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SN74F657NT
Octal Transceiver With Parity Generator/Checker and 3-State Outputs
VCC
ODD/EVEN
ERR
GND
GND
PARITY
3-State B Outputs Sink 64 mA and Source
15 mA Input Diodes for Termination Effects Package Options Include Plastic
Small-Outline Packages and Standard
Plastic 300-mil DIPs
descriptionThe SN74F657 contains eight noninverting
buffers with 3-state outputs and an 8-bit parity
generator/checker. It is intended for bus-oriented
applications. The buffers have a specified current
sinking capability of 24 mA at the A port and 64 mA
at the B port.
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 (OE) input is high, both the A and B ports are placed in a high-impedance state
(disabled). The ODD/EVEN 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/EVEN) 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
ODD/EVEN and the number of high bits on A port. When ODD/EVEN 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/EVEN
is low (for even parity) and the number of highs on B port is: Odd and the PARITY input is high, then ERR will be high signifying no error. Even and the PARITY input is high, then ERR will be low indicating an error.
The SN74F657 is characterized for operation from 0°C to 70°C.
