Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com

SP3243E_100_072309

9

Receivers

The receivers convert +5.0V EIA/TIA-232

levels to TTL or CMOS logic output levels. Re-

circuitry is enabled or when in shutdown. The

truth table logic of the SP3243 driver and receiver

outputs can be found in Table 2.

The SP3243E includes an additional non-in-

verting receiver with an output R

is an extra output that remains active and

monitors activity while the other receiver

outputs are forced into high impedance.

This allows a Ring Indicator (RI) signal from a

peripheral to be monitored without forward

biasing the TTL/CMOS inputs of the other

devices connected to the receiver outputs.

Table 2. SHUTDOWN Truth Tables

and SHUTDOWN = V

there is no activity present at the Receiver inputs.

Figure 11. Loopback Test Circuit for RS-232 Driver

Data Transmission Rates

Since receiver input is usually from a transmis-

sion line where long cable lengths and system

interference can degrade the signal, the inputs

have a typical hysteresis margin of 300mV. This

ensures that the receiver is virtually immune to

noisy transmission lines. Should an input be left

unconnected, an internal 5KΩ pulldown resistor

to ground will commit the output of the receiver

to a HIGH state.

Figure 13. Loopback Test results at 250Kbps

Figure 12. Loopback Test results at 1Mbps

SP3243

GND

C1+

C1-

C2+

C2-

V+

V-

0.1 µF

0.1 µF

0.1 µF

+

C2

C5

C1

+

+

C3

C4

+

+

0.1 µF

0.1 µF

TTL/CMOS

INPUTS

+3V to +5V

18

SHUTDOWN

5KΩ

5KΩ

TTL/CMOS

OUTPUTS

ONLINE

STATUS

To µP Supervisor

Circuit

1000pF

1000pF

Device: SP3243E

SHUTDOWN

TxOUT

RxOUT

R2OUT

0

High-Z

High-Z

Active

1

Active

Active

Active