Hardware Reference
In-Depth Information
Photo 1.5
Serial (RS-232 and USB) and parallel port I/O on a modern
motherboard. The DIP switch is used for setting various I/O options
Parallel versus serial I/O
Most microcomputer systems (including the PC) have provision for both par-
allel (e.g. a parallel printer) and serial (e.g. an RS-232 port) I/O. Parallel I/O
involves transferring data one (or more) bytes at a time between the micro-
computer and peripheral along multiple wires; usually eight plus a common
ground
connection). Serial I/O, on the other hand, involves transferring 1-bit
after another along a pair of lines (one of which is usually a ground connection).
In order to transmit a byte (or group of bytes) the serial method of I/O must
comprise a sequence or stream of bits. The stream of bits will continue until
all of the bytes concerned have been transmitted and additional bits may be
added to the stream in order to facilitate decoding and provide a means of error
detection.
Since data present on a microprocessor data bus exists in parallel form,
it should be apparent that a means of parallel-to-serial and serial-to-parallel
conversion will be required in order to implement a serial data link between
microcomputers and peripherals (see Figure 1.4).
Serial data may be transferred in either synchronous or asynchronous mode.
In the former case, all transfers are carried out in accordance with a com-
mon clock signal (the clock must be available at both ends of the transmission
path). Asynchronous operation involves transmission of data in packets: each
packet containing the necessary information required to decode the data which
it contains. Clearly this technique is more complex, but it has the considerable
advantage that a separate clock signal is not required.
As with parallel I/O, signals from serial I/O devices are invariably TTL com-
patible. It should be noted that, in general, such signals are unsuitable for
anything other than the shortest of transmission paths (e.g. between a keyboard
