MAX3291CSD-T_技术文档

19-1405; Rev 1; 4/99

RS-485/RS-422 Tra ns c e ive rs w ith Pre e m pha s is

for High-Spe e d, Long-Dis ta nc e Com m unic a tion

1/MAX392

________________Ge n e ra l De s c rip t io n

____________________________Fe a t u re s

The MAX3291/MAX3292 high-speed RS-485/RS-422

transceivers feature driver preemphasis circuitry, which

extends the distance and increases the data rate of reli-

able communication by reducing intersymbol interfer-

ence (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.

♦ 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

The MAX3291/MAX3292 are full-duplex devices that

operate from a single +5V supply and offer a low-cur-

rent 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-unit-

load receiver input impedance allows up to 128 trans-

ceivers on the bus.

♦ Pin-Compatible with ’75180, MAX489, MAX491

MAX3080, MAX3083, MAX3086, MAX1482

_______________Ord e rin g In fo rm a t io n

PART

TEMP. RANGE

0°C to +70°C

-40°C to +85°C

0°C to +70°C

-40°C to +85°C

PIN-PACKAGE

14 SO

MAX3291CSD

MAX3291CPD

MAX3291ESD

MAX3291EPD

MAX3292CSD

MAX3292CPD

MAX3292ESD

MAX3292EPD

14 Plastic DIP

14 SO

________________________Ap p lic a t io n s

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

Communications

14 Plastic DIP

14 SO

Telecommunications

14 Plastic DIP

14 SO

Industrial-Control Local Area Networks

14 Plastic DIP

Typ ic a l Op e ra t in g Circ u it a n d Fu n c t io n a l Dia g ra m

R

*

R

*

PSET

1µF

C

PSET

C

PSET

*

PEE

(PSET)

1

V

CC

V

CC

14

1

14

13

(V

CCD

)

(V

)

CCD

12

11

A

R = Z

O

9

10

Y

Z

O

2

5

RO

DI

B

4

3

4

5

DE

RE

DE

DI

10

R = Z

O

Z

11

12

B

A

O

2

RO

Y 9

( ) ARE FOR MAX3292

* MAX3292 ONLY

MAX3291

MAX3292

MAX3291

MAX3292

Z = THE CHARACTERISTIC

O

IMPEDANCE OF THE CABLE

7(6) GND

7(6)

GND

Pin Configuration appears at end of data sheet.

________________________________________________________________ Maxim Integrated Products

1

For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.

For small orders, phone 1-800-835-8769.

RS-485/RS-422 Tra ns c e ive rs w ith Pre e m pha s is

for High-Spe e d, Long-Dis ta nc e Com m unic a tion

ABSOLUTE MAXIMUM RATINGS

Supply Voltage (V , V

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

Operating Temperature Ranges

CC CCD

Control Input Voltage (RE, DE, PEE,

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

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

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

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

CC

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

CC

Continuous Power Dissipation (T = +70°C)

A

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

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

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.

DC ELECTRICAL CHARACTERISTICS

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

= 0 (MAX3292), V = V

(MAX3292), T = T

A

to T , unless otherwise

MAX

CC

PSET

CC

CCD

MIN

noted. Typical values are at V = +5V and T = +25°C.) (Note 1)

CC

A

PARAMETER

DRIVER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

1/MAX392

R = 27Ω

No load (Note 2)

1.5

5.0

Differential Driver Output

V

Figure 1

V

OD

5.25

Differential Driver Output with

Preemphasis

V

ODP

R = 27Ω

2.4

Differential Driver

Preemphasis Ratio

DPER

Figure 1, R = 27Ω (Note 3)

Figure 1, R = 27Ω (Note 4)

1.65

2.0

2.35

0.2

Change in Magnitude of

Differential Output Voltage

(Normal and Preemphasis)

∆V

,

OD

V

∆V

ODP

Driver Common-Mode Output

Voltage (Normal and

Preemphasis)

V

OC

Figure 1, R = 27Ω

V

/ 2

3

CC

Change in Magnitude of

Common-Mode Voltage

(Normal and Preemphasis)

∆V

Figure 1, R = 27Ω (Note 5)

0.3

OC

Change in Magnitude of

Common-Mode Output

Voltage (Normal to

Preemphasis)

∆V

Figure 1, R = 27Ω

50

mV

V

NP

2.4

DE, DI, RE

PEE

Input High Voltage

V

IH

3.75

Input Low Voltage

V

0.8

±2

V

DE, DI, RE, PEE

DE, DI, RE

IL

Input Current

I

IN

µA

PEE Input Current (MAX3291)

PSET Input Current (MAX3292)

I

-15

-30

70

-45

110

25

PEE

I

V

= V

PSET

PSET CC

V = V = +12V

Y

Z

DE = GND,

= GND or 5.25V

Output Leakage (Y and Z)

I

O

µA

V

CC

V = V = -7V

-25

Y

Z

Driver Short-Circuit Output

Current

I

-7V ≤ V

≤ +12V (Note 6)

±30

±250

mA

OSD

OUT

2

_______________________________________________________________________________________

RS-485/RS-422 Tra ns c e ive rs w ith Pre e m pha s is

for High-Spe e d, Long-Dis ta nc e Com m unic a tion

1/MAX392

DC ELECTRICAL CHARACTERISTICS (continued)

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

= 0 (MAX3292), V = V

(MAX3292), T = T

A

to T , unless otherwise

MAX

CC

PSET

CC

CCD

MIN

noted. Typical values are at V = +5V and T = +25°C.) (Note 1)

CC

A

PARAMETER

RECEIVER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

V

= +12V

= -7V

250

IN

DE = GND,

Input Current (A and B)

I ,

A B

µA

V

= GND or 5.25V

CC

V

IN

-150

Receiver Differential

Threshold Voltage

V

-7V ≤ V ≤ +12V

-200

3.5

200

mV

TH

CM

Receiver Input Hysteresis

∆V

V

A

= V = 0

35

mV

V

TH

B

Receiver Output High Voltage

Receiver Output Low Voltage

V

OH

I = -4mA, V - V = V

O A B TH

V

OL

I

O

= 4mA, V - V = -V

TH

0.4

±1

V

A

B

Three-State Output Current at

Receiver

I

0 ≤ V ≤ V

CC

0.1

µA

kΩ

OZR

O

Receiver Input Resistance

R

-7V ≤ V ≤ +12V

48

IN

CM

Receiver Output Short-Circuit

Current

I

0 ≤ V ≤ V

CC

±15

±95

mA

OSR

RO

SUPPLY CURRENT

No-Load Supply Current

I

2.0

0.1

3.0

1

mA

µA

RE = GND, DE = V

CC + CCD

CC

Supply Current in Shutdown

Mode

RE = V

DE = GND, V = V = 0 to V or

Y Z CC

CC,

I

SHDN

floating

SWITCHING CHARACTERISTICS

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

= 0 (MAX3292), V = V

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

CCD A

CC

PSET

CC

Typical values are at V = +5V and T = +25°C.)

CC

A

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

41

MAX

65

UNITS

t

DPLH

DPHL

Figures 3 and 5, R

= 54Ω,

DIFF

Driver Propagation Delay

ns

C

= C = 50pF

L2

L1

44

65

t

HL

LH

Driver Differential Output

Rise or Fall Time

Figures 3 and 5, R

= C = 50pF

= 54Ω,

DIFF

12

100

1

ns

µs

ns

C

L1

L2

MAX3291/MAX3292,

= 0

80

120

Figures 3 and 10,

R

PSET

Driver Preemphasis Interval

t

R

C

= 54Ω,

DIFF

PRE

MAX3292,

= 523kΩ

= C = 50pF

L1

L2

0.75

1.25

R

PSET

Preemphasis Voltage Level to

Normal Voltage Level Delay

Figures 3 and 10, R

= 54Ω,

DIFF

t

30

3

PTND

C

= C = 50pF

L2

L1

Differential Driver Output

Figures 3 and 5, R

= 54Ω,

DIFF

t

8

ns

DSKEW

Skew

t

- t

C

= C = 50pF

L1 L2

DPLH DPHL

Maximum Data Rate

f

10

Mbps

MAX

_______________________________________________________________________________________

3

RS-485/RS-422 Tra ns c e ive rs w ith Pre e m pha s is

for High-Spe e d, Long-Dis ta nc e Com m unic a tion

SWITCHING CHARACTERISTICS (continued)

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

= 0 (MAX3292), V = V

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

CCD A

CC

PSET

CC

Typical values are at V = +5V and T = +25°C.)

CC

A

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

Figures 2 and 6, S2 closed, R = 500Ω,

L

Driver Enable to Output High

Driver Enable to Output Low

Driver Disable Time from Low

Driver Disable Time from High

t

72

105

ns

DZH

C

= 100pF

L

Figures 2 and 6, S1 closed, R = 500Ω,

L

t

55

53

71

105

100

ns

DZL

DLZ

DHZ

C

= 100pF

L

Figures 2 and 6, S1 closed, R = 500Ω,

L

t

C

= 15pF

L

Figures 2 and 6, S2 closed, R = 500Ω,

L

t

C

= 15pF

L

t

49

52

85

RPLH

RPHL

Figures 7 and 9, C = 50pF, V = 2V,

L

ID

Receiver Propagation Delay

Receiver Output Skew

V

CM

= 0

1/MAX392

t

Figures 7 and 9, C = 100pF

3

RSKEW

L

t

- t

RPLH RPHL

Receiver Enable to Output

Low

Figures 2 and 8, R = 1kΩ, C = 100pF,

L

t

3

43

25

55

45

RZL

S1 closed

Receiver Enable to Output

High

Figures 2 and 8, R = 1kΩ, C = 100pF,

L

t

RZH

S2 closed

Receiver Disable Time from

Low

Figures 2 and 8, R = 1kΩ, C = 15pF,

L

t

RLZ

S1 closed

Receiver Disable Time from

High

Figures 2 and 8, R = 1kΩ, C = 15pF,

L

t

25

45

ns

RHZ

S2 closed

Figures 4 and 11 (Note 7)

Figures 2 and 6, R = 500Ω, C = 100pF,

Time to Shutdown

t

50

160

500

SHDN

Driver Enable from Shutdown

to Output High

L

t

6000

8750

DZH(SHDN)

S2 closed

Driver Enable from Shutdown

to Output Low

Figures 2 and 6, R = 500Ω, C = 100pF,

L

t

6000

850

30

8750

1500

ns

DZL(SHDN)

S1 closed

Receiver Enable from

Shutdown to Output High

Figures 2 and 8, R = 1kΩ, C = 100pF,

L

RZH(SHDN)

S2 closed

Receiver Enable from

Shutdown to Output Low

Figures 2 and 8, R = 1kΩ, C = 100pF,

L

t

RZL(SHDN)

S1 closed

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

/ V ).

OD

ODP

Note 4: ∆V

and ∆V are the changes in V and V , respectively, when the DI input changes. This specification reflects

DD OC

OC

ODP

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

Note 5: ∆V and ∆V are the changes in V and V , respectively, when the DI input changes state.

ODP

OC

OD

OC

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 ) is measured by monitoring R0 as in Figure 4.

SHDN

4

_______________________________________________________________________________________

RS-485/RS-422 Tra ns c e ive rs w ith Pre e m pha s is

for High-Spe e d, Long-Dis ta nc e Com m unic a tion

1/MAX392

__________________________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s

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

CC

A

PREEMPHASIS INTERVAL vs. R

PSET

(C

PSET

= 0.1µF)

TYPICAL PREEMPHASIS WAVEFORM

R

PRE

AND t

vs. t

PRE BAUD

2500

2250

2000

1750

1500

1250

1000

1200

1000

800

600

400

200

0

V - V

2V/

div

Y

Z

NOTE A

750

500

250

0

800

100ns/div

0

100 200 300 400 500 600 700

(kΩ)

900 1000

0

500

1000

(ns)

1500

2000

R

t

PSET

BAUD

RECEIVER PROPAGATION DELAY

vs. TEMPERATURE

DRIVER DIFFERENTIAL

OUTPUT VOLTAGE vs. TEMPERATURE

OUTPUT VOLTAGE vs. R

DIFF

60.0

57.5

55.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

4.25

C = 50pF

L

STRONG

STRONG (V

)

ODP

4.00

3.75

3.50

3.25

3.00

2.75

2.50

2.25

2.00

NORMAL

52.5

50.0

47.5

45.0

42.5

40.0

NORMAL (V

)

OD

R

DIFF

= 54Ω

100

-40 -20

0

20

40

60

80

10 20 30 40 50 60 70 80 90

(Ω)

-40 -20

0

20

40

60

80

TEMPERATURE (°C)

R

DIFF

TEMPERATURE (°C)

DRIVER PROPAGATION DELAY

vs. TEMPERATURE

RECEIVER PROPAGATION DELAY

50.0

47.5

45.0

C

L1

= C = 50pF

L2

42.5

40.0

37.5

35.0

32.5

30.0

2.5V/

div

V - V

A

B

5V/

div

RO

100

-40 -20

0

20

40

60

80

20ns/div

TEMPERATURE (°C)

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.

_______________________________________________________________________________________

5

RS-485/RS-422 Tra ns c e ive rs w ith Pre e m pha s is

for High-Spe e d, Long-Dis ta nc e Com m unic a tion

____________________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s (c o n t in u e d )

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

CC

A

RECEIVER OUTPUT LOW VOLTAGE

vs. TEMPERATURE

RECEIVER OUTPUT HIGH VOLTAGE

vs. TEMPERATURE

DRIVER PROPAGATION DELAY

0.30

0.25

4.60

4.55

4.50

4.45

4.40

4.35

4.30

4.25

4.20

4.15

4.10

I

RO

= 8mA

I = 8mA

RO

5V/

div

DI

0.20

0.15

2.5V/

div

V - V

Y

Z

0.10

0.05

0

100

-40 -20

0

20

40

60

80

-40 -20

0

20

40

60

80

20ns/div

1/MAX392

TEMPERATURE (°C)

OUTPUT SINK CURRENT vs.

RECEIVER OUTPUT LOW VOLTAGE

OUTPUT SOURCE CURRENT vs.

RECEIVER OUTPUT HIGH VOLTAGE

NO-LOAD SUPPLY CURRENT

vs. TEMPERATURE

70

60

50

40

30

20

10

0

30

25

20

15

10

5

3.00

2.75

2.50

2.25

2.00

1.75

1.50

1.25

1.00

0.75

0.50

0.25

0

100

0

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

OUTPUT LOW VOLTAGE (V)

0

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

OUTPUT HIGH VOLTAGE (V)

-40 -20

0

20

40

60

80

TEMPERATURE (°C)

6

_______________________________________________________________________________________

RS-485/RS-422 Tra ns c e ive rs w ith Pre e m pha s is

for High-Spe e d, Long-Dis ta nc e Com m unic a tion

1/MAX392

P in De s c rip t io n

NAME

PEE

FUNCTION

MAX3291

MAX3292

Preemphasis Enable Input. To enable preemphasis, leave PEE unconnected, connect to

1

—

V

CC

, or drive high. To enable strong-level-drive only mode, connect PEE to GND or drive

low.

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

) in paral-

PSET

—

1

PSET

lel with a capacitor (C

Operating Circuit.

) from PSET to V to set the preemphasis interval. See Typical

PSET CC

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

is low.

2

3

4

2

3

4

RO

RE

DE

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.

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.

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.

5

DI

6, 8, 13

8

6, 7

9

N.C.

GND

Y

No Connection. Not internally connected.

Ground

7

9

Noninverting Driver Output

Inverting Driver Output

Inverting Receiver Input

Noninverting Receiver Input

10

11

12

—

14

10

11

12

13

14

Z

B

A

V

CCD

Connect to V

CC

V

CC

Positive Supply: +4.75V ≤ V ≤ +5.25V

CC

Y

R

V

CC

S1

S2

R

L

V

ODP

OD

OUTPUT

UNDER TEST

C

L

R

V

OC

Z

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

_______________________________________________________________________________________

Figure 1. Driver DC Test Load

7

RS-485/RS-422 Tra ns c e ive rs w ith Pre e m pha s is

for High-Spe e d, Long-Dis ta nc e Com m unic a tion

5V

t < t

SHDN

t > t

SHDN

DE

C

L1

RE

Y

Z

DI

R

D

DIFF

V

ODP

t

RZH(SHDN)

C

L2

RO

DE = LOW

Figure 3. Driver Timing Test Circuit

Figure 4. Shutdown Timing Diagram

1/MAX392

5V

DI

1.5V

DE

1.5V

0

t

DPHL

DPLH

0

Z

P

Y

P

V

PH

Z

N

Y

N

t

DLZ

t

, t

DZL(SHDN) DZL

Y, Z

0 DIFFERENTIAL

OUTPUT NORMALLY LOW

2.3V

V * + 0.5V

OL

Z

N

Y

N

Z

P

Y

P

OUTPUT NORMALLY HIGH

V

DIFF

= V - V

Y Z

Y, Z

0

V

DIFF

0 DIFFERENTIAL

20%

V * - 0.5V

OH

2.3V

80%

20%

V

PL

t

, t

t

DHZ

DZH(SHDN) DZH

t

LH

t

HL

t

- t

SKEW = | PLH PHL |

Y - Z = V

|

N

N |

OD

*NOTE: V AND V ARE THE OUTPUT LEVELS IN FIGURE 2 WITH

OH

OL

Y - Z = V

P |

P

ODP

S2 AND S1 CLOSED, RESPECTIVELY.

Figure 5. Driver Propagation Delays

Figure 6. Driver Enable and Disable Times

3V

1.5V

RE

V

OH

0

RO

1.5V

t

1.5V

V

OL

t

, t

RLZ

RZL(SHDN) RZL

t

RPLH

RPHL

V

RO

CC

1.5V

V

+ 0.5V

- 0.5V

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

OL

A

B

+1V

-1V

0 DIFFERENTIAL

RO

V

OH

1.5V

0

t

- t

RSKEW = | RPLH RPHL |

t

, t

t

RHZ

RZH(SHDN) RZH

Figure 7. Receiver Propagation Delays

Figure 8. Receiver Enable and Disable Times

8

_______________________________________________________________________________________

RS-485/RS-422 Tra ns c e ive rs w ith Pre e m pha s is

for High-Spe e d, Long-Dis ta nc e Com m unic a tion

1/MAX392

t

PTND

Y

P

Z

P

80%

20%

Z

N

Y

N

B

A

RO

V

ID

R

ATE

0 DIFFERENTIAL

Z

N

C

L

Y

N

50%

Y

P

Z

P

t

PRE

Figure 9. Receiver Propagation Delay Test Circuit

Figure 10. Preemphasis Timing

V

CC

RE

1k

MAX3291

MAX3292

RO

DI

DE

Figure 11. Time-to-Shutdown Test Circuit

Fu n c t io n Ta b le s

RECEIVING

TRANSMITTING

INPUTSINPUTS

OUTPUT

INPUTS

OUTPUTS

DE

X

A-B

≥ 0.2V

≤ -0.2V

Open

X

RO

RE

0

DE

1

DI

1

Z

0

1

Y

RE

X

1

0

X

0

1

X

1

0

X

0

X

High-Z

1

High-Z

High-Z and

SHUTDOWN

High-Z and

SHUTDOWN

1

0

X

1

0

X

X = Don’t care

Z = High impedance

SHUTDOWN = Low-power shutdown; driver and receiver outputs are high impedance.

_______________________________________________________________________________________

9

RS-485/RS-422 Tra ns c e ive rs w ith Pre e m pha s is

for High-Spe e d, Long-Dis ta nc e Com m unic a tion

De t a ile d De s c rip t io n

The MAX3291/MAX3292 high-speed RS-485/RS-422

SIGNAL 1

transceivers feature driver preemphasis circuitry, which

extends the distance and increases the data rate of

reliable communication by reducing intersymbol inter-

BAUD PERIOD

ference (ISI) caused by long cables. The MAX3291 is

programmable for data rates of 5Mbps to 10Mbps,

V - V

A

B

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-cur-

rent 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-unit-

load receiver input impedance allows up to 128 trans-

ceivers on the bus.

t

ISI

SIGNAL 2

Figure 12. Inter-Symbol Interference among Two Data

Patterns: Signal 1 = 11111110, Signal 2 = 00000010

Inter-symbol interference (ISI) causes significant prob-

lems for UARTs if the total RS-485/RS-422 signal jitter

b e c ome s 10% or more of the b a ud p e riod . 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

s ta te , it ta ke s long e r to re a c h the ze ro-c ros s ing ,

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

1/MAX392

V - V

Y

Z

2.5V/div

5V/div

DI

much closer to the zero-crossing (V - V = 0) and it

A

B

takes much less time for the signal to reach the zero-

crossing (signal 2 in Figure 12). In other words, the

propagation delay depends upon the previous bit pat-

tern. This is inter-symbol interference (ISI).

0

1

0

1

0

1

100ns

Pre e mp ha s is re d uc e s ISI b y inc re a s ing the s ig na l

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

d a ta ra te s b e twe e n 5Mb p s a nd 10Mb p s . Whe n DI

changes from a logic low to a logic high, the differential

output switches to a strong high. At the end of the pre-

emphasis interval, the strong high returns to a normal

high level. Both levels meet RS-485/RS-422 specifica-

tions, 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 dif-

ferential output switches directly from the strong high to

the strong low. Similarly, this explanation applies when

DI transitions from high to low.

Figure 13. Typical Preemphasis Waveform with a 100ns

Preemphasis Interval

Ap p lic a t io n s In fo rm a t io n

Da t a Ra t e vs . Ca b le Le n g t h

In general, preemphasis allows either double the dis-

tance for a fixed data rate or double the data rate for a

fixe d e xis ting c a b le d is ta nc e ove r e xis ting RS-485

transceivers that do not feature preemphasis. Figure 14

shows that the MAX3291/MAX3292 transmits approxi-

mately 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.

10 ______________________________________________________________________________________

RS-485/RS-422 Tra ns c e ive rs w ith Pre e m pha s is

for High-Spe e d, Long-Dis ta nc e Com m unic a tion

1/MAX392

S e t t in g t h e P re e m p h a s is In t e rva l

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.

Eye Dia g ra m s

One simple method to quickly determine your circuit

configuration is to view an eye diagram. An eye dia-

gram 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 cross-

ing, and percent jitter is expressed as a percentage of

The MAX3292 has a resistor-programmable preemphasis

interval for more flexibility. For data rates less than 1Mbps,

use the following equation to calculate R

emphasis setting resistor):

(the pre-

PSET

one baud period, t . Figures 15 and 16 show typi-

BAUD

R

PSET

= 580 (t

- 100)

BAUD

cal 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 jit-

ter, but asymmetrical high and low driver output voltage

levels and time skews of non-ideal transceivers (driver

and receiver) also contribute to jitter.

where t

= one baud period in ns.

BAUD

For example, a baud rate of 500kbps produces a baud

period of 2µs (2µs = 2000ns).

R

= 580 (t

= 580 (2000 - 100) = 1.1MΩ

- 100)

PSET

BAUD

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

equation to calculate R

:

PSET

R

= 580 (t

- 100)(t

/ 1000)

PSET

BAUD

where t

= one baud period in ns.

BAUD

For example, a baud rate of 1Mbps produces a baud

period of 1µs (1µs = 1000ns).

R

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

PSET

(closest standard value is 523kΩ)

Set the preemphasis interval by connecting the R

PSET

resistor from PSET to V . Use a 0.1µF bypass capaci-

CC

tor (C

) from PSET to V . If PSET is connected

PSET

CC

directly to V

(R

= 0), the preemphasis interval

PSET

CC

reverts to the nominal 100ns value.

Figure 15. Eye Diagram of a Typical RS-485 Transceiver

Without Preemphasis, while Driving 1000 feet of Cable at

5Mbps

10,000

24-GAUGE

TWISTED PAIR

10% JITTER

PREEMPHASIS

485 DRIVER

LIMIT

1000

CONVENTIONAL

485 DRIVER

LIMIT

PREEMPHASIS REQUIRED

FOR ERROR-FREE

TRANSMISSION

100

0.1

1

10

DATA RATE (Mbps)

Figure 14. Preemphasis Driver Performance Compared to a

Conventional Driver Without Preemphasis at 10% Jitter

Figure 16. Eye Diagram of the MAX3292 with a Preemphasis

Interval of 175ns, while Driving 1000 feet of Cable at 5Mbps

______________________________________________________________________________________ 11

RS-485/RS-422 Tra ns c e ive rs w ith Pre e m pha s is

for High-Spe e d, Long-Dis ta nc e Com m unic a tion

Lin e Re p e a t e r

For line le ng ths g re a te r tha n wha t one MAX3291/

MAX3292 c a n d rive , us e the re p e a te r a p p lic a tion

shown in Figure 17.

% Jitter = (total jitter / t

) · 100

BAUD

When the total amount of time skew becomes 10% or

more of the b a ud p e riod , the d a ta e rror ra te c a n

increase sharply.

Figure 18 shows the system differential voltage for the

MAX3292 driving 4000 feet of 26AWG twisted-pair wire

into two 120Ω termination loads.

1 2 8 Tra n s c e ive rs o n t h e Bu s

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), allow-

ing up to 128 transceivers to be connected in parallel on

one c ommunic a tion line . Any c omb ina tion of the s e

devices and/or other RS-485 transceivers with a total of

32 unit-loads or less can be connected to the line.

Lin e Te rm in a t io n

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 combina-

tion, the cable system must be set up with care. There

are three basic steps:

1) The cable should only have two ends (no tree configu-

ration with long branches), which are terminated with

a s imp le re s is tor te rmina tion whos e va lue is the

Lo w -P o w e r S h u t d o w n Mo d e

Initiate low-power shutdown mode by bringing RE high

and DE low. In shutdown the MAX3291/MAX3292 typi-

cally draw only 1µA of supply current.

cable’s characteristic impedance (Z ). Avoid termina-

O

1/MAX392

tions 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

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.

the same impedance, equal to Z / 2.

O

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 reflec-

tions from the end of the stub (which are unavoid-

able, since the stubs are not terminated) settle in

much less than a baud period. If the application

re q uire s a b ra nc h muc h long e r tha n this , us e a

repeater (see the Line Repeater section).

Enable times t

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

power shutdown state. Enable times t and

and t in the Switching Characteris-

ZL

ZH

ZH(SHDN)

t

assume the parts are shut down. It takes dri-

ZL(SHDN)

vers and receivers longer to activate from the low-

power shutdown mode (t , t ) than from

the driver/receiver disable mode (t , t ).

ZH(SHDN) ZL(SHDN)

ZH ZL

MAX3291

MAX3292

5V/div

1V/div

DI

RECEIVER

INPUT

A

B

RO

RE

120Ω

R

DATA IN

V - V

A

B

DE

DI

RO

5V/div

Z

DATA OUT

D

Y

2µs/div

TYPICAL OPERATING CIRCUIT, R

= 1MΩ

PSET

Figure 18. MAX3292 System Differential Voltage Driving 4000

Figure 17. Line-Repeater Application

Feet, Using Two 120Ω Termination Resistors

12 ______________________________________________________________________________________

RS-485/RS-422 Tra ns c e ive rs w ith Pre e m pha s is

for High-Spe e d, Long-Dis ta nc e Com m unic a tion

1/MAX392

3) Don’t overload the cable with too many receivers.

Even though the MAX3291/MAX3292 receives pre-

sent only 1/4-unit load, placing 128 receivers on the

cable will attenuate the signal if spaced out along

the c a b le a nd , in a d d ition, c a us e re fle c tions if

clumped in one spot. The MAX3291/MAX3292 suc-

cessfully drive the cables to correct RS-485/RS-422

le ve ls with 128 re c e ive rs , b ut the p re e mp ha s is

effect is significantly diminished.

significantly less preemphasis. Determine the preempha-

sis ratio versus load by referring to the Driver Differential

Output Voltage vs. R

graph in the Typical Operating

DIFF

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

/ V

= V

/ V ).

STRONG

NORMAL

ODP OD

Figures 19 and 20 show typical network application cir-

cuits with proper termination.

The MAX3291/ MAX3292 are centered for a load imped-

ance of 54Ω, which corresponds to the parallel combina-

tion of the cable impedance and termination resistors. If

your cable impedance deviates somewhat from this

value, you still get the preemphasis effect (although the

P re e m p h a s is a t Lo w Da t a Ra t e s

(MAX3 2 9 2 )

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.

ideal preemphasis time, t

, may need adjustment).

PRE

However, if your cable impedance is significantly differ-

ent, the preemphasis ratio DPER changes, resulting in

DE

Z

DI

D

R = Z

O

D

DI

Y

B

DE

B

A

Z

Y

B

A

RO

RE

RO

RE

R

A

R

D

MAX3291

MAX3292

DI

DE

RO RE

Figure 19. Typical Half-Duplex RS-485 Network

A

Y

R = Z

O

R = Z

O

RO

RE

R

DI

D

B

Z

B

DE

RE

RO

R = Z

O

DI

R

D

Y

A

Y

Z

B

A

Y

Z

B

A

R

MAX3291

MAX3292

D

DI

D

DI

DE RE RO

NOTE: RE AND DE ON.

Figure 20. Typical Full-Duplex RS-485 Network

______________________________________________________________________________________ 13

RS-485/RS-422 Tra ns c e ive rs w ith Pre e m pha s is

for High-Spe e d, Long-Dis ta nc e Com m unic a tion

P in Co n fig u ra t io n

Ch ip In fo rm a t io n

TRANSISTOR COUNT: 2280

SUBSTRATE CONNECTED TO GND

TOP VIEW

MAX3291

MAX3292

PEE (PSET)

RO

1

2

3

4

5

6

7

14

V

CC

13 N.C. (PV

)

CC

RE

12

11

10

9

A

DE

B

DI

Z

N.C. (GND)

GND

Y

8

N.C.

1/MAX392

SO/DIP

( ) ARE FOR THE MAX3292 ONLY.

14 ______________________________________________________________________________________

RS-485/RS-422 Tra ns c e ive rs w ith Pre e m pha s is

for High-Spe e d, Long-Dis ta nc e Com m unic a tion

1/MAX392

________________________________________________________P a c k a g e In fo rm a t io n

______________________________________________________________________________________ 15

RS-485/RS-422 Tra ns c e ive rs w ith Pre e m pha s is

for High-Spe e d, Long-Dis ta nc e Com m unic a tion

___________________________________________P a c k a g e In fo rm a t io n (c o n t in u e d )

1/MAX392

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 ____________________Ma x im In t e g ra t e d P ro d u c t s , 1 2 0 S a n Ga b rie l Drive , S u n n yva le , CA 9 4 0 8 6 4 0 8 -7 3 7 -7 6 0 0

Printed USA

is a registered trademark of Maxim Integrated Products.


型号：	MAX3291CSD-T
厂家：	MAXIM INTEGRATED PRODUCTS
描述：	Line Transceiver, 1 Func, 1 Driver, 1 Rcvr, PDSO14, 0.150 INCH, MS-012B, SOIC-14 通信
文件：	总16页 (文件大小：286K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

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