81

Corporation

SIGNAL PROCESSING EXCELLENCE

SELECT A = 1

SELECT B = 0

LOOPBACK = 1

GND

2N2222

3.3KΩ

3.3KΩ

SP304

Figure 2. Typical Circuit

transmission line with other RS-232 devices

which are not powered by the same power

supplies, it is possible that a device that is not

powered will have a low impedance path to

ground at its driver output. The RS-232 stan-

dards require that with no power applied to the

device, the impedance from a transmitter output

to ground must be greater than 300

Ω. This can

be easily achieved as shown in Figure 1a, where

an external transistor is used as a switch to

isolate an SP304 from ground in the power off

condition. With V

switch is on, connecting ground (GND) for the

device to the circuit ground. In a power-off

condition, this transistor is switched off, thus

isolating the unit from circuit ground, and thereby

leaving the driver in a high impedance state.

Multiple SP304s can be connected as shown in

Figure 1b.

RS-422 OPERATION

The SP304 is a fully compliant RS-422 device

when operating in the RS-422 mode. Baud rate

and drive capability have been balanced to pro-

vide as much versatility as possible. The SP304

is both guaranteed for a 1Mbps data rate,

GND

2N2222

3.3kΩ

3.3kΩ

SP304

GND

2N2222

3.3kΩ

3.3kΩ

GND

SP304

SP304

Figure 1. Isolating the SP304 from Ground.

a) Single SP304

b) Multiple SP304

+4.0V for V

derated performance can be expected.

POWER SUPPLY SEQUENCING

There are no special power–up sequencing re-

quirements for the SP304.

GENERAL USAGE

RS-232 Operation

The SP304 is a fully compliant RS-232 device.

Its outputs are fully protected against shorts to

±20V with no external circuitry. If the potential

exists for momentary shorts to voltages greater

than

±20V, it is recommended that a 220Ω

resistor be wired in series with each driver

output. This will limit any damage from the

higher short-circuit current from these higher

voltage potentials. Voltage clamps such as back-

to-back Zener diodes can be used to clamp the

driver outputs to “safe” levels. Short circuit

current to ground is internally limited, and can

therefore be sustained infinitely. Under normal

operating conditions, the drivers can typically

source 7mA at

±5V output, which exceeds the

minimum RS-232 standards requirement.

If an SP304 transmitter output occupies a data