LTC491

8

491fa

S

APPLICATI

I FOR ATIO

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 cables

such as the Belden 9841, the conductor losses and

dielectric losses are of the same order of magnitude,

leading to relatively low over all loss (Figure 11).

When using low loss cables, Figure 12 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

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 it’s 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 13).

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.

0.1

0.1

1.0

10

1.0

10

100

LTC491 • F11

DATA RATE (bps)

10k

10

100

1k

10k

100k

1M

10M

LTC491 • F12

2.5M

Figure 12. Cable Length vs Data Rate

Figure 11. Attenuation vs Frequency for Belden 9481

Figure 13. Termination Effects

Rt

DRIVER

DX

RECEIVER

RX

Rt = 120

Ω

Rt = 47

Ω

Rt = 470

Ω

LTC491 • F13

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