74F670

The 74F670 is a 16-bit 3-State Register File organized as 4 words of 4 bits each. Separate Read and Write Address and Enable inputs are available, permitting simultaneous writing into one word location and reading from another location. The 4-bit word to be stored is presented to four data inputs.

The Write address inputs (WA and WB) of the 74F670 determine the location of the stored word. The Write Address inputs should only be changed when the Write Enable input (WE) is High for conventional operation. When the WE is Low, the data is entered into the addressed location.

The addressed location remains transparent to the data of the 74F670 while the WE is Low. Data supplied at the inputs will be read out in true(non-inverting) form from the 3-State outputs. Data and address inputs are inhibited when the WE is High. Direct acquisition of datastored in any of the four registers is made possible by individual Read Address inputs (RA, RB). The addressed word appears at the four outputs when the Read Enable (RE) is Low. Data outputs are in the high impedance "off" state when the RE is High. This permits outputs to be tied together to increase the word capacity to verylarge numbers.

Up to 128 devices can be stacked to increase the word size to 512 locations by tying the 3-State outputs together. Since the limiting factor for expansion is the output High current, further stacking is possible by tying pullup reisistors to the outputs to increase the I_OH current available. Design of the Read Enable signals of the 74F670 for the stacked devices must ensure that there is no overlap in the Low levels which cause more than one output to be active at the same time. Parallel expansion to generate n-bit words is accomplished by driving the Enable and address inputs of each device in parallel.