5

LTC487

TEST CIRCUITS

LTC487 • TA04

OUTPUT

UNDER TEST

S1

500

CC

V

Ω

S2

Figure 5. Driver Timing Test Load #2

DRIVER 1

LTC487 • TA03

DI

A

B

EN12

Figure 4. Driver Timing Test Circuit

LTC487 • TA02

A

B

R

R

OD

V

OC

V

Figure 3. Driver DC Test Load

S

APPLICATI

I FOR ATIO

Typical Application

A typical connection of the LTC487 is shown in Figure 6.

A twisted pair of wires connect up to 32 drivers and

receivers 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 optional shields around

the twisted pair help reduce unwanted noise, and are

connected to GND at one end.

Thermal Shutdown

The LTC487 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 LTC487

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.

Cable 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.

Figure 6. Typical Connection

EN12

4

LTC487 • TA07

120

Ω

DX

1

2

3

SHIELD

120

Ω

RX

RX

SHIELD

3

DX

EN12

4

2

EN12

4

1

2

3

RX

RX

3

DX

EN12

4

1

DX

1/4 LTC489

1/4 LTC487

1/4 LTC489

1/4 LTC487

2

1