6

LTC488/LTC489

TI E WAVEFOR

S

G

WITCHI

S

Figure 3. Receiver Propagation Delays

0V

0V

INPUT

A, B

1.5V

1.5V

RO

INPUT

488/9 F03

1.5V

RO

488/9 F04

0V

3V

1.5V

1.5V

0.5V

0.5V

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

0V

5V

1.5V

RO

EN OR

EN12

Figure 4. Receiver Enable and Disable Times

S

APPLICATI

I FOR ATIO

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 120

Ω. 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

120

Ω 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 dielec-

tric losses are of the same order of magnitude, leading to

relatively low overall loss (Figure 6).