SN74BCT657
OCTAL TRANSCEIVER WITH PARITY GENERATOR/CHECKER
AND 3-STATE OUTPUTS

SCBS079B - NOVEMBER 1991 - REVISED APRIL 1994

features

• State-of-the-Art BiCMOS Design Significantly Reduces I_CCZ
• ESD Protection Exceeds 2000 V Per MIL-STD-883C, Method 3015; Exceeds 200 V Using Machine Model (C = 200 pF, R = 0)
• 3-State B Outputs Sink 48 mA or 64 mA and Source 12 mA or 15 mA
• Package Options Include Plastic Small-Outline (DW) Packages and Standard Plastic 300-mil DIPs (NT)

description

The SN74BCT657 contains eight noninverting transceivers with 3-state outputs and an 8-bit parity generator/checker. It is intended for bus-oriented applications.

The transmit/receive (T/R\) input determines the direction of the data flow through the bidirectional transceivers. When T/R\ is high, data is transmitted from the A port to the B port. When T/R\ is low, data is received at the A port from the B port.

When the output-enable () input is high, both the A and B ports are placed in a high-impedance state (disabled). The ODD/ input allows the user to select between odd or even parity systems.

When transmitting from A port to B port (T/R\ high), PARITY is an output from the generator/checker. When receiving from B port to A port (T/R\ low), PARITY is an input.

When transmitting (T/R\ high), the parity-select (ODD/) input is made high or low as appropriate. The A port is then polled to determine the number of high bits. The PARITY output goes to the logic state determined by the parity-select (ODD/) input and the number of high bits on A port. When ODD/ is low (for even parity) and the number of high bits on A port is odd, the PARITY will be high, transmitting even parity. If the number of high bits on A port is even, the PARITY will be low, keeping even parity.

When in the receive mode (T/R\ low), the B port is polled to determine the number of high bits. If ODD/E is low (for even parity) and the number of highs on B port is as follows:

• Odd and the PARITY input is high, then will be high signifying no error.
• Even and the PARITY input is high, then will be low indicating an error.

The SN74BCT657 is characterized for operation from 0°C to 70°C.