SN74F657

    The 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 SN74F657 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 SN74F657 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 SN74F657 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) SN74F657, 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:
1. Odd and the PARITY input is high, then ERR will be high signifying no error.
2. Even and the PARITY input is high, then ERRwill be low indicating an error.

    The SN74F657 is characterized for operation from 0°C to 70°C.