Corporation

SIGNAL PROCESSING EXCELLENCE

92

and sink current capability. The RS423 line

drivers convert TTL logic levels into inverted

RS-423 output signals. All line drivers are inter-

nally protected against short circuits on their

outputs.

The RS-422 receivers convert the RS-422 dif-

ferential input signals into non–inverted TTL

logic levels. Receiver input thresholds are

±200mV. The RS-422 receivers can receive

input data up to 1Mbps. The RS-423 receivers

convert the RS-423 input signals into inverted

TTL output logic levels. The RS-423 receivers

have an input threshold of

±200mV, and can

receive data up to 100kbps.

A loopback test mode is provided that puts the

driver outputs into a high impedance tri–state

level, and routes the driver outputs to their

associated receiver inputs. In this configuration,

the signal path is non–inverting from the TTL

driver inputs to the receiver TTL outputs. This

operating mode allows the controlling system to

perform diagnostic self–test of the RS-422/

RS-423 transceiver circuitry at speeds up to

3kbps.

Figure 1. Control Input Configuration — SELECT A = 0, SELECT B = 0

LOOPBACK

GND

SUPPLY

+5V +12V –12V

SP306

28

1

15

14

7

8

SELECT A

SELECT B

22

SELECT A = 0

SELECT B = 0

LOOPBACK = 0

LOOPBACK

GND

SUPPLY

+5V +12V –12V

SP306

28

1

15

14

7

8

SELECT A

SELECT B

22

SELECT A = 0

SELECT B = 0

LOOPBACK = 1

RS-423 DRIVER

* Tri–state output

RS-423 RECEIVER

RS-422 RECEIVER

RS-422 DRIVER

* Tri–state output

APPLICATION INFORMATION

Control Logic Configuration

Software control of the SP306 is via two select

pins (7 and 8) and a loopback control pin (22).

SELECT A and SELECT B allow the user to

program the SP306 for four different interface

modes. Loopback mode can be selected in any

of these interface modes. The figures that follow

outline the various operating modes that are

supported by the SP306.