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FIGURE 1. Conceptual Input Section

FIGURE 2. Final Positions in a 74F403A

Resulting from a 64-Bit Serial Train

feed the stack. A LOW level on the TTS input initiates a

“fall-through” action. If the top location of the stack is

empty, data is loaded into the stack and the input register is

re-initialized. Note that this initialization is postponed until

PL is LOW again. Thus, automatic FIFO action is achieved

by connecting the IRF output to the TTS input.

An RS Flip-Flop (the Request Initialization Flip-Flop shown

in Figure 10) in the control section records the fact that

data has been transferred to the stack. This prevents multi-

ple entry of the same word into the stack despite the fact

the IRF and TTS may still be LOW. The Request Initializa-

tion Flip-Flop is not cleared until PL goes LOW. Once in the

stack, data falls through the stack automatically, pausing

only when it is necessary to wait for an empty next location.

In the 74F403A as in most modern FIFO designs, the MR

input only initializes the stack control section and does not

clear the data.

OUTPUT REGISTER (DATA EXTRACTION)

The Output Register receives 4-bit data words from the

bottom stack location, stores it and outputs data on a 3-

STATE 4-bit parallel data bus or on a 3-STATE serial data

bus. The output section generates and receives the neces-

sary status and control signals. Figure 3 is a conceptual

logic diagram of the output section.