Difference between serial buses and parallel buses

What are serial buses and parallel buses?

Literally, serial means sending data one by one, and parallel means sending data in groups.

The memory chip DDR is a parallel transmission. It has a set of data lines D0-D7, plus DQS, DQM. This set of lines is transmitted together. As long as one of the lines is wrong, the data cannot be transmitted correctly and needs to be retransmitted. The serial data is transmitted bit by bit, and there is no connection between bit and bit, don’t have situation that the next bit cannot be transmitted because of an error in this bit.

Old-fashioned devices in the early years all used parallel transmission, while current devices all use serial transmission. Why is parallel transmission replaced by serial transmission?

Parallel bus

Parallel buses need to consider the cooperativity of data because multiple data are transmitted at the same time, which leads to the fact that the frequency of parallel transmission cannot be made very high. In contrast, the serial bus has only one link, so the frequency can be made very high, and the transmission speed can be increased. The increased speed can make up for the defect that only one data can be transmitted at a time.

In addition, the data of two adjacent links of the parallel bus are transmitted at the same time, which will cause serious interference between them. The more parallel links, the stronger the interference. Therefore, the parallel bus needs to strengthen the anti-interference ability, otherwise the data may be damaged during the transmission. If the data fails during the transmission, the data needs to be realigned and then transmitted. However, if a data error occurs on a serial bus, it only needs to be retransmitted once. Due to the high frequency of the serial bus, the erroneous data can be retransmitted soon.

Thirdly, because the parallel bus is a multi-link data transmission, it needs a lot of wires and the interface needs a lot of pins. The parallel interface in the old computer is very large, the wiring is relatively wide, and many pins are needed. In this way, the installation is also very troublesome, because the wiring is inconvenient and the interface is large.

It is precisely because of these shortcomings that the computer bus has gradually changed from parallel transmission to serial transmission, such as USB, hard disk SATA, and so on.

It should be noted that the gold fingers on the bottom of the graphics card PCB are densely packed in a large row, and the interface is PCIE x16, which looks like a parallel bus, but is actually a serial bus. The serial bus can be used for multi-link transmission. Unlike parallel links, each of its links is independent data, which has nothing to do with each other and will not be interfered by other data.