Features

♦ Preemphasis Increases the Distance and Data

Rate of Reliable RS-485/RS-422 Communication

♦ Data Rate

Optimized for 5Mbps to 10Mbps (MAX3291)
Programmable up to 10Mbps (MAX3292)

♦ 100nA Low-Current Shutdown Mode
♦ Allow Up to 128 Transceivers on the Bus
♦ -7V to +12V Common-Mode Input Voltage Range
♦ Pin-Compatible with ’75180, MAX489, MAX491

MAX3080, MAX3083, MAX3086, MAX1482

2

PEE

(PSET)

12

13

R = Z

O

R = Z

O

R = Z

O

R = Z

O

14

1

1µF

R

PSET

*

C

PSET

*C

PSET

*

7(6)

GND

11

10

9

( ) ARE FOR MAX3292
* MAX3292 ONLY

Z

O

= THE CHARACTERISTIC

IMPEDANCE OF THE CABLE

RO

RE

DE

DI

3
4
5

9

5

13

7(6)

GND

4
3

DI
DE

RE

RO

2

10

11
12

V

CC

(V

CCD

)(V

CCD

)

PEE

(PSET)

14

1

1µF

R

PSET

*

V

CC

MAX3291
MAX3292

MAX3291
MAX3292

A

B

Z

Y

Y
Z

B

A

Typical Operating Circuit and Functional Diagram

PART

MAX3291CSD

MAX3291CPD
MAX3291ESD -40°C to +85°C

0°C to +70°C

0°C to +70°C

TEMP. RANGE PIN-PACKAGE

14 SO
14 Plastic DIP
14 SO

_______________Ordering Information

MAX3291EPD -40°C to +85°C 14 Plastic DIP
MAX3292CSD
MAX3292CPD
MAX3292ESD -40°C to +85°C

0°C to +70°C

0°C to +70°C 14 SO

14 Plastic DIP
14 SO

MAX3292EPD -40°C to +85°C 14 Plastic DIP

Pin Configuration appears at end of data sheet.

MAX3291/MAX3292

RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

DC ELECTRICAL CHARACTERISTICS

(Typical Operating Circuit, VCC= +5V ±5%, R

PSET

= 0 (MAX3292), VCC= V

CCD

(MAX3292), TA= T

MIN

to T

MAX

, unless otherwise

noted. Typical values are at V

CC

= +5V and TA= +25°C.) (Note 1)

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.

Supply Voltage (VCC, V

CCD

) .................................................+6V

Control Input Voltage (

RE, DE, PEE,

PSET, DI).................................................-0.3V to (V

CC

+ 0.3V)

Driver Output Voltage (Y, Z)................................-7.5V to +12.5V

Receiver Input Voltage (A, B)..............................-7.5V to +12.5V

Receiver Output Voltage (RO)....................-0.3V to (V

CC

+ 0.3V)
Continuous Power Dissipation (T

A

= +70°C)

14-Pin SO (derate 8.7mW/°C above +70°C).................695mW

14-Pin Plastic DIP (derate 10.0mW/°C above +70°C) ..800mW

Operating Temperature Ranges

MAX329_C_ D......................................................0°C to +70°C

MAX329_E_ D...................................................-40°C to +85°C

Storage Temperature Range.............................-65°C to +160°C

Lead Temperature (soldering, 10sec).............................+300°C

R = 27Ω

DE = GND,
VCC= GND or 5.25V

V

PSET

= V

CC

DE, DI, RE

DE, DI, RE, PEE
DE, DI, RE

CONDITIONS

V2.4V

ODP

Differential Driver Output with
Preemphasis

µA

-25

I

O

Output Leakage (Y and Z)

25

µA70 110I

PSET

PSET Input Current (MAX3292)

V

2.4

V

IH

Input High Voltage

V0.8V

IL

Input Low Voltage

µA±2I

IN

Input Current

µA-15 -30 -45I

PEE

PEE Input Current (MAX3291)

UNITSMIN TYP MAXSYMBOLPARAMETER

Figure 1, R = 27Ω (Note 3) V1.65 2.0 2.35DPER

Differential Driver
Preemphasis Ratio

Figure 1, R = 27Ω (Note 4) V0.2

∆VOD,

∆V

ODP

Change in Magnitude of
Differential Output Voltage
(Normal and Preemphasis)

Figure 1, R = 27Ω VV

CC

/ 2 3V

OC

Driver Common-Mode Output
Voltage (Normal and
Preemphasis)

Figure 1, R = 27Ω (Note 5) V0.3∆V

OC

Change in Magnitude of
Common-Mode Voltage
(Normal and Preemphasis)

Figure 1, R = 27Ω mV50∆V

NP

Change in Magnitude of
Common-Mode Output
Voltage (Normal to
Preemphasis)

-7V ≤ V

OUT

≤ +12V (Note 6) mA±30 ±250I

OSD

Driver Short-Circuit Output
Current

VY= VZ= +12V
VY= VZ= -7V

R = 27Ω

Figure 1

No load (Note 2)

V

5.25

V

OD

1.5 5.0

Differential Driver Output

PEE 3.75

DRIVER

MAX3291/MAX3292

RS-485/RS-422 Transceivers with Preemphasis

for High-Speed, Long-Distance Communication

_______________________________________________________________________________________ 3

DC ELECTRICAL CHARACTERISTICS (continued)

(Typical Operating Circuit, VCC= +5V ±5%, R

PSET

= 0 (MAX3292), V

CC

= V

CCD

(MAX3292), TA= T

MIN

to T

MAX

, unless otherwise

noted. Typical values are at V

CC

= +5V and TA= +25°C.) (Note 1)

SWITCHING CHARACTERISTICS

(Typical Operating Circuit, VCC= +5V ±5%, R

PSET

= 0 (MAX3292), VCC= V

CCD

(MAX3292), TA= +25°C, unless otherwise noted.

Typical values are at V

CC

= +5V and TA= +25°C.)

-7V ≤ VCM≤ +12V

RE = V

CC,

DE = GND, VY= VZ= 0 to VCCor

floating

RE = GND, DE = V

CC

0 ≤ VO≤ V

CC

-7V ≤ VCM≤ +12V
0 ≤ VRO≤ V

CC

CONDITIONS

mV-200 200V

TH

Receiver Differential
Threshold Voltage

µA0.1 1I

SHDN

Supply Current in Shutdown
Mode

mA2.0 3.0I

CC + ICCD

No-Load Supply Current

µA0.1 ±1I

OZR

Three-State Output Current at
Receiver

kΩ48R

IN

Receiver Input Resistance

mA±15 ±95I

OSR

Receiver Output Short-Circuit
Current

UNITSMIN TYP MAXSYMBOLPARAMETER

VA= VB= 0 mV35∆V

TH

Receiver Input Hysteresis

IO= -4mA, VA- VB= V

TH

V3.5V

OH

Receiver Output High Voltage

IO= 4mA, VA- VB= -V

TH

V0.4V

OL

Receiver Output Low Voltage

DE = GND,
VCC= GND or 5.25V

µA

-150

IA,

B

250

Input Current (A and B)

VIN= +12V
VIN= -7V

Driver Differential Output
Rise or Fall Time

Figures 3 and 5, R

DIFF

= 54Ω,

CL1= CL2= 50pF

ns

44 65t

DPHL

Driver Propagation Delay

CONDITIONS

t

HL

12

Figures 3 and 5, R

DIFF

= 54Ω,

CL1= CL2= 50pF

ns

t

LH

Differential Driver Output
Skew  t

DPLH

- t

DPHL



Figures 3 and 10, R

DIFF

= 54Ω,

CL1= CL2= 50pF

ns30t

PTND

Preemphasis Voltage Level to
Normal Voltage Level Delay

t

DSKEW

Figures 3 and 5, R

DIFF

= 54Ω,

CL1= CL2= 50pF

38ns

Mbps10f

MAX

Maximum Data Rate

41 65t

DPLH

UNITSMIN TYP MAXSYMBOLPARAMETER

RECEIVER

SUPPLY CURRENT

Driver Preemphasis Interval t

PRE

80 100 120

Figures 3 and 10,
R

DIFF

= 54Ω,

C

L1

= CL2= 50pF

ns

0.75 1 1.25 µs

MAX3292,
R

PSET

= 523kΩ

MAX3291/MAX3292,
R

PSET

= 0

MAX3291/MAX3292

RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication

4 _______________________________________________________________________________________

SWITCHING CHARACTERISTICS (continued)

(Typical Operating Circuit, VCC= +5V ±5%, R

PSET

= 0 (MAX3292), VCC= V

CCD

(MAX3292), TA= +25°C, unless otherwise noted.

Typical values are at V

CC

= +5V and TA= +25°C.)

Note 1: All currents into the device are positive; all currents out of the device are negative. All voltages are referenced to device

ground unless otherwise noted.

Note 2: Guaranteed by design.
Note 3: DPER is defined as (V

ODP

/ VOD).

Note 4: ∆V

ODP

and ∆VOCare the changes in VDDand VOC, respectively, when the DI input changes. This specification reflects

constant operating conditions. When operating conditions shift, the maximum value may be momentarily exceeded.

Note 5: ∆V

ODP

and ∆VOCare the changes in VODand VOC, respectively, when the DI input changes state.

Note 6: Maximum current level applies to peak current just prior to foldback-current limiting; minimum current level applies during

current limiting.

Note 7: Shutdown is enabled by bringing RE high and DE low. If the enable inputs are in this state for less than 50ns, the device is

guaranteed not to enter shutdown. If the enable inputs are in this state for at least 500ns, the device is guaranteed to have
entered shutdown. Time to shutdown for the device (t

SHDN

) is measured by monitoring R0 as in Figure 4.

Figures 2 and 6, S2 closed, RL= 500Ω,
C

L

= 100pF

Figures 2 and 6, RL= 500Ω, CL= 100pF,
S2 closed

CONDITIONS

ns

ns6000 8750t

DZH(SHDN)

Driver Enable from Shutdown
to Output High

UNITSMIN TYP MAXSYMBOLPARAMETER

Figures 4 and 11 (Note 7) ns50 160 500t

SHDN

Time to Shutdown

Figures 2 and 8, RL= 1kΩ, CL= 100pF,
S1 closed

ns34355t

RZL

Receiver Enable to Output
Low

Figures 2 and 8, RL= 1kΩ, CL= 15pF,
S1 closed

ns

Figures 2 and 8, RL= 1kΩ, CL= 100pF,
S2 closed

25 45t

RLZ

Receiver Disable Time from
Low

ns34355t

RZH

Receiver Enable to Output
High

Figures 2 and 8, RL= 1kΩ, CL= 15pF,
S2 closed

ns25 45t

RHZ

Receiver Disable Time from
High

Figures 2 and 6, RL= 500Ω, CL= 100pF,
S1 closed

ns6000 8750t

DZL(SHDN)

Driver Enable from Shutdown
to Output Low

Figures 2 and 8, RL= 1kΩ, CL= 100pF,
S1 closed

ns30 1500t

RZL(SHDN)

Receiver Enable from
Shutdown to Output Low

Figures 2 and 8, RL= 1kΩ, CL= 100pF,
S2 closed

ns850 1500t

RZH(SHDN)

Receiver Enable from
Shutdown to Output High

Figures 7 and 9, CL= 100pF ns3t

RSKEW

Receiver Output Skew
 t

RPLH

- t

RPHL



Driver Enable to Output High t

DZH

72 105

Figures 7 and 9, CL= 50pF, VID= 2V,
VCM= 0

Receiver Propagation Delay

t

RPHL

52 85

ns

49 85t

RPLH

Figures 2 and 6, S2 closed, RL= 500Ω,
C

L

= 15pF

Driver Disable Time from High t

DHZ

71 100 ns

Figures 2 and 6, S1 closed, RL= 500Ω,
C

L

= 15pF

Driver Disable Time from Low t

DLZ

53 100 ns

Figures 2 and 6, S1 closed, RL= 500Ω,
C

L

= 100pF

Driver Enable to Output Low t

DZL

55 105 ns

MAX3291/MAX3292

RS-485/RS-422 Transceivers with Preemphasis

for High-Speed, Long-Distance Communication

_______________________________________________________________________________________

5

TYPICAL PREEMPHASIS WAVEFORM

MAX3291/2-01

100ns/div

V

Y

- V

Z

2V/
div

2500

0

0 1000

PREEMPHASIS INTERVAL vs. R

PSET

(C

PSET

= 0.1µF)

250

500

2250

MAX3291/2-02

R

PSET

(kΩ)

PREEMPHASIS INTERVAL (ns)

200100 300 400 600500

800

700 900

2000
1750

750

1500
1250
1000

NOTE A

0

400

200

800

NOTE A

600

1000

1200

0 1000500 1500 2000

R

PRE

AND t

PRE

vs. t

BAUD

MAX3291/92 toc 03

t

BAUD

(ns)

R

PRE

(kΩ)

0

1.0

0.5

2.0

1.5

2.5

3.0

4.0

3.5

4.5

10 30 40 5020 60 70 80 90

100

DRIVER DIFFERENTIAL

OUTPUT VOLTAGE vs. R

DIFF

MAX3291/2-04

R

DIFF

(Ω)

DRIVER DIFFERENTIAL OUTPUT VOLTAGE (V)

STRONG

NORMAL

30.0

35.0

32.5

40.0

37.5

42.5

47.5

45.0

50.0

-40 0 20-20 40 60 80

100

DRIVER PROPAGATION DELAY

vs. TEMPERATURE

MAX3291/2-07

TEMPERATURE (°C)

PROPAGATION DELAY (ns)

C

L1

= C

L2

= 50pF

2.00

2.50

2.25

3.00

2.75

3.25

3.50

4.00

3.75

4.25

-40 0 20-20 40 60 80

100

DRIVER DIFFERENTIAL

OUTPUT VOLTAGE vs. TEMPERATURE

MAX3291/2-05

TEMPERATURE (°C)

DIFFERENTIAL OUTPUT VOLTAGE (V)

STRONG (V

ODP

)

R

DIFF

= 54Ω

NORMAL (VOD)

40.0

45.0

42.5

50.0

47.5

52.5

57.5

55.0

60.0

-40 0 20-20 40 60 80

100

RECEIVER PROPAGATION DELAY

vs. TEMPERATURE

MAX3291/2-06

TEMPERATURE (°C)

PROPAGATION DELAY (ns)

CL = 50pF

RECEIVER PROPAGATION DELAY

MAX3291/2-08

20ns/div

V

A

- V

B

RO

2.5V/
div

5V/
div

__________________________________________Typical Operating Characteristics

(VCC= +5V, TA= +25°C, unless otherwise noted.)

Note A: Dotted line represents region in which preemphasis may not work in systems with excessive power-supply noise. See

Preemphasis at Low Data Rates

.

MAX3291/MAX3292

RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication

6 _______________________________________________________________________________________

____________________________________Typical Operating Characteristics (continued)

(VCC= +5V, TA= +25°C, unless otherwise noted.)

0

0.75

0.50

0.25

1.75

1.50

1.25

1.00

2.00

2.75

2.50

2.25

3.00

-40 0 20-20 40 60 80

100

NO-LOAD SUPPLY CURRENT

vs. TEMPERATURE

MAX3291/2-12

TEMPERATURE (°C)

NO-LOAD SUPPLY CURRENT (mA)

0

20

10

40

30

60

50

70

0 1.0 1.50.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

OUTPUT SINK CURRENT vs.

RECEIVER OUTPUT LOW VOLTAGE

MAX3291/2-13

OUTPUT LOW VOLTAGE (V)

OUTPUT SINK CURRENT (mA)

0

0.10

0.05

0.15

0.20

0.25

0.30

-40 0 20-20 40 60 80

100

RECEIVER OUTPUT LOW VOLTAGE

vs. TEMPERATURE

MAX3291/2-10

TEMPERATURE (°C)

OUTPUT LOW VOLTAGE (V)

IRO = 8mA

0

5

15

10

25

20

30

0 1.0 1.50.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

OUTPUT SOURCE CURRENT vs.

RECEIVER OUTPUT HIGH VOLTAGE

MAX3291/2-14

OUTPUT HIGH VOLTAGE (V)

OUTPUT SOURCE CURRENT (mA)

4.10

4.25

4.20

4.15

4.35

4.30

4.45

4.40

4.55

4.50

4.60

-40 0 20-20 40 60 80

100

RECEIVER OUTPUT HIGH VOLTAGE

vs. TEMPERATURE

MAX3291/2-11

TEMPERATURE (°C)

OUTPUT HIGH VOLTAGE (V)

IRO = 8mA

DRIVER PROPAGATION DELAY

MAX3291/2-09

20ns/div

V

Y

- V

Z

DI

2.5V/
div

5V/
div

MAX3291/MAX3292

RS-485/RS-422 Transceivers with Preemphasis

for High-Speed, Long-Distance Communication

_______________________________________________________________________________________ 7

Pin Description

Preemphasis Enable Input. To enable preemphasis, leave PEE unconnected, connect to
VCC, or drive high. To enable strong-level-drive only mode, connect PEE to GND or drive
low.

PEE1 —

Preemphasis Set Input. Sets the preemphasis interval. Connect a resistor (R

PSET

) in paral-

lel with a capacitor (C

PSET

) from PSET to VCCto set the preemphasis interval. See

Typical

Operating Circuit

.

PSET— 1

Receiver Output Enable. Drive RE low to enable RO; RO is high impedance when RE is
high. Drive RE high and DE low to enter low-power shutdown mode.

RE

3 3

Receiver Output. When RE is low and if A - B ≥ 200mV, RO is high; if A - B ≤ -200mV, RO
is low.

RO2 2

Driver Input. With DE high, a low on DI forces the noninverting output low and the inverting
output high. Similarly, a high on DI forces the noninverting output high and the inverting
output low.

DI5 5

GroundGND7 6, 7

No Connection. Not internally connected.N.C.6, 8, 13 8

Driver Output Enable. Drive DE high to enable the driver outputs. These outputs are high
impedance when DE is low. Drive RE high and DE low to enter low-power shutdown mode.

DE4 4

Noninverting Driver OutputY9 9

Inverting Receiver InputB11 11

Connect to V

CC

V

CCD

— 13

Noninverting Receiver InputA12 12

Inverting Driver OutputZ10 10

PIN

MAX3291

FUNCTIONNAME

MAX3292

Figure 1. Driver DC Test Load

OUTPUT

UNDER TEST

R

L

S1

S2

V

CC

C

L

Figure 2. Driver or Receiver Enable/Disable Timing Test Load

Positive Supply: +4.75V ≤ VCC≤ +5.25VV

CC

14 14

Y

Z

R

V

OD

V

ODP

R

V

OC

MAX3291/MAX3292

RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication

8 _______________________________________________________________________________________

DI

DE

D

5V

Y

Z

C

L1

C

L2

R

DIFF

V

ODP

Figure 3. Driver Timing Test Circuit Figure 4. Shutdown Timing Diagram

DI

5V

0

Z

N

Y

P

Z

P

Y

N

Z

N

Z

P

Y

P

Y

N

0 DIFFERENTIAL

0 DIFFERENTIAL

1.5V
t

DPLH

20%

80%

80%

t

DPHL

1.5V

20%

t

HL

t

LH

V

DIFF

= VY - V

Z

V

DIFF

t

SKEW = | tPLH

- t

PHL

|

|

YN - ZN | = V

OD

|

YP - ZP | = V

ODP

Figure 5. Driver Propagation Delays Figure 6. Driver Enable and Disable Times

RO

V

OH

V

OL

A

+1V

B

-1V

0 DIFFERENTIAL

1.5V
t

RPHL

t

RPLH

1.5V

t

RSKEW = | tRPLH

- t

RPHL

|

Figure 7. Receiver Propagation Delays Figure 8. Receiver Enable and Disable Times

t < t

SHDN

RE

RO

DE = LOW

5V

DE

1.5V

0

t

Y, Z

DZL(SHDN)

2.3V

Y, Z

0

2.3V

t

DZH(SHDN)

t > t

SHDN

, t

DZL

t

DLZ

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

, t

DZH

t

DHZ

t

RZH(SHDN)

1.5V

VOL* + 0.5V

V

OH

* - 0.5V

V

PH

V

PL

*NOTE: V
S2 AND S1 CLOSED, RESPECTIVELY.

AND VOL ARE THE OUTPUT LEVELS IN FIGURE 2 WITH

OH

3V

RE

V

CC

RO

RO

1.5V 1.5V

0

, t

t

RZL(SHDN)

1.5V

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

1.5V

0

t

, t

RZH(SHDN)

RZL

RZH

t

RLZ

V

+ 0.5V

OL

V

- 0.5V

OH

t

RHZ

INPUTS

MAX3291/MAX3292

RS-485/RS-422 Transceivers with Preemphasis

for High-Speed, Long-Distance Communication

_______________________________________________________________________________________ 9

R

B

RO

C

L

ATE

A

V

ID

R

Figure 9. Receiver Propagation Delay Test Circuit Figure 10. Preemphasis Timing

RE

DI
DE

1k

V

CC

RO

MAX3291
MAX3292

Figure 11. Time-to-Shutdown Test Circuit

Function Tables

DI

RE

DE

1X 1

INPUTS

Z

0

OUTPUTS

Y

1

TRANSMITTING

1 00X 1

High-Z High-Z

High-Z and

SHUTDOWN

X1 0

X0 0

A-B

RE

DE

OUTPUT

RO

1
0≤ -0.2V0 X
1

High-Z and

SHUTDOWN

X1 0

Open0 X

≥ 0.2V0 X

X = Don’t care
Z = High impedance
SHUTDOWN = Low-power shutdown; driver and receiver outputs are high impedance.

INPUTS

RECEIVING

High-ZX1 1

t

PTND

Y

P

Z

N

0 DIFFERENTIAL

Y

N

80%

20%

50%

Z

P

t

PRE

Z

P

Y

N

Z

N

Y

P

MAX3291/MAX3292

Detailed Description

The MAX3291/MAX3292 high-speed RS-485/RS-422
transceivers feature driver preemphasis circuitry, which
extends the distance and increases the data rate of
reliable communication by reducing intersymbol inter­ference (ISI) caused by long cables. The MAX3291 is
programmable for data rates of 5Mbps to 10Mbps,
while the MAX3292 is programmable for data rates up
to 10Mbps by using a single external resistor.

The MAX3291/MAX3292 are full-duplex devices that
operate from a single +5V supply and offer a low-cur­rent shutdown mode that reduces supply current to
100nA. They feature driver output short-circuit current
limiting and a fail-safe receiver input that guarantees a
logic-high output if the input is open circuit. A 1/4-unit­load receiver input impedance allows up to 128 trans­ceivers on the bus.

Inter-symbol interference (ISI) causes significant prob­lems for UARTs if the total RS-485/RS-422 signal jitter
becomes 10% or more of the baud period. ISI is
caused by the effect of the cable’s RC time constant on
different bit patterns. If a series of ones is transmitted,
followed by a zero, the transmission-line voltage rises
to a high value at the end of the string of ones (signal 1
in Figure 12). As the signal moves towards the zero
state, it takes longer to reach the zero-crossing,
because its starting voltage is farther from the zero
crossing. On the other hand, if the data pattern has a
string of zeros followed by a one and then another zero,
the one-to-zero transition starts from a voltage that is
much closer to the zero-crossing (VA- VB= 0) and it
takes much less time for the signal to reach the zero­crossing (signal 2 in Figure 12). In other words, the
propagation delay depends upon the previous bit pat­tern. This is inter-symbol interference (ISI).

Preemphasis reduces ISI by increasing the signal
amplitude at every transition edge for about one baud
period, counteracting the effects of the cable (see the
section

Setting the Preemphasis Interval

). Figure 13
shows a typical preemphasis waveform optimized for
data rates between 5Mbps and 10Mbps. When DI
changes from a logic low to a logic high, the differential
output switches to a strong high. At the end of the pre­emphasis interval, the strong high returns to a normal
high level. Both levels meet RS-485/RS-422 specifica­tions, and the strong levels are typically 1.9 times larger
than the normal levels. If DI switches back to a logic
low before the end of the preemphasis interval, the dif­ferential output switches directly from the strong high to
the strong low. Similarly, this explanation applies when
DI transitions from high to low.

Applications Information

Data Rate vs. Cable Length

In general, preemphasis allows either double the dis­tance for a fixed data rate or double the data rate for a
fixed existing cable distance over existing RS-485
transceivers that do not feature preemphasis. Figure 14
shows that the MAX3291/MAX3292 transmits approxi­mately twice as far at the same data rate or twice as
fast at the same cable length as a conventional RS-485
transceiver without preemphasis for 10% jitter.

RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication

10 ______________________________________________________________________________________

SIGNAL 2

SIGNAL 1

V

A

- V

B

BAUD PERIOD

t

ISI

Figure 12. Inter-Symbol Interference among Two Data
Patterns: Signal 1 = 11111110, Signal 2 = 00000010

100ns

0010101

1

10

V

Y

- V

Z

DI

2.5V/div

5V/div

Figure 13. Typical Preemphasis Waveform with a 100ns
Preemphasis Interval

Setting the Preemphasis Interval

The MAX3291 has an internal fixed preemphasis interval
of 100ns. Use the MAX3291 for existing designs requiring
industry-standard ’75180 pin-compatibility at data rates of
5Mbps to 10Mbps.

The MAX3292 has a resistor-programmable preemphasis
interval for more flexibility. For data rates less than 1Mbps,
use the following equation to calculate R

PSET

(the pre-

emphasis setting resistor):

R

PSET

= 580 (t

BAUD

- 100)

where t

BAUD

= one baud period in ns.

For example, a baud rate of 500kbps produces a baud
period of 2µs (2µs = 2000ns).

R

PSET

= 580 (t

BAUD

- 100)

R

PSET

= 580 (2000 - 100) = 1.1MΩ

For data rates of 1Mbps to 10Mbps, use the following
equation to calculate R

PSET

:

R

PSET

= 580 (t

BAUD

- 100)(t

BAUD

/ 1000)

where t

BAUD

= one baud period in ns.

For example, a baud rate of 1Mbps produces a baud
period of 1µs (1µs = 1000ns).

R

PSET

= 580 (1000 - 100)(1000 / 1000) = 522kΩ

(closest standard value is 523kΩ)

Set the preemphasis interval by connecting the R

PSET

resistor from PSET to VCC. Use a 0.1µF bypass capaci­tor (C

PSET

) from PSET to VCC. If PSET is connected

directly to VCC(R

PSET

= 0), the preemphasis interval

reverts to the nominal 100ns value.

Eye Diagrams

One simple method to quickly determine your circuit
configuration is to view an eye diagram. An eye dia­gram is a scope photo (voltage vs. time) showing the
transitions of a pseudo-random bit string displaying at
least one bit interval. Use an eye diagram to quickly
calculate the total jitter of a circuit configuration. Jitter is
the total time variation at the zero-volt differential cross­ing, and percent jitter is expressed as a percentage of
one baud period, t

BAUD

. Figures 15 and 16 show typi­cal eye diagrams for a non-preemphasis device and
the MAX3291/MAX3292. ISI and jitter are often used
interchangeably; however, they are not exactly the
same thing. ISI usually makes up the majority of the jit­ter, but asymmetrical high and low driver output voltage
levels and time skews of non-ideal transceivers (driver
and receiver) also contribute to jitter.

MAX3291/MAX3292

RS-485/RS-422 Transceivers with Preemphasis

for High-Speed, Long-Distance Communication

______________________________________________________________________________________ 11

10,000

100

0.1 1 10
DATA RATE (Mbps)

CABLE LENGTH (FEET)

1000

10% JITTER

PREEMPHASIS

485 DRIVER

LIMIT

CONVENTIONAL

485 DRIVER

LIMIT

PREEMPHASIS REQUIRED
FOR ERROR-FREE
TRANSMISSION

24-GAUGE
TWISTED PAIR

Figure 14. Preemphasis Driver Performance Compared to a
Conventional Driver Without Preemphasis at 10% Jitter

Figure 15. Eye Diagram of a Typical RS-485 Transceiver
Without Preemphasis, while Driving 1000 feet of Cable at
5Mbps

Figure 16. Eye Diagram of the MAX3292 with a Preemphasis
Interval of 175ns, while Driving 1000 feet of Cable at 5Mbps

MAX3291/MAX3292

% Jitter = (total jitter / t

BAUD

) · 100

When the total amount of time skew becomes 10% or
more of the baud period, the data error rate can
increase sharply.

128 Transceivers on the Bus

The standard RS-485 receiver input impedance is 12kΩ
(one unit load), and the standard driver can drive up to
32 unit loads. The MAX3291/MAX3292 transceivers have
a 1/4-unit-load receiver input impedance (48kΩ), allow­ing up to 128 transceivers to be connected in parallel on
one communication line. Any combination of these
devices and/or other RS-485 transceivers with a total of
32 unit-loads or less can be connected to the line.

Low-Power Shutdown Mode

Initiate low-power shutdown mode by bringing RE high
and DE low. In shutdown the MAX3291/MAX3292 typi­cally draw only 1µA of supply current.

Simultaneously driving RE and DE is allowed; the parts
are guaranteed not to enter shutdown if RE is high and
DE is low for less than 80ns. If the inputs are in this
state for at least 300ns, the parts are guaranteed to
enter shutdown.

Enable times tZHand tZLin the

Switching Characteris-

tics

tables correspond to when the part is not in the low-

power shutdown state. Enable times t

ZH(SHDN)

and

t

ZL(SHDN)

assume the parts are shut down. It takes dri­vers and receivers longer to activate from the low­power shutdown mode (t

ZH(SHDN)

, t

ZL(SHDN)

) than from

the driver/receiver disable mode (tZH, tZL).

Line Repeater

For line lengths greater than what one MAX3291/
MAX3292 can drive, use the repeater application
shown in Figure 17.

Figure 18 shows the system differential voltage for the
MAX3292 driving 4000 feet of 26AWG twisted-pair wire
into two 120Ω termination loads.

Line Termination

The MAX3291/MAX3292 are targeted for applications
requiring the best combination of long cable length and
lowest bit-error rate. In order to achieve this combina­tion, the cable system must be set up with care. There
are three basic steps:

1) The cable should only have two ends (no tree configu­ration with long branches), which are terminated with
a simple resistor termination whose value is the
cable’s characteristic impedance (ZO). Avoid termina­tions anywhere else along the cable. This ensures that
there are no reflections at the end of the cable, and
that all transmitters (whether they are located at the
ends of the cable or somewhere along the length) see
the same impedance, equal to ZO/ 2.

2) Make all branches or stubs short enough so that
twice the propagation delay along the stub (down
and back) is significantly less than one baud period
(around 15% or less). This ensures that the reflec­tions from the end of the stub (which are unavoid­able, since the stubs are not terminated) settle in
much less than a baud period. If the application
requires a branch much longer than this, use a
repeater (see the

Line Repeater

section).

RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication

12 ______________________________________________________________________________________

Figure 17. Line-Repeater Application

2µs/div

V

A

- V

B

RO

DI

RECEIVER

INPUT

1V/div

5V/div

TYPICAL OPERATING CIRCUIT, R

PSET

= 1MΩ

5V/div

Figure 18. MAX3292 System Differential Voltage Driving 4000
Feet, Using Two 120

Ω

Termination Resistors

MAX3291
MAX3292

A

RO
RE

DE

DI

D

R

120Ω

B

Z

120Ω

Y

DATA IN

DATA OUT

MAX3291/MAX3292

RS-485/RS-422 Transceivers with Preemphasis

for High-Speed, Long-Distance Communication

______________________________________________________________________________________ 13

DI RO

RE

Z

Z

Z

Y

B

A

YBA

RE

RE

RO

RO

DI

DI

DE

DE

DE

D

D

R

R

Y
B

A

R = Z

O

R = Z

O

D

R

MAX3291
MAX3292

Figure 19. Typical Half-Duplex RS-485 Network

3) Don’t overload the cable with too many receivers.
Even though the MAX3291/MAX3292 receives pre­sent only 1/4-unit load, placing 128 receivers on the
cable will attenuate the signal if spaced out along
the cable and, in addition, cause reflections if
clumped in one spot. The MAX3291/MAX3292 suc­cessfully drive the cables to correct RS-485/RS-422
levels with 128 receivers, but the preemphasis
effect is significantly diminished.

The MAX3291/ MAX3292 are centered for a load imped­ance of 54Ω, which corresponds to the parallel combina­tion of the cable impedance and termination resistors. If
your cable impedance deviates somewhat from this
value, you still get the preemphasis effect (although the
ideal preemphasis time, t

PRE

, may need adjustment).
However, if your cable impedance is significantly differ­ent, the preemphasis ratio DPER changes, resulting in

significantly less preemphasis. Determine the preempha­sis ratio versus load by referring to the Driver Differential
Output Voltage vs. R

DIFF

graph in the

Typical Operating

Characteristics

. Read the strong and normal levels from
the graph (remember that the horizontal units are half
your cable impedance) and divide the two numbers to
get DPER (DPER = V

STRONG

/ V

NORMAL

= V

ODP

/ VOD).
Figures 19 and 20 show typical network application cir­cuits with proper termination.

Preemphasis at Low Data Rates

(MAX3292)

At low data rates (<1Msps), preemphasis operation is
not guaranteed because it is highly dependent on the
system power-supply noise. Minimize this noise by
increasing bypass capacitance and using a power
supply with a fast transient response.

R = Z

O

R = Z

O

R

D

RO

RE

DE

DI

A

B

Y

R = Z

O

R = Z

O

DI

DI DIRO RO

RO

DE DE

DE

RE

RE

RE

Z

Z

Z

Z

Y

Y

Y

AA

A

BB

B

D

D

D

R

R

R

MAX3291
MAX3292

NOTE: RE AND DE ON.

Figure 20. Typical Full-Duplex RS-485 Network

MAX3291/MAX3292

RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication

14 ______________________________________________________________________________________

Chip Information

TRANSISTOR COUNT: 2280
SUBSTRATE CONNECTED TO GND

Pin Configuration

14
13
12
11
10

9
8

1
2
3
4
5
6
7

V

CC

N.C. (PVCC)
A
BDE

RE

RO

PEE (PSET)

TOP VIEW

MAX3291
MAX3292

Z
Y
N.C.

( ) ARE FOR THE MAX3292 ONLY.

GND

N.C. (GND)

DI

SO/DIP

MAX3291/MAX3292

RS-485/RS-422 Transceivers with Preemphasis

for High-Speed, Long-Distance Communication

______________________________________________________________________________________ 15

________________________________________________________Package Information

SOICN.EPS

MAX3291/MAX3292

RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication

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

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© 1999 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

___________________________________________Package Information (continued)

PDIPN.EPS