9

LTC1387

SWITCHI G WAVEFOR S

A, B

R

LTC1387 • F11

2.4V

0.8V

1.3V

1.7V

Figure 11. RS232 Receiver Propagation Delays

1.5V

3V

0V

RXEN

1.5V

R

5V

0.5V

OUTPUT NORMALLY LOW

0V

LTC1387 • F12

1.5V

1.5V

OUTPUT NORMALLY HIGH

0.5V

R

Figure 12. Receiver Enable and Disable Times

S

APPLICATI

I FOR ATIO

Basic Theory of Operating

The LTC1387 is a single 5V supply, single-port logic

reconfigurable RS485/RS232 transceiver with an onboard

charge pump. The interface port offers a flexible combina-

tion of an RS485 driver and an RS485 receiver or two

RS232 drivers and two RS232 receivers. The RS485

transceiver and the RS232 transceivers are designed to

share the same I/O pins. A logic input 485/232 controls the

selection between RS485 and RS232 transceiver modes.

The RS485 transceiver supports both RS485 and RS422

standards, whereas the RS232 transceivers support both

RS232 and EIA562 standards. With four additional control

inputs: ON, DXEN, RXEN and SLEW, the LTC1387 can

easily be reconfigured via software to adapt to various

communication needs including a one-signal-line RS232

I/O mode. Four examples of interface port connections are

shown in Figures 13 through 16.

Both the interface drivers and the receivers feature three-

state outputs. Driver outputs are forced into high

impedance when the driver is disabled, in the shutdown

mode or with the power off. The driver outputs can be

forced beyond power supply levels without damage up to

±18V. The receiver inputs can withstand ±25V without

damage. The receiver input resistance is typically 24k in

RS485 mode, shutdown mode or power off but drops to

5k in RS232 mode.

In RS485 mode, the DXEN and RXEN control the three-

state outputs of the driver and receiver respectively. The

SLEW input is active during RS485 mode and the logic

level controls the differential driver slew rate. This pin has

an internal 5

µA pull-up current source during the RS485

mode. A high logic selects fast differential driver slew rate

and a low logic selects slow slew rate. In slow slew mode,

the maximum signal bandwidth is reduced, minimizing