Why the current serial is faster than parallel transmission

Overview: The parallel interface speed is faster than the serial interface, which was the case a few years ago.
In the case of low actual clock frequency, parallel port because it can transmit several bits at the same time, the rate is really faster than the serial port.
However, with the development of technology, the clock frequency is more and more high, the mutual interference between parallel conductors is more and more serious. Parallel interface Because there are many parallel and tight wires, but the clock frequency increases to a certain extent, the transmitted data can not be recovered.
And the serial port because of less wire, inter-line interference is easy to control, but can continuously improve the clock frequency to improve the transmission rate. And the terminal of the serial port will also be relatively small. This is why high-speed transmissions are now serially used."Crowds" is the old saying, but the computer field has occurred in many lines than the 1 lines of strange things. In terms of communication speed, cost and communication quality, the current serial transmission mode is better than the parallel transmission mode.

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In the last two years, one of the most widely heard words could be serial transmissions. From the technology development situation, the serial transmission method has completely replaced the parallel transmission mode momentum, the USB replaces the IEEE 1284,sata replaces the PATA,PCI Express to replace the PCI ... From the principle point of view, the parallel transmission is better than the serial transmission mode. In layman's words, the parallel transmission path is like a wide avenue with multiple lanes, while serial transmission is a country road that allows only one car to pass. For example, the old and typical standard parallel port (Parallel port) and serial port (commonly known as COM port), the parallel interface has 8 data lines, high data transmission rate, and the serial interface has only 1 data lines, data transmission speed is low. The parallel port can transmit one byte at the time of the serial port transmitting 1 bits. When the parallel port completes the word "advanced" transfer task, the serial port only transmits the first letter "a" of the word.

Figure 1: Parallel interface speed is 8 times times the serial interface

So why is the serial transmission way better now? The underlying causes behind the representation are analyzed from parallel, serial changes, and technical features.

One, the parallel transmission technology encounters the development predicament

The bus and interface in the computer is the "aorta" that transmits data between the host and the external device, and as the processor speed climbs, the data transmission speed of the bus and interface also needs to be increased gradually, otherwise it will become the bottleneck of the computer development.
Let's take a look at the bus situation first. 1981 the first PC in the ISA bus as the symbol of the open architecture, the data bus is 8-bit, operating frequency of 8.33MHz, which at that time was considered "advanced technology", so the ISA bus also has another name "at the bus", at 286, the ISA's bit width increased to 16 bits, To maintain compatibility with 8-bit ISA, the operating frequency is still 8.33MHz. This technology has been used in the 386 system.

In the 486 era, there were also PCI and VESA two faster bus standards, they have the same bit width (32 bit), but the PCI bus can be run asynchronously with the processor, when the processor frequency increases, the PCI bus frequency can still remain unchanged, the choice of 25MHz, Three frequencies of 30MHz and 33MHz. While the VESA bus is working synchronously with the processor, the VESA-bus-type peripheral devices work more frequently, with less adaptability, and quickly lose competitiveness as processor frequency increases. PCI bus standard becomes the king of Pentium Era PC bus, hard disk controller, sound card to network card and video card all use PCI slots.

Figure 2:

Parallel data transmission technology has always been an important means to improve data transmission rate, but further development has encountered obstacles. First, because the premise of the parallel transmission method is to use the same timing signal transmission, the same timing to receive the signal, and excessive increase in clock frequency will be difficult to make the data transmission timing and clock, the wiring length is slightly different, the data will be in the same time as the clock, the increase in clock frequency also easily cause interference between the signal lines. Therefore, the parallel mode is difficult to achieve high-speed. In addition, increasing the bit width will undoubtedly lead to an increase in the number of cabling on the motherboard and the expansion board, with the cost rising.

In the external interface, we know that the IEEE 1284 parallel port rate of up to 300kb/s, the transmission of graphics data using compression technology can be increased to 2mb/s, and RS-232C standard serial port data transmission rate is usually only 20kb/s, parallel port data transmission rate is undoubtedly to win a chip. As a result, the parallel port has been the preferred connection for the printer for more than 10 years. For a stylus printer that transmits only text, the transfer speed of the IEEE 1284 parallel port can be said to be more than sufficient. However, the situation has changed for printers that have been speeding up in recent years. The author uses Epson 6200L (parallel port and USB interface) when printing 2MB picture, parallel port and USB interface speed difference is not obvious, but when printing 7.5MB size picture file, from click "Print" to the final paper, using the USB interface for 18 seconds, while using the parallel port, It took 33 seconds. From this test results, it can be seen that the current parallel port for today's application needs, there is indeed a bottleneck.

Do you know? Three types of interfaces for IEEE 1284

The early parallel port is a ring port, and IEEE 1284 uses a design-resistant D-connector. IEEE 1284 defines three connectors such as D-Sub, Centronics, and MDR-36 (Figure 3). We see the printer cable, one end of the D-sub connector, used to connect to the host, and the other end is a centronics connector with a locking device to connect to the printer. The connection is not only convenient, but also very reliable. The D-Sub Connector has a 25 root cuttings pin, while the Centronics connector has 36 root cuttings pins, and the extra 11 root is basically a redundant signal ground. The MDR (mini Delta Ribbon) connector is also a 36 root cuttings pin, a small size connector designed for small devices such as digital cameras and zip drives, and is rarely used in practice.

Figure 3: Three parallel port connectors of different sizes

Second, USB, let serial transmission bath fire Reborn

Looking back at the parallel interface and serial interface described earlier, we know that the IEEE 1284 parallel port rate can reach 300kb/s, while the RS-232C standard serial port data transmission rate is usually only 20kb/s, parallel port data transmission rate is undoubtedly to win. External interface in order to obtain a higher quality of communication, it is also necessary to look for RS-232 alternatives.

In 1995, USB interfaces introduced by several companies, including Compaq, Intel, Microsoft and NEC, first appeared on the PC and entered a large-scale practical phase from 1998 onwards. USB is more than 100 times times faster than rs-232c, breaking the speed bottleneck of serial port communication, and has good compatibility and ease of use. The self-adaptability of the USB device communication rate makes it possible to automatically select HS (high-speed, high speed, 480Mbps), FS (full-speed, full velocity, 12Mbps) and LS (low-speed, low speed, 1.5Mbps) according to the motherboard settings One of three modes. The USB bus also has automatic device detection capabilities, and after the device is plugged in, the operating system software automatically detects, installs, and configures the device, eliminating the hassle of having to turn off the PC when adding or removing devices. USB interface is able to obtain a high data transmission rate, mainly because it abandoned the conventional single-ended signal transmission mode, instead of using differential signal (differential signal) transmission technology, effectively overcome the antenna effect on the signal transmission line formation interference, and transmission lines between the crosstalk. The two data cables in the USB interface are twisted together to form a twisted pair structure (Figure 4).

Figure 4: USB with differential mode signal transfer mode

Figure 5: Differential transmission with better anti-jamming performance

Figure 5 is a choke coil consisting of two signal lines wrapped around a toroidal ferrite core. In the single-ended signal transmission mode, the line is affected by electromagnetic radiation interference and generate common mode current, the magnetic field is superimposed into a higher line impedance, so that although the interference is reduced, but the effective signal is also attenuated. In differential transmission mode, the common mode interference is offset by the core, but no additional line impedance is generated. In other words, the use of common mode choke coil in differential transmission mode can not only achieve the purpose of anti-jamming, but also does not affect the signal transmission.

In differential signal transmission system, the transmission line can obtain good anti-jamming performance without shielding, and reduce the connection cost. However, due to the relatively low signal level of the USB interface 3.3V, the maximum communication distance is only 5 meters. The USB specification also limits the number of layers in the physical layer to no more than 7 layers, which means that users can place a USB device up to 30 meters from the host by using up to 5 connectors.

In order to solve the long-distance transmission problem, extend the application range of USB, some manufacturers added new features on the USB specification, such as powered USB and extreme USB, which increased the USB power supply capacity, the latter extended the USB transmission distance.

Three, differential signal technology: The key to open the door of high-speed transmission of the signal

Computer history is the pursuit of faster speed history, with the increase in bus frequency, all signal transmission has encountered the same problem: the more electromagnetic interference between the lines, the higher the probability of data transmission failure, the traditional single-ended signal transmission technology can not adapt to the needs of high-speed bus. Differential signal technology began to be used in a variety of high-speed bus, we have been aware that the secret of high-speed USB signal transmission is the use of differential signal transmission mode.

Differential signal technology is a data transmission and interface technology that appeared in the the 1990s, compared with the traditional single-ended transmission mode, it has the characteristics of low power consumption, low bit error rate, low crosstalk and low radiation, the transmission medium can be copper PCB connection, also can be balanced cable, the maximum transmission rate can reach 1.923Gbps. Intel advocates the third generation of I/O technology (3GIO), the core technology of its physical layer is differential signaling technology. So what is the difference between signal technology and what's going on?

Figure 6: Differential signal transmission circuit

As we all know, in the traditional single-ended (single-ended) communication, a line to transmit a bit. The high level is expressed as "1" and the low level is expressed as "0". In the event of interference during data transmission, the high and low level signal is fully likely to cause a significant disturbance to the critical value, and the signal will go wrong once the higher or lower signal exceeds the critical value (Figure 7).

Figure 7: Single-ended signal transmission

In the differential circuit, the output level is the positive voltage of the logic "1", the output negative voltage is the logic "0", and the output "0" voltage is meaningless, it does not mean "1", also does not mean "0". In the differential communication shown in Figure 7, the interference signal will enter the adjacent two signal lines simultaneously, when two of the same interference signal into the receiver side of the differential amplifier of the two inverting input, the output voltage is 0. Therefore, the differential signal technology has a strong immunity to the interference signal.

Figure 8: Differential signal transmission

For this reason, in the actual circuit as long as the use of low-voltage differential signal (Voltage differential SIGNAL,LVDS), the amplitude of about 350mV can meet the requirements of near-distance transmission. Assuming that the load resistance is 100ω, using LVDS transmission data, if the twisted pair length is 10 meters, the transmission rate can reach 400Mbps, when the cable length increases to 20 meters, the rate drops to 100Mbps, and when the cable length is 100 meters, the rate can only reach about 10Mbps.

In the near-distance data transmission, LVDS can not only achieve high transmission performance, but also a low-cost solution. LVDS devices can be manufactured using a cost-efficient CMOS process, and a very high rate is achieved with the use of 3 types of cables and connectors with low costs. At the same time, because the LVDS can use a lower signal voltage, and the driver adopts constant current source mode, its power will hardly change with the frequency, which makes it possible to improve data transmission rate and reduce power consumption. As a result, LVDS technology is used in USB, SATA, PCI Express, and HyperTransport, while the LCD control circuitry transmits pixel brightness control signals to the LCD screen, as well as the LVDS approach.

Four, the new serial era has arrived

Differential transmission technology Not only breaks the speed bottleneck, but also saves space by using small connections. In recent years, in addition to USB and FireWire, there are many serial connection standards characterized by differential signal transmission, almost covering the motherboard bus and external I/O ports, showing the trend from parallel to the new serial era, the application of serial interface technology in 2005 will enter the heyday (Figure 9).

Figure 9: All I/O technologies will be serially

1.LVDS technology to break the chipset transmission bottleneck

As the speed of the computer increases, the speed of communication between the CPU and the North Bridge chip, between the North Bridge and the South Bridge, and the various device buses connected to the chipset affects the overall performance of the computer. However, the FR4 printed circuit board has been used since the existence of the skin effect and dielectric loss caused by inter-code interference, limiting the transmission rate of ascension.

In the case where the rate of traditional parallel synchronous digital signals is going to reach the limit, designers turn to the high-speed serial signal to find a way out, because serial bus technology not only can achieve higher performance, but also can minimize the number of chip pins, simplify board wiring, reduce manufacturing costs. Intel's PCI Express, AMD Hypertansport, and Rambus's redwood and other I/O bus standards invariably use low-voltage differential signaling (LVDS) as a new generation of high-speed signal level standards.

As shown in a typical PCI Express Channel 9, the two sides of the communication consist of two differential pairs consisting of a duplex channel, a pair for sending, and a pair for receiving. 4 Physical Lines form the PCI Express x1. x1, x2, x4, and x16 are defined in the PCI Express standard. The PCI Express x16 has the most physical lines (16x4=64).

Figure 10:PCI Express x1 Data Channel

Even with the lowest-provisioned X1 system, the data can be transmitted at 2.5GHz simultaneously in two directions, with a bandwidth of 5Gbps and exceeding the traditional PCI bus 1.056Gbps (32bitx33mhz) bandwidth. Moreover, the PCI bus is a shared bus through the bridge, and the PCI Express uses the "end-to-end connection" (Figure 11), but also allows each device to enjoy the bus bandwidth alone, so it can achieve higher performance than PCI.

Figure 11:pci Express end-to-end connection eliminates the bridge path

AMD's HyperTransport technology is very similar to PCI Express, with the same LVDS data channel, which is first used for fast communication between North and South Bridges. Operating frequency range from 200MHz to 1GHz, the bit width can be flexibly selected according to the bandwidth requirements of 2, 4, 8, 16 or 32 bits. The HyperTransport is first used for fast communication between the North and South bridges, and will be used for all inter-chip connections in the future.

2.SATA, plug the hard disk into the wings

Before ATA33, 40 parallel data lines were used, which limited the increase of signal frequency due to crosstalk between data lines. Therefore, starting from ATA66, the ATA data cable adds 1 ground lines between the two lines to reduce mutual interference. When the ground is increased, there is still a distributed capacitance C2 between the data line and the ground (Figure 12), or the interference problem can not be solved completely, so that the maximum operating frequency of the Pata interface stays on 133MHz. In addition to the root cause of signal interference, PATA also has problems that do not support hot-swappable and fault-tolerant.

Figure 12: Inter-line crosstalk of parallel ATA

SATA was launched by Intel on IDF2000, and since then, Intel's industry giants such as APT, Dell, IBM, Seagate and Maxtor formally launched the SATA 1.0 specification in 2001. On the spring IDF2002, the SATA 2.0 specification has been announced.

The SATA interface consists of 4 data lines and 3 ground wires, with a total of 7 physical connections. The current SATA 1.0 standard, the data transfer rate is 150mb/s, and the ATA133 interface 133mb/s speed slightly improved, but the future SATA 2.0/3.0 can be lifted to 300mb/s and 600mb/s. From the current drive speed growth trend, the SATA standard can meet the requirements of at least the next few years.

3.FireWire, Image transmission

FireWire was drafted by Apple Computer in 1986 and was introduced in 1995 by the American Institute of Electrical and Electronics Engineers (IEEE) as an IEEE 1394, another high-speed serial communication standard other than USB. FireWire's earliest application target is to transmit digital image signals to video recording devices, which have been used in DV, DC, DVD, DVR, TV set-top box and home game consoles.

The FireWire transmission line has 6 cables, two pairs of twisted pairs forming two separate channels, and the other two are power lines and ground. Sony has improved FireWire by abandoning the power and ground lines to form a streamlined firewire with only two pairs of twisted-pair wires, and named I. Link.

FireWire data rates are comparable to USB, with a single channel bandwidth of 400Mbps and a communication distance of 4.5 meters. However, the IEEE 1394b standard has extended the single channel bandwidth to 800Mbps, and in the new IEEE 1394-2000 standard, the maximum data transfer rate is determined to be 1.6Gbps, and the maximum length of the connecting cable between adjacent devices can be extended to 100 meters.

Five, "serial" can red to what day?

After reading this article, if someone asks about a better question about serial transmission and parallel transmission, you might blurt out: good serial communication! However, the popularity of serial transmission is due to the conversion of single-ended signal transmission to differential signal transmission, and increase the operating frequency of the controller, and the "parallel communication speed at the same frequency higher" the basic principle is never wrong, by increasing the bit width to improve the data transmission rate of the parallel strategy will play an important role. Of course, the premise is that there are better measures to solve the problems of parallel transmission.

Technological progress is endless, and no technology can ever be applied. Computer technology in the future into the THz era, the signal transmission speed requirements will be higher, differential transmission technology can meet the requirements? Is there a need for another better technique to accomplish another breakthrough in frequency? Wait and see!

Why the current serial is faster than parallel transmission