10
LTC1387
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
EMI and signal reflection problems. Slow slew rate sys-
tems can often use improperly terminated or even
unterminated cables with acceptable results. The RS485
differential input receiver features an offset input thresh-
old of –80mV at 0V common mode voltage. This allows
the receiver output to have a known High output state
when the inputs are open or shorted. The receiver also
features an input hysteresis of 40mV. The charge pump
can be kept active regardless of the state of DXEN and
RXEN pins by keeping the ON pin High. This improves the
receiver response time by removing the 100µs charge
pump start-up time.
In RS232 mode, the drivers and receivers can be selected
from the Function Tables with control inputs ON, RXEN
and DXEN. The receivers feature a typical 0.6V input
hysteresis.
All the interface driver outputs are fault protected by a
current limiting and thermal shutdown circuit. The thermal
shutdown circuit disables both the RS232 and RS485
driver outputs when the die temperature reaches 150°C.
The thermal shutdown circuit reenables the drivers when
the die temperature cools to 135°C.
A loopback mode enables internal connections from driver
outputs to receiver inputs for self-test. The driver outputs
are not isolated from the external loads during loopback.
The charge pump generates boosted positive voltage V
DD
and negative voltage V
EE
for true RS232 levels from a
single 5V V
CC
supply. The charge pump requires four
0.1µF capacitors.
R
A
R
B
D
Y
D
Z
A
B
Y
Z
R
A
R
B
D
Y
D
Z
/SLEW
ON
V
CC
RXEN
DXEN
485/232
CONTROLLER
DX/RX
RXEN
DXEN
MODE
INTERFACE
A
B
RS232 RS485
RS485
RS232 RS232 RS485 RS485 SHUTDOWN
TRANSMIT MODE RECEIVE MODE TRANSMIT MODE RECEIVE MODE MODE
RXEN = 0 RXEN = 1 RXEN = 0 RXEN = 1 RXEN = 0
DXEN = 1 DXEN = 0 DXEN = 1 DXEN = 0 DXEN = 0
MODE = 0 MODE = 0 MODE = 1 MODE = 1 MODE = X
1387 • F13
LTC1387
Figure 13. Half-Duplex RS232 (1-Channel), Half-Duplex RS485
