1.

General description

The 74ABT657 high-performance BiCMOS device combines low static and dynamic

power dissipation with high speed and high output drive.

The 74ABT657 is an octal transceiver featuring non-inverting buffers with 3-state outputs

and an 8-bit parity generator/checker, and is intended for bus-oriented applications. The

buffers have a guaranteed current sinking capability of 64 mA. The Transmit/Receive input

(pin T/R) determines the direction of the data flow through the bidirectional transceivers.

Transmit (active HIGH) enables data from A ports to B ports; Receive (active LOW)

enables data from B ports to A ports.

When Output Enable input (pin OE) is HIGH, both A and B ports are high-impedance. The

parity select input (pin ODD/EVEN) allows the user to generate either an odd or even

parity output, depending on the system. Pin PARITY is an output from the

generator/checker when transmitting from port A to port B (pin T/R = HIGH) and an input

when receiving from port B to port A port (pin T/R = LOW).

In transmit mode (pin T/R = HIGH) port A is polled to determine the number of HIGH

inputs on port A. Pin PARITY output goes to the logic state determined by the setting of

pin ODD/EVEN and by the number of HIGH inputs on port A. For example, if pin

ODD/EVEN is set LOW (even parity) and the number of HIGH inputs on port A is odd, pin

PARITY output goes HIGH, transmitting even parity. If the number of HIGH inputs on port

A is even, pin PARITY output goes LOW, keeping even parity.

In receive mode (pin T/R = LOW) port B is polled to determine the number of HIGH inputs

on port B. If pin ODD/EVEN is LOW (even parity) and the number of HIGH inputs on port

B is:

2.

Features and benefits

74ABT657

Octal transceiver with parity generator/checker; 3-state

Rev. 03 — 15 March 2010

Product data sheet