19-1405; Rev 1; 4/99
RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication
____________________________Features
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.
♦ Preemphasis Increases the Distance and Data
Rate of Reliable RS-485/RS-422 Communication
________________________Applications
Long-Distance, High-Speed RS-485/RS-422
Communications
Telecommunications
Industrial-Control Local Area Networks
♦ 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
_______________Ordering Information
PART
TEMP. RANGE
MAX3291CSD
0°C to +70°C
MAX3291CPD
MAX3291ESD
MAX3291EPD
MAX3292CSD
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
0°C to +70°C
14 Plastic DIP
14 SO
14 Plastic DIP
14 SO
MAX3292CPD
MAX3292ESD
MAX3292EPD
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
14 Plastic DIP
14 SO
14 Plastic DIP
PIN-PACKAGE
14 SO
Typical Operating Circuit and Functional Diagram
RPSET*
RPSET*
1µF
PEE
(PSET)
RE
DE
DI
13
A 12
2
(VCCD)
R = ZO
R = ZO
9
10
Y
Z 10
R = ZO
Y 9
MAX3291
MAX3292
7(6)
GND
R = ZO
11
12
DI
4 DE
3
RE
4
5
13
5
Z
B 11
3
CPSET*
VCC 14
1
PEE
(PSET)
(VCCD)
RO
1µF
CPSET*
VCC 14
1
B
2
A
MAX3291
MAX3292
7(6)
GND
RO
( ) ARE FOR MAX3292
* MAX3292 ONLY
ZO = THE CHARACTERISTIC
IMPEDANCE OF THE CABLE
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.
MAX3291/MAX3292
________________General Description
MAX3291/MAX3292
RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (VCC, VCCD) .................................................+6V
Control Input Voltage (RE, DE, PEE,
PSET, DI) .................................................-0.3V to (VCC + 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 (VCC + 0.3V)
Continuous Power Dissipation (TA = +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
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, VCC = +5V ±5%, RPSET = 0 (MAX3292), VCC = VCCD (MAX3292), TA = TMIN to TMAX, unless otherwise
noted. Typical values are at VCC = +5V and TA = +25°C.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
DRIVER
R = 27Ω
1.5
Differential Driver Output
VOD
Figure 1
Differential Driver Output with
Preemphasis
VODP
R = 27Ω
2.4
Differential Driver
Preemphasis Ratio
DPER
Figure 1, R = 27Ω (Note 3)
1.65
Change in Magnitude of
Differential Output Voltage
(Normal and Preemphasis)
∆VOD,
∆VODP
Figure 1, R = 27Ω (Note 4)
Driver Common-Mode Output
Voltage (Normal and
Preemphasis)
VOC
∆VOC
Figure 1, R = 27Ω (Note 5)
Change in Magnitude of
Common-Mode Output
Voltage (Normal to
Preemphasis)
∆VNP
Figure 1, R = 27Ω
2.4
PEE
3.75
Input Low Voltage
VIL
DE, DI, RE, PEE
IIN
DE, DI, RE
PEE Input Current (MAX3291)
IPEE
PSET Input Current (MAX3292)
IPSET
Output Leakage (Y and Z)
IO
Driver Short-Circuit Output
Current
IOSD
2
-15
VPSET = VCC
DE = GND,
VCC = GND or 5.25V
2.0
2.35
V
0.2
V
3
V
0.3
V
50
DE, DI, RE
VIH
mV
V
0.8
V
±2
µA
-30
-45
µA
70
110
µA
VY = VZ = +12V
25
VY = VZ = -7V
-25
-7V ≤ VOUT ≤ +12V (Note 6)
V
V
VCC / 2
Change in Magnitude of
Common-Mode Voltage
(Normal and Preemphasis)
Input Current
5.25
Figure 1, R = 27Ω
Input High Voltage
5.0
No load (Note 2)
±30
_______________________________________________________________________________________
±250
µA
mA
RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication
(Typical Operating Circuit, VCC = +5V ±5%, RPSET = 0 (MAX3292), VCC = VCCD (MAX3292), TA = TMIN to TMAX, unless otherwise
noted. Typical values are at VCC = +5V and TA = +25°C.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
250
-150
µA
200
mV
RECEIVER
Input Current (A and B)
IA, B
DE = GND,
VCC = GND or 5.25V
Receiver Differential
Threshold Voltage
VTH
-7V ≤ VCM ≤ +12V
Receiver Input Hysteresis
∆VTH
VA = VB = 0
Receiver Output High Voltage
VOH
IO = -4mA, VA - VB = VTH
Receiver Output Low Voltage
VOL
IO = 4mA, VA - VB = -VTH
Three-State Output Current at
Receiver
IOZR
0 ≤ VO ≤ VCC
Receiver Input Resistance
RIN
-7V ≤ VCM ≤ +12V
Receiver Output Short-Circuit
Current
IOSR
0 ≤ VRO ≤ VCC
VIN = +12V
VIN = -7V
-200
35
mV
3.5
V
0.1
0.4
V
±1
µA
48
kΩ
±15
±95
mA
2.0
3.0
mA
0.1
1
µA
SUPPLY CURRENT
No-Load Supply Current
Supply Current in Shutdown
Mode
ICC + ICCD RE = GND, DE = VCC
ISHDN
RE = VCC, DE = GND, VY = VZ = 0 to VCC or
floating
SWITCHING CHARACTERISTICS
(Typical Operating Circuit, VCC = +5V ±5%, RPSET = 0 (MAX3292), VCC = VCCD (MAX3292), TA = +25°C, unless otherwise noted.
Typical values are at VCC = +5V and TA = +25°C.)
PARAMETER
Driver Propagation Delay
Driver Differential Output
Rise or Fall Time
Driver Preemphasis Interval
Preemphasis Voltage Level to
Normal Voltage Level Delay
Differential Driver Output
Skew tDPLH - tDPHL
Maximum Data Rate
SYMBOL
tDPLH
tDPHL
tHL
tLH
tPRE
CONDITIONS
MIN
Figures 3 and 5, RDIFF = 54Ω,
CL1 = CL2 = 50pF
Figures 3 and 5, RDIFF = 54Ω,
CL1 = CL2 = 50pF
Figures 3 and 10,
RDIFF = 54Ω,
CL1 = CL2 = 50pF
MAX3291/MAX3292,
RPSET = 0
MAX3292,
RPSET = 523kΩ
TYP
MAX
41
65
44
65
12
ns
ns
80
100
120
ns
0.75
1
1.25
µs
tPTND
Figures 3 and 10, RDIFF = 54Ω,
CL1 = CL2 = 50pF
30
tDSKEW
Figures 3 and 5, RDIFF = 54Ω,
CL1 = CL2 = 50pF
3
fMAX
UNITS
10
ns
8
ns
Mbps
_______________________________________________________________________________________
3
MAX3291/MAX3292
DC ELECTRICAL CHARACTERISTICS (continued)
MAX3291/MAX3292
RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication
SWITCHING CHARACTERISTICS (continued)
(Typical Operating Circuit, VCC = +5V ±5%, RPSET = 0 (MAX3292), VCC = VCCD (MAX3292), TA = +25°C, unless otherwise noted.
Typical values are at VCC = +5V and TA = +25°C.)
PARAMETER
SYMBOL
Driver Enable to Output High
tDZH
Driver Enable to Output Low
TYP
MAX
UNITS
Figures 2 and 6, S2 closed, RL = 500Ω,
CL = 100pF
72
105
ns
tDZL
Figures 2 and 6, S1 closed, RL = 500Ω,
CL = 100pF
55
105
ns
Driver Disable Time from Low
tDLZ
Figures 2 and 6, S1 closed, RL = 500Ω,
CL = 15pF
53
100
ns
Driver Disable Time from High
tDHZ
Figures 2 and 6, S2 closed, RL = 500Ω,
CL = 15pF
71
100
ns
Figures 7 and 9, CL = 50pF, VID = 2V,
VCM = 0
49
85
52
85
Figures 7 and 9, CL = 100pF
3
Receiver Propagation Delay
Receiver Output Skew
tRPLH - tRPHL
tRPLH
tRPHL
tRSKEW
CONDITIONS
MIN
ns
ns
Receiver Enable to Output
Low
tRZL
Figures 2 and 8, RL = 1kΩ, CL = 100pF,
S1 closed
3
43
55
ns
Receiver Enable to Output
High
tRZH
Figures 2 and 8, RL = 1kΩ, CL = 100pF,
S2 closed
3
43
55
ns
Receiver Disable Time from
Low
tRLZ
Figures 2 and 8, RL = 1kΩ, CL = 15pF,
S1 closed
25
45
ns
Receiver Disable Time from
High
tRHZ
Figures 2 and 8, RL = 1kΩ, CL = 15pF,
S2 closed
25
45
ns
160
500
ns
Time to Shutdown
tSHDN
Figures 4 and 11 (Note 7)
50
Driver Enable from Shutdown
to Output High
tDZH(SHDN)
Figures 2 and 6, RL = 500Ω, CL = 100pF,
S2 closed
6000
8750
ns
Driver Enable from Shutdown
to Output Low
tDZL(SHDN)
Figures 2 and 6, RL = 500Ω, CL = 100pF,
S1 closed
6000
8750
ns
Receiver Enable from
Shutdown to Output High
tRZH(SHDN)
Figures 2 and 8, RL = 1kΩ, CL = 100pF,
S2 closed
850
1500
ns
Receiver Enable from
Shutdown to Output Low
tRZL(SHDN)
Figures 2 and 8, RL = 1kΩ, CL = 100pF,
S1 closed
30
1500
ns
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 (VODP / VOD).
Note 4: ∆VODP and ∆VOC are the changes in VDD and 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: ∆VODP and ∆VOC are the changes in VOD and 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 (tSHDN) is measured by monitoring R0 as in Figure 4.
4
_______________________________________________________________________________________
RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication
PREEMPHASIS INTERVAL vs. RPSET
(CPSET = 0.1µF)
2000
1000
1750
800
1500
RPRE (kΩ)
PREEMPHASIS INTERVAL (ns)
2250
2V/
div
1200
MAX3291/2-02
MAX3291/2-01
VY - VZ
RPRE AND tPRE vs. tBAUD
2500
MAX3291/92 toc 03
TYPICAL PREEMPHASIS WAVEFORM
1250
NOTE A
1000
600
NOTE A
400
750
500
200
250
0
0
3.5
NORMAL
3.0
2.5
2.0
1.5
1.0
0.5
STRONG (VODP)
4.00
3.75
3.50
3.25
3.00
2.75
NORMAL (VOD)
2.50
RDIFF = 54Ω
2.00
10
20
30
40
50
60
70
90 100
80
-40
-20
RDIFF (Ω)
0
CL = 50pF
57.5
55.0
52.5
50.0
47.5
45.0
40
60
80
-40
100
-20
20
40
60
80
100
RECEIVER PROPAGATION DELAY
MAX3291/2-08
MAX3291/2-07
CL1 = CL2 = 50pF
0
TEMPERATURE (°C)
TEMPERATURE (°C)
50.0
PROPAGATION DELAY (ns)
60.0
40.0
20
DRIVER PROPAGATION DELAY
vs. TEMPERATURE
47.5
2000
42.5
2.25
0
1500
RECEIVER PROPAGATION DELAY
vs. TEMPERATURE
PROPAGATION DELAY (ns)
4.0
1000
DRIVER DIFFERENTIAL
OUTPUT VOLTAGE vs. TEMPERATURE
4.25
DIFFERENTIAL OUTPUT VOLTAGE (V)
STRONG
500
tBAUD (ns)
MAX3291/2-05
4.5
MAX3291/2-04
DRIVER DIFFERENTIAL OUTPUT VOLTAGE (V)
DRIVER DIFFERENTIAL
OUTPUT VOLTAGE vs. RDIFF
0
RPSET (kΩ)
MAX3291/2-06
0 100 200 300 400 500 600 700 800 900 1000
100ns/div
45.0
42.5
40.0
2.5V/
div
VA - VB
37.5
5V/
div
RO
35.0
32.5
30.0
-40
-20
0
20
40
60
80
100
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
MAX3291/MAX3292
__________________________________________Typical Operating Characteristics
(VCC = +5V, TA = +25°C, unless otherwise noted.)
____________________________________Typical Operating Characteristics (continued)
(VCC = +5V, TA = +25°C, unless otherwise noted.)
RECEIVER OUTPUT LOW VOLTAGE
vs. TEMPERATURE
0.25
0.20
0.15
0.10
IRO = 8mA
4.55
OUTPUT HIGH VOLTAGE (V)
2.5V/
div
VY - VZ
IRO = 8mA
OUTPUT LOW VOLTAGE (V)
5V/
div
DI
4.60
MAX3291/2-10
MAX3291/2-09
0.30
RECEIVER OUTPUT HIGH VOLTAGE
vs. TEMPERATURE
MAX3291/2-11
DRIVER PROPAGATION DELAY
4.50
4.45
4.40
4.35
4.30
4.25
4.20
0.05
4.15
0
4.10
-40
20ns/div
-20
0
20
40
60
80
100
-40
TEMPERATURE (°C)
1.25
1.00
0.75
0.50
0.25
0
40
30
20
0
20
40
60
TEMPERATURE (°C)
80
100
40
60
80
100
30
25
20
15
10
5
0
0
-20
20
MAX3291/2-14
MAX3291/2-13
50
10
-40
6
60
0
OUTPUT SOURCE CURRENT vs.
RECEIVER OUTPUT HIGH VOLTAGE
OUTPUT SOURCE CURRENT (mA)
1.75
1.50
70
OUTPUT SINK CURRENT (mA)
MAX3291/2-12
3.00
2.75
2.50
2.25
2.00
-20
TEMPERATURE (°C)
OUTPUT SINK CURRENT vs.
RECEIVER OUTPUT LOW VOLTAGE
NO-LOAD SUPPLY CURRENT
vs. TEMPERATURE
NO-LOAD SUPPLY CURRENT (mA)
MAX3291/MAX3292
RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication
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)
_______________________________________________________________________________________
RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication
PIN
NAME
FUNCTION
—
PEE
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.
—
1
PSET
Preemphasis Set Input. Sets the preemphasis interval. Connect a resistor (RPSET) in parallel with a capacitor (CPSET) from PSET to VCC to set the preemphasis interval. See Typical
Operating Circuit.
2
2
RO
Receiver Output. When RE is low and if A - B ≥ 200mV, RO is high; if A - B ≤ -200mV, RO
is low.
3
3
RE
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.
4
4
DE
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.
5
5
DI
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.
6, 8, 13
8
N.C.
No Connection. Not internally connected.
7
6, 7
GND
Ground
9
9
Y
Noninverting Driver Output
10
10
Z
Inverting Driver Output
11
11
B
Inverting Receiver Input
12
12
A
Noninverting Receiver Input
—
13
VCCD
14
14
VCC
MAX3291
MAX3292
1
Connect to VCC
Positive Supply: +4.75V ≤ VCC ≤ +5.25V
Y
R
VOD
VODP
RL
OUTPUT
UNDER TEST
R
VOC
S1
VCC
CL
S2
Z
Figure 1. Driver DC Test Load
Figure 2. Driver or Receiver Enable/Disable Timing Test Load
_______________________________________________________________________________________
7
MAX3291/MAX3292
Pin Description
MAX3291/MAX3292
RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication
5V
DE
DI
t < tSHDN
CL1
RE
Y
D
VODP
t > tSHDN
RDIFF
Z
tRZH(SHDN)
CL2
RO
DE = LOW
Figure 3. Driver Timing Test Circuit
Figure 4. Shutdown Timing Diagram
5V
DI
1.5V
0
5V
1.5V
tDPHL
tDPLH
YN
YN
tDZL(SHDN), tDZL
VPH
tDLZ
OUTPUT NORMALLY HIGH
Y, Z
80%
tLH
VOL* + 0.5V
YP
VDIFF = VY - VZ
80%
OUTPUT NORMALLY LOW
2.3V
ZN
ZP
0 DIFFERENTIAL
20%
1.5V
Y, Z
0 DIFFERENTIAL
VDIFF
1.5V
0
ZP
YP
ZN
DE
VOH* - 0.5V
2.3V
0
20%
tDZH(SHDN), tDZH
tHL
tSKEW = | tPLH - tPHL |
| YN - ZN | = VOD
| YP - ZP | = VODP
VPL
tDHZ
*NOTE: VOH AND VOL ARE THE OUTPUT LEVELS IN FIGURE 2 WITH
S2 AND S1 CLOSED, RESPECTIVELY.
Figure 5. Driver Propagation Delays
Figure 6. Driver Enable and Disable Times
3V
VOH
RO
VOL
1.5V
RE
1.5V
0
1.5V
tRPHL
1.5V
tRPLH
tRZL(SHDN), tRZL
VCC
RO
tRLZ
1.5V OUTPUT NORMALLY LOW
VOL + 0.5V
A +1V
OUTPUT NORMALLY HIGH
0 DIFFERENTIAL
B -1V
RO
tRSKEW = | tRPLH - tRPHL |
Figure 7. Receiver Propagation Delays
8
VOH - 0.5V
1.5V
0
tRZH(SHDN), tRZH
tRHZ
Figure 8. Receiver Enable and Disable Times
_______________________________________________________________________________________
RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication
MAX3291/MAX3292
tPTND
YP
ZP
80%
ZN
B
VID
ATE
RR
RO
A
YN
20%
0 DIFFERENTIAL
CL
YN
50%
ZN
YP
ZP
tPRE
Figure 9. Receiver Propagation Delay Test Circuit
Figure 10. Preemphasis Timing
VCC
RE
DI
1k
MAX3291
MAX3292
RO
DE
Figure 11. Time-to-Shutdown Test Circuit
Function Tables
RECEIVING
TRANSMITTING
INPUTS
RE
DE
INPUTSINPUTS
OUTPUTS
DI
Z
Y
X
1
1
0
1
X
1
0
1
0
0
1
0
0
X
X
High-Z
High-Z
High-Z and
SHUTDOWN
OUTPUT
RE
DE
A-B
RO
0
X
≥ 0.2V
1
0
X
≤ -0.2V
0
0
X
Open
1
1
1
X
High-Z
X
High-Z and
SHUTDOWN
1
0
X = Don’t care
Z = High impedance
SHUTDOWN = Low-power shutdown; driver and receiver outputs are high impedance.
_______________________________________________________________________________________
9
MAX3291/MAX3292
RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication
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.
10
SIGNAL 1
BAUD PERIOD
VA - VB
tISI
SIGNAL 2
Figure 12. Inter-Symbol Interference among Two Data
Patterns: Signal 1 = 11111110, Signal 2 = 00000010
VY - VZ
2.5V/div
5V/div
DI
0
1
1
1
0
0
100ns
0
1
0
1
Figure 13. Typical Preemphasis Waveform with a 100ns
Preemphasis Interval
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
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, tBAUD. 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.
RPSET = 580 (tBAUD - 100)
RPSET = 580 (2000 - 100) = 1.1MΩ
For data rates of 1Mbps to 10Mbps, use the following
equation to calculate RPSET:
RPSET = 580 (tBAUD - 100)(tBAUD / 1000)
where tBAUD = one baud period in ns.
For example, a baud rate of 1Mbps produces a baud
period of 1µs (1µs = 1000ns).
RPSET = 580 (1000 - 100)(1000 / 1000) = 522kΩ
(closest standard value is 523kΩ)
Set the preemphasis interval by connecting the RPSET
resistor from PSET to VCC. Use a 0.1µF bypass capacitor (CPSET) from PSET to VCC. If PSET is connected
directly to VCC (RPSET = 0), the preemphasis interval
reverts to the nominal 100ns value.
10,000
CABLE LENGTH (FEET)
24-GAUGE
TWISTED PAIR
Figure 15. Eye Diagram of a Typical RS-485 Transceiver
Without Preemphasis, while Driving 1000 feet of Cable at
5Mbps
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
MAX3291/MAX3292
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 RPSET (the preemphasis setting resistor):
RPSET = 580 (tBAUD - 100)
where tBAUD = one baud period in ns.
For example, a baud rate of 500kbps produces a baud
period of 2µs (2µs = 2000ns).
MAX3291/MAX3292
RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication
Line Repeater
% Jitter = (total jitter / tBAUD) · 100
When the total amount of time skew becomes 10% or
more of the baud period, the data error rate can
increase sharply.
For line lengths greater than what one MAX3291/
MAX3292 can drive, use the repeater application
shown in Figure 17.
128 Transceivers on the Bus
Figure 18 shows the system differential voltage for the
MAX3292 driving 4000 feet of 26AWG twisted-pair wire
into two 120Ω termination loads.
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 tZH and tZL in the Switching Characteristics tables correspond to when the part is not in the lowpower shutdown state. Enable times t ZH(SHDN) and
tZL(SHDN) assume the parts are shut down. It takes drivers and receivers longer to activate from the lowpower shutdown mode (tZH(SHDN), tZL(SHDN)) than from
the driver/receiver disable mode (tZH, tZL).
MAX3291
MAX3292
R
120Ω
B
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).
5V/div
DI
RECEIVER
INPUT
A
RO
RE
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:
DATA IN
VA - VB
1V/div
RO
5V/div
DE
Z
DI
D
120Ω
Y
DATA OUT
2µs/div
TYPICAL OPERATING CIRCUIT, RPSET = 1MΩ
Figure 17. Line-Repeater Application
12
Figure 18. MAX3292 System Differential Voltage Driving 4000
Feet, Using Two 120Ω Termination Resistors
______________________________________________________________________________________
RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication
significantly less preemphasis. Determine the preemphasis ratio versus load by referring to the Driver Differential
Output Voltage vs. RDIFF 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 = VSTRONG / VNORMAL = VODP / VOD).
Figures 19 and 20 show typical network application circuits with proper termination.
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
Preemphasis at Low Data Rates
(MAX3292)
At low data rates (
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