串行传输：克服并行传输的局限

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"有关‘串行传输’的英文资料" 串行传输是一种通信方式，与并行传输相对，它通过单一的数据线按顺序传输数据位。在并行传输中，多个数据位同时传输，可以提供快速的数据交换，但需要大量的连接线，这增加了硬件成本和复杂性。例如，一个典型的8位并行数据端口需要8条数据线，加上至少一条握手线和多条地线，通常每条信号线都会有一个单独的地回路线，因此可能需要20芯的连接电缆。对于短距离通信，这种多芯电缆是可行的，但对于长距离链接，它的成本会变得非常高昂，因为除了多芯电缆的成本，每条信号线可能还需要独立的驱动器和接收器电路。串行传输则解决了这个问题，它只需要一条单线链接，按顺序传输每一位数据。每个数据位被分配一个固定的时间间隔，在这个间隔内完成传输。这种方式大大减少了所需的物理连接，降低了硬件成本，尤其适用于长距离通信和无线通信场景。例如，如果部分链路通过无线电链接，如通过太空卫星，串行传输只需一个射频信道就能传输所有数据，相比于并行传输的每个数据位都需要一个独立的射频通道，串行传输更加经济且实际操作上更可行。串行传输有两种主要类型：同步串行和异步串行。同步串行传输中，数据位以固定的时钟速率连续发送，接收端和发送端共享一个精确的时钟源，确保数据能正确对齐。异步串行传输则使用起始位、停止位和可能的校验位来确定数据的边界，允许接收端和发送端之间存在时钟频率差异。在串行传输中，数据的传输速率通常以波特率（Bits Per Second, bps）表示，它定义了每秒传输的数据位数。为了提高效率和可靠性，串行传输还可能使用各种编码技术，如曼彻斯特编码或差分曼彻斯特编码，这些编码方法能在单线上同时传输数据和时钟信息，减少了信号干扰的可能性。串行传输是一种高效、节省资源的通信方式，特别适合于需要长距离传输或资源有限的应用，如物联网设备、远程传感器网络以及现代高速通信标准如USB、串行ATA (SATA) 和串行外设接口 (SPI) 等。

Serial Transmission

Alan Clements

Although a parallel input-output scheme can provide fast data transfer and is

simple in operation, it has the disadvantage of requiring a large number of intercon-

nections. As an example a typical 8 bit parallel data port uses 8 data lines, plus one or

two handshake lines and one or more ground return lines. It is fairly common practice

to provide a separate ground return line for each signal line, so an 8 bit port could

typically use a 20core interconnection cable. Whilst such a multiway cable is quite

acceptable for short distance links, up to perhaps a few meters, it becomes too expen-

sive for long distance links where, in addition to the cost of the multicore cable, sepa-

rate driver and receiver circuits may be required on each of the 10 signal lines. Where

part of the link is to be made via a radio link, perhaps through a space satellite, sepa-

rate radio frequency channels would be required for each data bit and this becomes

unacceptable.

An alternative to the parallel transfer of data is a serial system in which the states

of the individual data bits are transmitted in sequence over a single wire link. Each bit

is allocated a fixed time slot. At the receiving end the individual bit states are detected

and stored in separate flip-flop stages, so that the data may be reassembled to produce

a parallel data word. The advantage of this serial method of transmission is that it re-

quires only one signal wire and a ground return, irrespective of the number of bits in

the data word being transmitted. The main disadvantage is that the rate at which data

can be transferred is reduced in comparison with a parallel data transfer, since the bits

are dealt with in sequence and the larger the number of bits in the word, the slower the

maximum transfer speed becomes. For most applications however, a serial data

stream can provide a perfectly adequate data transfer rate. This type of communica-

tion system is well suited for radio or telephone line links, since only one communica-

tion channel is required to carry the data.

We have seen that in the CPU system data is normally transferred in parallel

across the、main data bus, so if the input-output data is to be in serial form, then a

parallel to serial data conversion process is required between the CPU data bus and

the external I／O line. The conversion from parallel data to the serial form could be

achieved by simply using a multiplexer switch which selects each data bit in turn and

connects it to the output line for a fixed time period. This basic scheme is shown in

Fig. 12-1．Amore practical technique makes use of a shift register to convert the par-

allel data into serial form.

A shift register consists of a series of D type flip-flops connected in a chain, with

the Q output of one flip-flop driving the D input of the next in the chain. All of the

flip-flops are clocked simultaneously by a common clock pulse. When the clock pulse

occurs the data stored in each flip-flop is transferred to the next flip-flop to the right in

the chain. Thus for each clock pulse the data word is effectively stepped along the

shift register by one stage. At the end of the chain the state of the output flip-flop will

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