7

LTC490

490fb

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 cables

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 7).

When using low loss cables, Figure 8 can be used as a

guideline for choosing the maximum line length for a given

data rate. With lower quality PVC cables, the dielectric loss

factor can be 1000 times worse. PVC twisted pairs have

APPLICATIO S I FOR ATIO

terrible losses at high data rates (>100kbs), and greatly

reduce the maximum cable length. At low data rates

however, they are acceptable and much more economical.

Cable Termination

The proper termination of the cable is very important.

If the cable is not terminated with its characteristic

impedance, distorted waveforms will result. In severe

cases, distorted (false) data and nulls will occur.

A quick look at the output of the driver will tell how well the

cable is terminated. It is best to look at a driver connected

to the end of the cable, since this eliminates the possibility

of getting reflections from two directions. Simply look at

the driver output while transmitting square wave data. If

the cable is terminated properly, the waveform will look

like a square wave (Figure 9). If the cable is loaded

excessively (47

Ω), the signal initially sees the surge

impedance of the cable and jumps to an initial amplitude.

The signal travels down the cable and is reflected back out

of phase because of the mistermination. When the re-

flected signal returns to the driver, the amplitude will be

lowered. The width of the pedestal is equal to twice the

electrical length of the cable (about 1.5ns/foot). If the

Figure 7. Attenuation vs Frequency for Belden 9841

Figure 8. RS485 Cable Length Specification. Applies for 24

Gauge, Polyethylene Dielectric Twisted Pair

Figure 9. Termination Effects

FREQUENCY (MHz)

0.1

0.1

1.0

10

1.0

10

100

LTC490 • F07

DATA RATE (bps)

10k

10

100

1k

10k

100k

1M

10M

LTC490 • F08

2.5M

Rt

DRIVER

DX

RECEIVER

RX

Rt = 120

Ω

Rt = 47

Ω

Rt = 470

Ω

LTC490 • F09

PROBE HERE