13
LTC1321/LTC1322/LTC1335
SWITCHI G WAVEFOR S
U
W
Figure 12. Receiver Enable and Disable Times
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
Basic Theory of Operation
The LTC1321/LTC1322/LTC1335 each have two interface
ports. Each port may be configured as single-ended EIA562
transceiver(s) or differential RS485 transceiver by forcing
the port’s selection input to a LOW or HIGH, respectively.
The LTC1321 provides one EIA562 driver and one EIA562
receiver per port to maintain same pinout as SP301. The
LTC1322 and LTC1335 each provide two drivers and two
receivers per port. Additionally, the LTC1321 and LTC1322
single-ended ports are RS232 compatible with higher V
DD
and V
EE
supply levels.
All the interface drivers feature three-state outputs. Inter-
face outputs are forced into high impedance 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 thermal
shutdown circuit disables both the EIA562 and RS485
driver outputs when the die temperature reaches 150°C.
The thermal shutdown circuit enables the drivers when the
die temperature cools to 135°C.
In RS485 mode, shutdown mode or with the power off,
the input resistance of the receiver is 24k. The input
resistance drops to 6.3k in EIA562 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 left open.
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 is a high state. Both the current sources are
disabled in the EIA562 mode.
For LTC1335, a logic LOW at the OE pin enables all the
receiver outputs and a logic HIGH disables all the
receiver outputs. An internal 4µA current source pulls
the OE pin LOW if left open.
A loopback mode enables internal connections from
driver outputs to receiver inputs for self-test when the
1.5V
3V
0V
OE
f = 1MHz: t
r
≤ 10ns: t
f
≤ 10ns
1.5V
R
V
OL
5V
t
ZL
t
LZ
0.5V
OUTPUT NORMALLY LOW
0V
1321/22/35 F12
1.5V
1.5V
t
ZH
V
OH
OUTPUT NORMALLY HIGH
t
HZ
0.5V
R
Figure 11. EIA/TIA-562 Receiver Propagation Delays
V
IH
A OR B
R
V
OL
V
OH
1321/22/35 F11
t
PHL
t
PLH
V
IL
2.4V
0.8V
1.3V
1.7V
