74F657

    The 74F657 is an octal transceiver featuring noninverting buffers with 3state outputs and an 8bit parity generator/checker, and is intended for busoriented applications. The buffers have a guaranteed current sinking capability of 24mA at the A ports and 64mA at the B ports.The transmit/receive (T/R) input 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.The output enable (OE) input disables both the A and Bports by placing them in a high impedance condition when the OE input is high.

    The parity select  (ODD/EVEN) input of the 74F657 gives the user the option of odd or even parity systems.

    The parity (PARITY) pin of the 74F657 is an output from the generator/checker when transmitting from the port A to B (T/R = high) and an input when receiving from port B to A port ( T/R = low). 

    When transmitting (T/R = high) the parity select (ODD/EVEN) input of the 74F657 is set, then the A port data is polled to determined the number of high bits. The parity (PARITY) output then goes to the logic state determined by the parity select (ODD/EVEN) setting and by the number of high bits on port A.

    For example, if the parity select (ODD/EVEN) is set low (even parity), and the number of high bits on port A is odd, then the parity (PARITY) output will be high, transmitting even parity. If the number of high bits on port  A is even, then the parity (PARITY) output will be low, keeping even parity.

    When in receive mode (T/R = low) the B port is polled to determine the number of high bits. If parity select (ODD/EVEN) is low (even parity) and the number of highs on port B is:

(1) odd and the parity (PARITY) input is high, then ERROR will be high, significantly no error.

(2) even and the parity (PARITY) input is high, then ERROR will be asserted low,  indicating an error.