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19-1405; Rev 1; 4/99
RS-485/RS-422 Transceivers with Preemphasis for High-Speed, Long-Distance Communication
________________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.
____________________________Features
o Preemphasis Increases the Distance and Data Rate of Reliable RS-485/RS-422 Communication o Data Rate Optimized for 5Mbps to 10Mbps (MAX3291) Programmable up to 10Mbps (MAX3292) o 100nA Low-Current Shutdown Mode o Allow Up to 128 Transceivers on the Bus o -7V to +12V Common-Mode Input Voltage Range o Pin-Compatible with '75180, MAX489, MAX491 MAX3080, MAX3083, MAX3086, MAX1482
MAX3291/MAX3292
_______________Ordering Information
PART MAX3291CSD TEMP. RANGE 0C to +70C 0C to +70C -40C to +85C -40C to +85C 0C to +70C 0C to +70C -40C to +85C -40C to +85C PIN-PACKAGE 14 SO 14 Plastic DIP 14 SO 14 Plastic DIP 14 SO 14 Plastic DIP 14 SO 14 Plastic DIP
________________________Applications
Long-Distance, High-Speed RS-485/RS-422 Communications Telecommunications Industrial-Control Local Area Networks
MAX3291CPD MAX3291ESD MAX3291EPD MAX3292CSD MAX3292CPD MAX3292ESD MAX3292EPD
Typical Operating Circuit and Functional Diagram
RPSET* RPSET*
1F PEE (PSET) CPSET* VCC 14 1 (VCCD) RO RE DE DI 2 3 4 5 Z 10 Y9 R = ZO R = ZO 11 12 B A 13 R = ZO R = ZO 9 10 Y Z A 12 B 11 PEE (PSET) CPSET* VCC 14 1 (VCCD)
1F
13 5
DI
4 DE 3 RE 2 RO ( ) ARE FOR MAX3292 * MAX3292 ONLY ZO = THE CHARACTERISTIC IMPEDANCE OF THE CABLE
MAX3291 MAX3292
7(6) GND
MAX3291 MAX3292
7(6) GND
Pin Configuration appears at end of data sheet.
________________________________________________________________ Maxim Integrated Products 1
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RS-485/RS-422 Transceivers with Preemphasis for High-Speed, Long-Distance Communication MAX3291/MAX3292
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 = +70C) 14-Pin SO (derate 8.7mW/C above +70C).................695mW 14-Pin Plastic DIP (derate 10.0mW/C above +70C) ..800mW Operating Temperature Ranges MAX329_C_ D......................................................0C to +70C MAX329_E_ D ...................................................-40C to +85C Storage Temperature Range .............................-65C to +160C Lead Temperature (soldering, 10sec) .............................+300C
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 = +25C.) (Note 1) PARAMETER DRIVER Differential Driver Output Differential Driver Output with Preemphasis Differential Driver Preemphasis Ratio Change in Magnitude of Differential Output Voltage (Normal and Preemphasis) Driver Common-Mode Output Voltage (Normal and Preemphasis) Change in Magnitude of Common-Mode Voltage (Normal and Preemphasis) Change in Magnitude of Common-Mode Output Voltage (Normal to Preemphasis) Input High Voltage Input Low Voltage Input Current PEE Input Current (MAX3291) PSET Input Current (MAX3292) Output Leakage (Y and Z) Driver Short-Circuit Output Current 2 VOD VODP DPER VOD, VODP Figure 1 R = 27 Figure 1, R = 27 (Note 3) R = 27 No load (Note 2) 2.4 1.65 2.0 2.35 1.5 5.0 5.25 V V V SYMBOL CONDITIONS MIN TYP MAX UNITS
Figure 1, R = 27 (Note 4)
0.2
V
VOC
Figure 1, R = 27
VCC / 2
3
V
VOC
Figure 1, R = 27 (Note 5)
0.3
V
VNP
Figure 1, R = 27 DE, DI, RE PEE DE, DI, RE, PEE DE, DI, RE -15 VPSET = VCC DE = GND, VCC = GND or 5.25V VY = VZ = +12V VY = VZ = -7V 30
50
mV
VIH VIL IIN IPEE IPSET IO IOSD
2.4 3.75 0.8 2 -30 70 -45 110 25 -25 250
V V A A A A mA
-7V VOUT +12V (Note 6)
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RS-485/RS-422 Transceivers with Preemphasis for High-Speed, Long-Distance Communication MAX3291/MAX3292
DC ELECTRICAL CHARACTERISTICS (continued)
(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 = +25C.) (Note 1) PARAMETER RECEIVER Input Current (A and B) Receiver Differential Threshold Voltage Receiver Input Hysteresis Receiver Output High Voltage Receiver Output Low Voltage Three-State Output Current at Receiver Receiver Input Resistance Receiver Output Short-Circuit Current 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 2.0 0.1 3.0 1 mA A IA, B VTH VTH VOH VOL IOZR RIN IOSR DE = GND, VCC = GND or 5.25V -7V VCM +12V VA = VB = 0 IO = -4mA, VA - VB = VTH IO = 4mA, VA - VB = -VTH 0 VO VCC -7V VCM +12V 0 VRO VCC 48 15 95 0.1 3.5 0.4 1 VIN = +12V VIN = -7V -200 35 250 -150 200 A mV mV V V A k mA SYMBOL CONDITIONS MIN TYP MAX UNITS
SWITCHING CHARACTERISTICS
(Typical Operating Circuit, VCC = +5V 5%, RPSET = 0 (MAX3292), VCC = VCCD (MAX3292), TA = +25C, unless otherwise noted. Typical values are at VCC = +5V and TA = +25C.) PARAMETER Driver Propagation Delay Driver Differential Output Rise or Fall Time SYMBOL tDPLH tDPHL tHL tLH CONDITIONS Figures 3 and 5, RDIFF = 54, CL1 = CL2 = 50pF Figures 3 and 5, RDIFF = 54, CL1 = CL2 = 50pF Figures 3 and 10, RDIFF = 54, CL1 = CL2 = 50pF MAX3291/MAX3292, RPSET = 0 MAX3292, RPSET = 523k 80 0.75 MIN TYP 41 44 12 100 1 30 3 10 8 120 1.25 MAX 65 65 UNITS ns ns ns s ns ns Mbps
Driver Preemphasis Interval
tPRE
Preemphasis Voltage Level to Normal Voltage Level Delay Differential Driver Output Skew tDPLH - tDPHL Maximum Data Rate
tPTND tDSKEW fMAX
Figures 3 and 10, RDIFF = 54, CL1 = CL2 = 50pF Figures 3 and 5, RDIFF = 54, CL1 = CL2 = 50pF
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3
RS-485/RS-422 Transceivers with Preemphasis for High-Speed, Long-Distance Communication MAX3291/MAX3292
SWITCHING CHARACTERISTICS (continued)
(Typical Operating Circuit, VCC = +5V 5%, RPSET = 0 (MAX3292), VCC = VCCD (MAX3292), TA = +25C, unless otherwise noted. Typical values are at VCC = +5V and TA = +25C.) PARAMETER Driver Enable to Output High Driver Enable to Output Low Driver Disable Time from Low Driver Disable Time from High Receiver Propagation Delay Receiver Output Skew tRPLH - tRPHL Receiver Enable to Output Low Receiver Enable to Output High Receiver Disable Time from Low Receiver Disable Time from High Time to Shutdown Driver Enable from Shutdown to Output High Driver Enable from Shutdown to Output Low Receiver Enable from Shutdown to Output High Receiver Enable from Shutdown to Output Low SYMBOL tDZH tDZL tDLZ tDHZ tRPLH tRPHL tRSKEW tRZL tRZH tRLZ tRHZ tSHDN tDZH(SHDN) tDZL(SHDN) tRZH(SHDN) tRZL(SHDN) CONDITIONS Figures 2 and 6, S2 closed, RL = 500, CL = 100pF Figures 2 and 6, S1 closed, RL = 500, CL = 100pF Figures 2 and 6, S1 closed, RL = 500, CL = 15pF Figures 2 and 6, S2 closed, RL = 500, CL = 15pF Figures 7 and 9, CL = 50pF, VID = 2V, VCM = 0 Figures 7 and 9, CL = 100pF Figures 2 and 8, RL = 1k, CL = 100pF, S1 closed Figures 2 and 8, RL = 1k, CL = 100pF, S2 closed Figures 2 and 8, RL = 1k, CL = 15pF, S1 closed Figures 2 and 8, RL = 1k, CL = 15pF, S2 closed Figures 4 and 11 (Note 7) Figures 2 and 6, RL = 500, CL = 100pF, S2 closed Figures 2 and 6, RL = 500, CL = 100pF, S1 closed Figures 2 and 8, RL = 1k, CL = 100pF, S2 closed Figures 2 and 8, RL = 1k, CL = 100pF, S1 closed 50 3 3 MIN TYP 72 55 53 71 49 52 3 43 43 25 25 160 6000 6000 850 30 55 55 45 45 500 8750 8750 1500 1500 MAX 105 105 100 100 85 85 UNITS ns ns ns ns ns ns ns ns ns ns ns ns ns ns 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
__________________________________________Typical Operating Characteristics
(VCC = +5V, TA = +25C, unless otherwise noted.)
TYPICAL PREEMPHASIS WAVEFORM
MAX3291/2-01
MAX3291/MAX3292
PREEMPHASIS INTERVAL vs. RPSET (CPSET = 0.1F)
MAX3291/2-02
RPRE AND tPRE vs. tBAUD
MAX3291/92 toc 03
2500 2250 PREEMPHASIS INTERVAL (ns) 2000 1750
1200 1000 800 RPRE (k) 600 NOTE A 400 200 0
1500 1250 1000 750 500 250 0 NOTE A
VY - VZ
2V/ div
100ns/div
0 100 200 300 400 500 600 700 800 900 1000 RPSET (k)
0
500
1000 tBAUD (ns)
1500
2000
DRIVER DIFFERENTIAL OUTPUT VOLTAGE vs. RDIFF
MAX3291/2-04
DRIVER DIFFERENTIAL OUTPUT VOLTAGE vs. TEMPERATURE
MAX3291/2-05
RECEIVER PROPAGATION DELAY vs. TEMPERATURE
CL = 50pF 57.5 PROPAGATION DELAY (ns) 55.0 52.5 50.0 47.5 45.0 42.5
MAX3291/2-06
DRIVER DIFFERENTIAL OUTPUT VOLTAGE (V)
4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 10 20 30 40 50 60 70 80 NORMAL STRONG
4.25 DIFFERENTIAL OUTPUT VOLTAGE (V) 4.00 3.75 3.50 3.25 3.00 2.75 2.50 2.25 2.00 RDIFF = 54 -40 -20 0 20 40 60 80 NORMAL (VOD) STRONG (VODP)
60.0
40.0 100 -40 -20 0 20 40 60 80 100 TEMPERATURE (C) TEMPERATURE (C)
90 100
RDIFF ()
DRIVER PROPAGATION DELAY vs. TEMPERATURE
MAX3291/2-07
RECEIVER PROPAGATION DELAY
MAX3291/2-08
50.0 47.5 PROPAGATION DELAY (ns) 45.0 42.5 40.0 37.5 35.0 32.5 30.0 -40 -20 0 20 40 60 80 CL1 = CL2 = 50pF
VA - VB
2.5V/ div 5V/ div
RO
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.
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5
RS-485/RS-422 Transceivers with Preemphasis for High-Speed, Long-Distance Communication MAX3291/MAX3292
____________________________________Typical Operating Characteristics (continued)
(VCC = +5V, TA = +25C, unless otherwise noted.)
DRIVER PROPAGATION DELAY
MAX3291/2-09
RECEIVER OUTPUT LOW VOLTAGE vs. TEMPERATURE
MAX3291/2-10
RECEIVER OUTPUT HIGH VOLTAGE vs. TEMPERATURE
4.55 OUTPUT HIGH VOLTAGE (V) 4.50 4.45 4.40 4.35 4.30 4.25 4.20 4.15 IRO = 8mA
MAX3291/2-11
0.30 IRO = 8mA 0.25 OUTPUT LOW VOLTAGE (V) 0.20 0.15 0.10 0.05 0
4.60
DI VY - VZ
5V/ div 2.5V/ div
4.10 -40 -20 0 20 40 60 80 100 -40 -20 0 20 40 60 80 100 TEMPERATURE (C) TEMPERATURE (C)
20ns/div
NO-LOAD SUPPLY CURRENT vs. TEMPERATURE
2.75 2.50 2.25 2.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25 0 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
MAX3291/2-12
OUTPUT SINK CURRENT vs. RECEIVER OUTPUT LOW VOLTAGE
MAX3291/2-13
OUTPUT SOURCE CURRENT vs. RECEIVER OUTPUT HIGH VOLTAGE
MAX3291/2-14
3.00 NO-LOAD SUPPLY CURRENT (mA)
70 60 OUTPUT SINK CURRENT (mA) 50 40 30 20 10 0 0
30 OUTPUT SOURCE CURRENT (mA) 25 20 15 10 5 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)
6
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RS-485/RS-422 Transceivers with Preemphasis for High-Speed, Long-Distance Communication
Pin Description
PIN NAME MAX3291 1 MAX3292 -- 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. 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. Receiver Output. When RE is low and if A - B 200mV, RO is high; if A - B -200mV, RO is low. 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. No Connection. Not internally connected. Ground Noninverting Driver Output Inverting Driver Output Inverting Receiver Input Noninverting Receiver Input Connect to VCC Positive Supply: +4.75V VCC +5.25V FUNCTION
MAX3291/MAX3292
--
1
PSET
2 3 4
2 3 4
RO RE DE
5 6, 8, 13 7 9 10 11 12 -- 14
5 8 6, 7 9 10 11 12 13 14
DI N.C. GND Y Z B A VCCD VCC
Y R VOD VODP R Z VOC OUTPUT UNDER TEST CL S2 RL S1 VCC
Figure 1. Driver DC Test Load
Figure 2. Driver or Receiver Enable/Disable Timing Test Load
7
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RS-485/RS-422 Transceivers with Preemphasis for High-Speed, Long-Distance Communication MAX3291/MAX3292
5V DE DI Y D Z VODP RDIFF CL2 RO t < tSHDN CL1 RE tRZH(SHDN) t > tSHDN
DE = LOW
Figure 3. Driver Timing Test Circuit
Figure 4. Shutdown Timing Diagram
5V DI 0 ZN 0 DIFFERENTIAL YN ZP VDIFF = VY - VZ VDIFF 0 DIFFERENTIAL 20% 80% 80% 20% ZN YP 1.5V tDPLH YP tDPHL ZP YN 1.5V DE
5V 1.5V 0 tDZL(SHDN), tDZL Y, Z 2.3V OUTPUT NORMALLY LOW VOL* + 0.5V tDLZ VPH 1.5V
Y, Z 2.3V 0
OUTPUT NORMALLY HIGH VOH* - 0.5V tDHZ VPL
tLH tSKEW = | tPLH - tPHL | | YN - ZN | = VOD | YP - ZP | = VODP
tHL
tDZH(SHDN), tDZH
*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 tRPHL tRPLH 1.5V tRZL(SHDN), tRZL VCC RO tRLZ VOL + 0.5V RE 0 1.5V 1.5V
1.5V OUTPUT NORMALLY LOW OUTPUT NORMALLY HIGH
A +1V 0 DIFFERENTIAL B -1V tRSKEW = | tRPLH - tRPHL | RO 0 tRZH(SHDN), tRZH tRHZ 1.5V VOH - 0.5V
Figure 7. Receiver Propagation Delays
8
Figure 8. Receiver Enable and Disable Times
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RS-485/RS-422 Transceivers with Preemphasis for High-Speed, Long-Distance Communication MAX3291/MAX3292
tPTND YP 80% B ATE VID A RR RO CL ZN 0 DIFFERENTIAL YN ZP tPRE 50% ZN YP 20% YN ZP
Figure 9. Receiver Propagation Delay Test Circuit
Figure 10. Preemphasis Timing
VCC RE
1k
DI DE
MAX3291 MAX3292
RO
Figure 11. Time-to-Shutdown Test Circuit
Function Tables
TRANSMITTING INPUTS RE X X 0 1 DE 1 1 0 0 DI 1 0 X X Z 0 1 High-Z OUTPUTS Y 1 0 High-Z RE 0 0 0 1 1 High-Z and SHUTDOWN DE X X X 1 0 RECEIVING INPUTS INPUTS A-B 0.2V -0.2V Open X X OUTPUT RO 1 0 1 High-Z High-Z and SHUTDOWN
X = Don't care Z = High impedance SHUTDOWN = Low-power shutdown; driver and receiver outputs are high impedance.
_______________________________________________________________________________________ 9
RS-485/RS-422 Transceivers with Preemphasis for High-Speed, Long-Distance Communication 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.
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
DI
5V/div
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.
10
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RS-485/RS-422 Transceivers with Preemphasis for High-Speed, Long-Distance Communication
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 2s (2s = 2000ns). RPSET = 580 (tBAUD - 100) RPSET = 580 (2000 - 100) = 1.1M 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 1s (1s = 1000ns). RPSET = 580 (1000 - 100)(1000 / 1000) = 522k (closest standard value is 523k) Set the preemphasis interval by connecting the RPSET resistor from PSET to VCC. Use a 0.1F 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 24-GAUGE TWISTED PAIR CABLE LENGTH (FEET) 10% JITTER PREEMPHASIS 485 DRIVER LIMIT 1000 CONVENTIONAL 485 DRIVER LIMIT PREEMPHASIS REQUIRED FOR ERROR-FREE TRANSMISSION 100 0.1 1 DATA RATE (Mbps) 10
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.
MAX3291/MAX3292
Figure 15. Eye Diagram of a Typical RS-485 Transceiver Without Preemphasis, while Driving 1000 feet of Cable at 5Mbps
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
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RS-485/RS-422 Transceivers with Preemphasis for High-Speed, Long-Distance Communication MAX3291/MAX3292
% 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.
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 120 termination loads.
128 Transceivers on the Bus
The standard RS-485 receiver input impedance is 12k (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 (48k), 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.
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).
Low-Power Shutdown Mode
Initiate low-power shutdown mode by bringing RE high and DE low. In shutdown the MAX3291/MAX3292 typically draw only 1A 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
A RO RE
DI RECEIVER INPUT
5V/div
R
120 B
DATA IN
VA - VB
1V/div
DE Z DI
RO 5V/div
D
120 Y
DATA OUT
2s/div TYPICAL OPERATING CIRCUIT, RPSET = 1M
Figure 17. Line-Repeater Application
Figure 18. MAX3292 System Differential Voltage Driving 4000 Feet, Using Two 120 Termination Resistors
12
______________________________________________________________________________________
RS-485/RS-422 Transceivers with Preemphasis for High-Speed, Long-Distance Communication MAX3291/MAX3292
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 54, 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. 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.
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.
DE DI Z Z R = ZO Y B RO RE Z Y B A B A A R = ZO Y
D
DE
D
DI
R R D
R
RO RE
MAX3291 MAX3292 Figure 19. Typical Half-Duplex RS-485 Network
DI
DE
RO
RE
A RO RE DE Z DI
Y R = ZO R = ZO Z DE B R = ZO R = ZO Y Z B A Y Z B A A RE RO
R
B
D
DI
D
Y
R
R D
DI NOTE: RE AND DE ON. DE RE RO
R D
DI DE RE RO
MAX3291 MAX3292
Figure 20. Typical Full-Duplex RS-485 Network
______________________________________________________________________________________ 13
RS-485/RS-422 Transceivers with Preemphasis for High-Speed, Long-Distance Communication MAX3291/MAX3292
Pin Configuration
TOP VIEW
Chip Information
TRANSISTOR COUNT: 2280 SUBSTRATE CONNECTED TO GND
MAX3291 MAX3292
PEE (PSET) 1 RO RE 2 3 14 VCC 13 N.C. (PVCC) 12 A 11 B 10 Z 9 8 Y N.C.
DE 4 DI 5 N.C. (GND) 6 GND 7
SO/DIP
( ) ARE FOR THE MAX3292 ONLY.
14
______________________________________________________________________________________
RS-485/RS-422 Transceivers with Preemphasis for High-Speed, Long-Distance Communication
________________________________________________________Package Information
SOICN.EPS
MAX3291/MAX3292
______________________________________________________________________________________
15
RS-485/RS-422 Transceivers with Preemphasis for High-Speed, Long-Distance Communication MAX3291/MAX3292
___________________________________________Package Information (continued)
PDIPN.EPS
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 1999 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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