Datasheet

LTC2870/LTC2871
11
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pin FuncTions
PIN NAME
LTC2870
QFN
LTC2870
TSSOP
LTC2871
QFN
LTC2871
TSSOP DESCRIP
TION
485/232 4 7 Interface Select Input. A logic low enables RS232 mode and a high enables RS485 mode. The
mode determines which transceiver inputs and outputs are accessible at the LTC2870 pins as
well as which is controlled by the driver and receiver enable pins. Do not float.
RXEN 5 8 Receiver Enable. A logic high disables RS232 and RS485 receivers leaving receiver outputs
Hi-Z. A logic low enables the RS232 or RS485 receivers, depending on the state of the
interface select input 485/232 . Do not float.
DXEN 6 9 Driver Enable. A logic low disables the RS232 and RS485 drivers leaving the driver output in
a Hi-Z state. A logic high enables the RS232 or RS485 drivers, depending on the state of the
interface select input 485/232. Do not float.
RX232 11 15 RS232 Receiver Enable. A logic high disables the RS232 receivers and input termination
resistors leaving the RS232 receiver outputs in a Hi-Z state. A logic low enables the RS232
receivers and resistors, subject to the state of the CH2 pin. Do not float.
RX485 5 9 RS485 Receiver Enable. A logic high disables the RS485 receiver leaving the RS485 receiver
output in a Hi-Z state. A logic low enables the RS485 receiver and resistors, subject to the
state of the CH2 pin. Do not float.
DX232 10 14 RS232 Driver Enable. A logic low disables the RS232 drivers leaving the RS232 driver outputs
in a Hi-Z state. A logic high enables the RS232 drivers. Do not float.
DX485 6 10 RS485 Driver Enable. A logic low disables the RS485 driver leaving the RS485 driver output in
a Hi-Z state. A logic high enables the RS485 driver. Do not float.
H/F 27 2 37 3 RS485 Half-Duplex Select Input. A logic low is used for full-duplex operation where pins A and
B are the receiver inputs and pins Y and Z are the driver outputs. A logic high is used for half-
duplex operation where pins Y and Z are both the receiver inputs and driver outputs and pins
A and B do not serve as the receiver inputs. The impedance on A and B and state of differential
termination between A and B is independent of the state of H/F. The H/F pin has no effect on
RS232 operation. Do not float.
TE485 28 3 38 4 RS485 Termination Enable. A logic high enables a 120Ω resistor between pins AandB and
also between pins Y and Z. A logic low opens the resistors, leaving A/B and Y/Z unterminated.
The LTC2870 termination resistors are never enabled in RS232 mode. Do not float.
FEN 9 12 13 17 Fast Enable. A logic high enables fast enable mode. In fast enable mode the integrated DC/
DC converter is active independent of the state of driver, receiver, and termination enable pins
allowing faster circuit enable times than are otherwise possible. A logic low disables fast
enable mode leaving the state of the DC/DC converter dependent on the state of driver, receiver,
and termination enable control inputs. The DC/DC converter powers down only when FEN is
low and all drivers, receivers, and terminators are disabled (refer to Table 1). Do not float.
LB 26 1 36 2 Loopback Enable. A logic high enables logic loopback diagnostic mode, internally routing
the driver input logic levels to the receiver output pins. This applies to both RS232 channels
as well as the RS485 driver/receiver. The targeted receiver must be enabled for the loopback
signal to be available on its output. A logic low disables loopback mode. In Loopback mode,
signals are not inverted from driver inputs to receiver outputs. Do not float.
CH2 4 8 RS232 Channel 2 Disable. A logic high disables RS232 receiver 2 and RS232 driver 2
independent of the state of RX232 and DX232 pins. In this state, the disabled driver output
becomes Hi-Z and the 5kΩ load resistor on the disabled receiver input is opened.
A logic low allows both RS232 transceiver channels to be enabled or disabled together based
on the RX232
and DX232 pins. Has no effect on RS485 operation. Do not float.
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