Speak & Spell

Using standard 74154 four-to-sixteen line decoders, the Speak & Spell can be tricked into thinking that a button has been pressed on the keyboard. The PNP transistors shown take the place of the key contacts, to short the controlling lines. Thus, the letter A can be spoken by pressing the A button, or by sending a binary 1001 on the four high-order lines to the decoder circuits; both actions short the L1 and L4 lines.