I/O-Friendly Keyboard

038 1/0-FRIENDLY KEYBOARD COMPUTERS Not all computers have a keyboard, yet it is often essential to have the use of one. Two circuits are presented here that enable a keyboard facility to be produced with the aid of only six or seven I/o lines. Fig. 1 shows a circuit based on a 74HCT148 and a 74HCT138 that can serve 56 or 64 keys. The circuit in Fig. 2, based on a 74HCT147 and a 74HCT138, can address 72 keys via seven I/o lines. The choice between the two circuits depends on the number of available i/o lines and the wanted number of keys. In either circuit, the key rows are selected by the bits on the A, B or C input of the HCT138. The combination of these bits deter- mines which of the outputs YO-Y7 goes low. As long as no key is depressed, the inputs of the HCT148 in Fig. 1 or the HCT147 in Fig. 2 are high. When a key is pressed, the inverted binary infor- mation at the output of the ics show which key it is. In Fig. 1, the 0 input of the HCT148 is not used, because the code a...