Datasheet

11
LTC1334
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Basic Theory of Operation
The LTC1334 has two interface ports. Each port may be
configured as a pair of single-ended RS232 transceivers
or as a differential RS485 transceiver by forcing the
port’s selection input to a low or high, respectively. The
LTC1334 provides two RS232 drivers and two RS232
receivers or one RS485 driver and one RS485 receiver
per port. All the interface drivers feature three-state
outputs. Interface outputs are forced into high imped-
ance when the driver is disabled, in the shutdown mode
or with the power off.
All the interface driver outputs are fault-protected by a
current limiting and thermal shutdown circuit. The ther-
mal 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 130°C.
In RS485 mode, shutdown mode or with the power off, the
input resistance of the receiver is 24k. The input resistance
drops to 5k in RS232 mode.
A logic low at the ON/OFF pin shuts down the device and
forces all the outputs into a high impedance state. A logic
high enables the device. An internal 4µA current source to
V
CC
pulls the ON/OFF pin high if it is left open.
Figure 11. RS232/RS485 Interfaces
1
28
V
CC
5V
LTC1334 • F11
120Ω
12
21
LTC1334
RX OUT
18
5V
13
10
11
9
8
14
17
16
RS485 I/O
RS232 DR OUT
19
5V
20
15
V
EE
5V
≥ ±5V INTO
3kΩ LOAD
5
4
27
26
24
RX OUT
6
RS232 RX IN
23
2
3
RX OUT
7
DR IN
DR ENABLE
DR IN
DR IN
22
RS232 DR OUT
RS232 RX IN
V
DD
C1
C2
0.1µF
0.1µF
0.1µF0.1µF
0.1µF
In RS485 mode, an internal 4µA current source pulls the
driver enable pin high if left open. The RS485 receiver has
a 4µA current source at the noninverting input. If both the
RS485 receiver inputs are open, the output goes to a high
state. Both the current sources are disabled in the RS232
mode. The receiver output B is inactive in RS485 mode
and has a 50k pull-up resistor to provide a known output
state in this mode.
A loopback mode enables internal connections from driver
outputs to receiver inputs for self-test when the LB pin has
a low logic state. The driver outputs are not isolated from
the external loads. This allows transmitter verification
under the loaded condition. An internal 4µA current source
pulls the LB pin high if left open and disables the loopback
configuration.
RS232/RS485 Applications
The LTC1334 can support both RS232 and RS485 levels
with a single 5V supply as shown in Figure 11.
Multiprotocol Applications
The LTC1334 is well-suited for software controlled inter-
face mode selection. Each port has a selection pin as
shown in Figure 12. The single-ended transceivers sup-
port both RS232 and EIA562 levels. The differential trans-
ceivers support both RS485 and RS422.