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MAX3291CSD

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
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 inter-
ference (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-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.
Inter-symbol interference (ISI) causes significant prob-
lems 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 (V
A
- V
B
= 0) and it
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).
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 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.
Applications Information
Data Rate vs. Cable Length
In general, preemphasis allows either double the dis-
tance 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 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.
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 V
CC
. Use a 0.1µF bypass capaci-
tor (C
PSET
) from PSET to V
CC
. If PSET is connected
directly to V
CC
(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 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
one baud period, t
BAUD
. Figures 15 and 16 show typi-
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.
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), allow-
ing 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 typi-
cally 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 t
ZH
and t
ZL
in 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 dri-
vers and receivers longer to activate from the low-
power shutdown mode (t
ZH(SHDN)
, t
ZL(SHDN)
) than from
the driver/receiver disable mode (t
ZH
, t
ZL
).
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 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 simple resistor termination whose value is the
cable’s characteristic impedance (Z
O
). Avoid termina-
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
the same impedance, equal to Z
O
/ 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 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
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
120Ω
120Ω
DATA IN
DATA OUT
R
D
RO
RE
DE
DI
A
B
Z
Y
MAX3291
MAX3292
Figure 18. MAX3292 System Differential Voltage Driving 4000
Feet, Using Two 120
Ω
Termination Resistors
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

MAX3291CSD

Mfr. #:
Buy MAX3291CSD
Manufacturer:
Maxim Integrated
Description:
RS-422/RS-485 Interface IC RS-485/RS-422 Transceivers with Preemphasis for High-Speed, Long-Distance Communication
Lifecycle:
New from this manufacturer.
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