MadSci Network: Computer Science |
Well Mikael, you actually almost figured out the answer by yourself when you said that you have to pause between one letter and the next. Yes, you need a pause or other signal. Since there's two major ways that computers handle this, and I'm not sure which you mean, I'll explain both... The first sort of signalling is called asynchronous, and is usually used in conjunction with input/output devices such as tape drives, modems, and similar. It relies on the fact that on the external media (the disk surface, the magnetic tape, the phone line) there are usually actually *three* states (a signal indicating "0", a signal indicating "1", and no detectable signal). (I am intentionally skipping over things like phase encoding in modems and so on). Then at the start of each chunk of information, there is a "preamble", and at the end, there's a "trailer". Often on older/slower modems, you will find "start" and "stop" bits - that's what they do. So for example, you might be using 2 start and 1 stop bit on a modem - so what is actually transmitted for an 8-bit sequence 01101101 would be: (silence) (start) (start) (0) (1) (1) (0) (1) (1) (0) (1) (stop) (silence)... (next start bit) and your modem would listen for *11* pieces of info. Similar techniques are used in disk drives and tape drives to indicate the start/end of blocks of data - the start/stop bits serve as the "pause" from a telegraph. The second sort of signalling is "clocked" or "synchronous", where we don't worry about pauses because an external signal keeps everything in sync. The metaphor here would be the strong beat of disco/dance music - it's very hard to get out of step unless you're *really* uncool (which if you're royalty or a politician gets you an unflattering clip on TV). Here, in addition to the actual signal being sent, there's a *second* totally seperate clocking signal, that basically chants out "BIt!" "Bit!" "Bit!", and the hardware that is sending/receiving the bits just takes the bits at the specified times. This is how it's usually done inside the computer, and where things like "bus speed" come from - that refers to how fast the clock signal for the bus is (which is usually different from the CPU clock, but usually a simple multiple like 3, 4, 5, 3/2, and so on, so the CPU and bus can synchronize fairly regularly - if the CPU clock were 8.684 times as fast as the bus clock, things would be ugly to design - CPU 9 times the bus is a LOT easier).
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