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 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/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:
- 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 ERR\ will be low
indicating an error.
The SN74F657 is characterized for operation from 0°C to