LTC490

6

Figure 8. RS485 Cable Length Specification. Applies for 24

Gauge, Polyethylene Dielectric Twisted Pair.

S

APPLICATI

I FOR ATIO

one driver can be connected on the bus, the cable can be

terminated only at the receiving end. The optional shields

around the twisted pair help reduce unwanted noise, and

are connected to GND at one end.

The LTC490 can also be used as a line repeater as shown

in Figure 6. If the cable length is longer than 4000 feet, the

LTC490 is inserted in the middle of the cable with the

receiver output connected back to the driver input.

LTC490 • TA07

120

Ω

LTC490

DRIVER

DX

RX

2

3

5

6

7

8

RECEIVER

DATA IN

DATA OUT

Figure 6. Line Repeater

Thermal Shutdown

The LTC490 has a thermal shutdown feature which pro-

tects the part from excessive power dissipation. If the

outputs of the driver are accidently shorted to a power

supply or low impedance, source, up to 250mA can flow

through the part. The thermal shutdown circuit disables

the driver outputs when the internal temperature reaches

150

°C and turns them back on when the temperature

cools to 130

°C. If the outputs of two or more LTC490

drivers are shorted directly, the driver outputs can not

supply enough current to activate the thermal shutdown.

Thus, the thermal shutdown circuit will not prevent con-

tention faults when two drivers are active on the bus at the

same time.

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 overall loss (Figure 7).

FREQUENCY (MHz)

0.1

0.1

1.0

10

1.0

10

100

LTC490 • TA08

Figure 7. Attenuation vs Frequency for Belden 9841

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

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.

DATA RATE (bps)

10k

10

100

1k

10k

100k

1M

10M

LTC490 • TA09

2.5M