5
LTC487
TEST CIRCUITS
LTC487 • TA04
OUTPUT
UNDER TEST
C
L
S1
500
CC
V
Ω
S2
Figure 5. Driver Timing Test Load #2
DRIVER 1
LTC487 • TA03
DI
A
B
EN12
R
DIFF
C
L1
C
L2
Figure 4. Driver Timing Test Circuit
LTC487 • TA02
A
B
R
R
OD
V
OC
V
Figure 3. Driver DC Test Load
U
S
A
O
PPLICATI
W
U
U
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