7

LTC491

S

APPLICATI

I FOR ATIO

Typical Application

A typical connection of the LTC491 is shown in Figure 9.

Two twisted pair wires connect up to 32 driver/receiver

pairs for full 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 imped-

ance, typically 120

Ω. The input impedance of a receiver is

LTC491 • TA10

120

Ω

120

Ω

LTC491

DRIVER

RECEIVER

LTC491

DRIVER

DX

RX

DX

RX

2

5

9

10

11

12

RECEIVER

9

10

11

12

5

2

120

Ω

3

4

3

4

120

Ω

RECEIVER

LTC491

DRIVER

9

10

11

12

5

4

3

2

DX

RX

Figure 9. Typical Connection

LTC491 • TA11

120

Ω

LTC491

DRIVER

DX

RX

2

5

9

10

11

12

RECEIVER

DATA IN

DATA OUT

3

4

120

Ω

Figure 10. Line Repeater

typically 20k

Ω to GND, or 0.6 unit RS-485 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.

The LTC491 can also be used as a line repeater as shown

in Figure 10. If the cable length is longer than 4000 feet, the

LTC491 is inserted in the middle of the cable with the

receiver output connected back to the driver input.