Question 13.7: Design an interface to an 8-bit serial printer. Sketch the w......
Design an interface to an 8-bit serial printer. Sketch the waveforms required to transmit the single ASCII code for an asterisk (*). Note: ASCII is the 7-bit code that was given in Chapter 1. The ASCII code for an asterisk is 010 1010. Let’s make the unused eighth bit (MSB) a zero.
The circuit design and waveforms are shown in Figure 13–23.
The 74165 is chosen for the job because it is an 8-bit register that can be parallel loaded and then shifted synchronously by the clock input to provide the serial output to the printer.
During pulse 0, the register is loaded with the ASCII code for an asterisk (the LSB is put into D_{7} because we want it to come out first). The clock input is then enabled by a LOW on {\overline{{C E}}} (Clock Enable). Each positive pulse on C_{p} from then on will shift the data bits one position to the right. After the eighth clock pulse (0 to 7), the printer will have received all 8 serial data bits. Then, the {\overline{{C E}}} line is brought HIGH to disable the synchronous clock input. To avoid any racing problems, the printer will read the Q_{7} line at each negative edge of C_{p} so that the level will definitely be a stable HIGH or LOW, as shown in Figure 13–23.
At this point, you may be wondering how, practically, we are going to electronically provide the necessary signals on the {\overline{{C E}}} and {\overline{{PL}}} lines. An exact degree of timing must be provided on these lines to ensure that the register– printer interface communicates properly. These signals will be provided by a microprocessor and are called the handshaking signals.
Microprocessor theory and programming are advanced digital topics and are not discussed in this book. For now, it is important for us to realize that these signals are required and to be able to sketch their timing diagrams.
