LTC488 데이터 시트보기 (PDF) - Linear Technology
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LTC488 Datasheet PDF : 12 Pages
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LTC488/LTC489
APPLICATIONS INFORMATION
Typical Application
A typical connection of the LTC488/LTC489 is shown in
Figure 5. Two twisted-pair wires connect up to 32 driver/
receiver pairs for half-duplex data transmission. There are
no restrictions on where the chips are connected to the
wires, and it isn’t necessary to have the chips connected
at the ends. However, the wires must be terminated only
at the ends with a resistor equal to their characteristic
impedance, typically 120Ω. The input impedance of a
receiver is typically 20k to GND, or 0.5 unit RS485 load,
so in practice 50 to 60 transceivers can be connected to
the same wires. The optional shields around the twisted-
pair help reduce unwanted noise, and are connected to
GND at one end.
Cables and Data Rate
The transmission line of choice for RS485 applications is a
twisted-pair. There are coaxial cables (twinaxial) made for
this purpose that contain straight-pairs, but these are less
flexible, more bulky, and more costly than twisted-pairs.
Many cable manufacturers offer a broad range of 120Ω
cables designed for RS485 applications.
Losses in a transmission line are a complex combination
of DC conductor loss, AC losses (skin effect), leakage, and
AC losses in the dielectric. In good polyethylene cable such
as the Belden 9841, the conductor losses and dielectric
losses are of the same order of magnitude, leading to
relatively low overall loss (Figure 6).
When using low loss cables, Figure 7 can be used as a
guideline for choosing the maximum line length for a given
EN
4
3
1 DX
DX
1/4 LTC486
120Ω
SHIELD
SHIELD
12
EN
1
2
3
1/4 LTC488 OR
1/4 LTC489
2
RX
EN
12
DX
4
1/4 LTC486
EN
1
3
DX
RX
Figure 5. Typical Connection
2
120Ω
RX
1/4 LTC488 OR
3
RX
1/4 LTC489
1
4889 F05
10
10k
1k
1
100
0.1
0.1
1
10
FREQUENCY (MHz)
100
4889 F06
Figure 6. Attenuation vs Frequency for Belden 9841
10
10k
100k
1M
DATA RATE (bps)
2.5M 10M
4889 F07
Figure 7. Cable Length vs Data Rate
4889fb
7
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