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| 型號(hào)： | MAX3291 |
| 廠商： | Maxim Integrated Products, Inc. |
| 英文描述： | 32-Bit Digital Signal Controller with Flash 128-LQFP -40 to 85 |
| 中文描述： | RS-485/RS-422收發(fā)器，帶有預(yù)加重，用于高速、遠(yuǎn)距離通信 |
| 文件頁(yè)數(shù)： | 13/16頁(yè) |
| 文件大小： | 285K |
| 代理商： | MAX3291 |
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M
RS-485/RS-422 Transceivers with Preemphasis
for High-Speed, Long-Distance Communication
______________________________________________________________________________________
13
DI
RO
RE
Z
Z
Z
Y
B
A
Y
A
RE
RE
RO
RO
DI
DI
DE
DE
DE
D
D
R
R
Y
B
A
R = Z
O
R = Z
O
D
R
MAX3291
MAX3292
Figure 19. Typical Half-Duplex RS-485 Network
3) Don’t overload the cable with too many receivers.
Even though the MAX3291/MAX3292 receives pre-
sent 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 suc-
cessfully 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 imped-
ance of 54
, 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
ideal preemphasis time, t
PRE
, may need adjustment).
However, if your cable impedance is significantly differ-
ent, the preemphasis ratio DPER changes, resulting in
significantly less preemphasis. Determine the preempha-
sis ratio versus load by referring to the Driver Differential
Output Voltage vs. R
DIFF
graph in the Typical Operating
Characteristics. Read the strong and normal levels from
the graph (remember that the horizontal units are half
your cable impedance) and divide the two numbers to
get DPER (DPER = V
STRONG
/ V
NORMAL
= V
ODP
/ V
OD
).
Figures 19 and 20 show typical network application cir-
cuits with proper termination.
Preemphasis at Low Data Rates
(MAX 3292)
At low data rates (<1Msps), preemphasis operation is
not guaranteed because it is highly dependent on the
system power-supply noise. Minimize this noise by
increasing bypass capacitance and using a power
supply with a fast transient response.
R = Z
O
R = Z
O
R
D
RO
RE
RE
DE
DI
A
B
Y
R = Z
O
R = Z
O
DI
DI
DI
RO
RO
RO
DE
DE
DE
RE
RE
Z
Z
Z
Z
Y
Y
Y
A
A
A
B
B
B
D
D
D
R
R
R
MAX3291
MAX3292
NOTE: RE AND DE ON.
Figure 20. Typical Full-Duplex RS-485 Network
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