LTC488/LTC489

7

4889fb

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

APPLICATIONS INFORMATION

Figure 5. Typical Connection

Figure 7. Cable Length vs Data Rate

Figure 6. Attenuation vs Frequency for Belden 9841

120Ω

120Ω

3

RX

2

1

4889 F05

DX

1

3

SHIELD

RX

EN

DX

1/4 LTC486

12

SHIELD

4

12

3

2

DX

DX

1/4 LTC486

1

EN

1/4 LTC488 OR

1/4 LTC489

RX

3

1

2

4

RX

1/4 LTC488 OR

1/4 LTC489

EN

EN

FREQUENCY (MHz)

0.1

0.1

1

10

1

10

100

4889 F06

DATA RATE (bps)

10k

10

100

1k

10k

100k

1M

10M

4889 F07

2.5M