These devices consist of bus-transceiver circuits, D-type flip-flops, and control circuitry arranged for multiplexed transmission of data directly from the data bus or from the internal storage registers. Output-enable (OEAB and OEBA\) inputs are provided to control the transceiver functions. The select-control (SAB and SBA) inputs are provided to select whether real-time or stored data is transferred. A low input level selects real-time data, and a high input level selects stored data. Figure 1 illustrates the four fundamental bus-management functions that can be performed with the 'ABT651 devices.
Data on the A or B bus, or both, can be stored in the internal D flip-flops by low-to-high transitions at the appropriate clock (CLKAB or CLKBA) inputs, regardless of the select- or enable-control pins. When SAB and SBA are in the real-time transfer mode, it also is possible to store data without using the internal D-type flip-flops by simultaneously enabling OEAB and OEBA\. In this configuration, each output reinforces its input. When all the other data sources to the two sets of bus lines are at high impedance, each set remains at its last state.
To ensure the high-impedance state during power up or power down, OEBA\ should be tied to VCC through a pullup resistor; the minimum value of the resistor is determined by the current-sinking capability of the driver (B to A). OEAB should be tied to GND through a pulldown resistor; the minimum value of the resistor is determined by the current-sourcing capability of the driver (A to B).
The SN54ABT651 is characterized for operation over the full military temperature range of -55°C to 125°C. The SN74ABT651 is characterized for operation from -40°C to 85°C.
The data output functions may be enabled or disabled by a variety of level combinations at OEAB or OEBA\. Data input functions are always enabled; i.e., data at the bus terminals is stored on every low-to-high transition of the clock inputs.
When select control is low, clocks can occur simultaneously if allowances are made for propagation delays from A to B (B to A) plus setup and hold times. When select control is high, clocks must be staggered to load both registers.
Figure 1. Bus-Management Functions