What Is Serial Communication?

Serial communication, as one of the computer communication methods, mainly plays the role of data transmission between the host and peripherals and the host. Serial communication has the characteristics of few transmission lines and low cost, and is mainly suitable for close-range human-machine exchange, real-time In the communication work of monitoring and other systems, long-distance transmission can also be achieved with the help of the existing telephone network. Therefore, the serial communication interface is a common interface in computer systems. ^[1]

Serial communication technology refers to a communication method in which both parties communicate bit by bit and follow the sequence. In serial communication, data is transmitted bit by bit. Each bit of data occupies a fixed length of time, and a few pieces can be used.

The serial transmission of data on a single one-bit wide transmission line, called bit-by-bit, is called serial communication. in

Serial communication workplaces are often in complex environments such as high power / outdoors, and the distance between the parties to the communication is generally long, so they are vulnerable to interference. For serial communication, when the baud rate is constant, the transmission time of data bits is relatively short. Due to the characteristics of data bit sampling / acquisition in serial communication, bit information is disturbed, and the entire byte data is error information. ^[3]

The reality of serial communication isolation

Serial communication due to its working characteristics (susceptible to bitwise transmission, long-distance information exchange), applications (industrial control in harsh environments, outdoor, etc.), level matching between devices (inconsistent working levels of devices on both sides, etc.) , Need to do the corresponding isolation and protection. Through isolation, the following purposes are achieved. (1) Device protection, protection and isolation In today's rapid development of electronic devices, low-power, high-package chips are widely used. The low-voltage working conditions of microprocessors and the high-voltage working environment of peripheral devices have different development processes. The current microprocessor chip levels are mostly low-voltage devices such as 1.8V, 3.3V, and 5.0V, and with the continuous emergence of digital ICs with different operating voltages, the necessity of logic level conversion is more prominent. For example, the 3.3V input and output I / O of the STM32 controller and the traditional serial communication interface core environment have different development processes. The current microprocessor chip levels are mostly low-voltage devices such as 1.8V, 3.3V, and 5.0V, and with the continuous emergence of digital ICs with different operating voltages, the necessity of logic level conversion has become more prominent. For example, the 3.3V input and output I / O of the STM32 controller and the TTL level of the traditional serial communication interface chip MAX232 are input standards, which are incompatible. Therefore, in order to achieve level matching between the controller and the communication interface chip, and to protect the controller pins from being damaged due to excessively high or low operating voltages, it is particularly necessary to add an isolation device. ^[2]

(2) Shielding interference and line isolation. Because more serial communication equipment works in the harsh environment of the industrial site or the long-distance transmission of the power distribution system, the obvious interference signal is often induced on the line in long-line communication. Causes occasional errors in the communication process, which in turn affects the reliable operation of the entire system. Sources of interference signals include space radiation, crosstalk, and system noise. For example, in RS-232C communication, because it uses a single-ended signal transmission mode, when the ground potentials between the different ground wires of the two sides of the communication are not the same, a common-mode interference voltage will be introduced, causing communication instability. ^[2]

In serial communication, the effect of radiated interference can be reduced by shielding the communication line, and the effect of common-mode interference voltage can be reduced by differential signal transmission, but the level conversion for the protection of the device and the line designed to reduce interference Isolation is still essential. ^[2]

Application of serial communication isolation method

(1) Discrete device isolation technology

In the need of isolation design, the level-shift isolation method between devices can be implemented by using only discrete devices. The ultimate purpose of level conversion is to achieve the levels on both sides of the working unit according to their needs. The discrete device isolation method mainly uses a reasonable combination of resistors and transistors, so that the input / output levels can be matched. It is a conventional and effective method to realize the double-side level conversion by using the switching function of the MOS tube. This isolation method is generally common ground isolation, which only completes level conversion to protect the function of the device and is not electrically isolated between systems. ^[2]

(2) Photocoupler isolation technology

Photocouplers, or optocouplers for short, are a class of devices that use light as a medium to implement electrical signal transmission. Its working principle is to encapsulate the light emitter (light emitting device) and photoreceptor (photosensitive device) inside the chip, and control the light emitting device to emit light by an electric signal applied to the input end. When the photoreceptor is internally illuminated, it generates an electrical signal and drives the output. At the end, "electricity-optical-electricity" conversion is realized. Because the electrical signals on both sides of the optocoupler are completely isolated, and light is used as the transmission medium inside, the input / output of the optocoupler is insulated, which can complete the isolated transmission of the unidirectional signal, which is widely used in digital circuits. The application of the ordinary optocoupler (TLP521) in the isolation circuit is limited by the characteristics of the device. Its transmission characteristics are good at low frequencies and the distortion of high frequency signals is severe. In the actual circuit test, the 115kbp serial communication frequency can be basically adapted through circuit device parameter matching and circuit structure optimization. From Toshiba Semiconductor's optocoupler product line, it can be seen that its communication rate covers 20kbps to 50Mbps, so when high-speed communication transmission, high-speed optocouplers should be selected according to design needs. ^[2]

(3) New isolation technology

With the rapid development of products, new devices are emerging one after another. The mainstream chip vendors Texas Instruments (TI), Arnold Semiconductor (ADI) and Silicon Labs (Silicon Labs) have developed different types of digital isolators such as capacitive isolation, magnetic coupling isolation, and RF isolation. ^[2]

Capacitive isolation

Capacitive isolation uses the filling material between the capacitor plates as the insulating material as the isolation layer, and completes the signal transmission through the change of the internal electric field. TI's ISO72x series is the application of typical capacitive isolation technology. In the capacitance isolation function, the signal transmission channel is divided into "low frequency channel" and "high frequency channel". The low-frequency signal is modulated by the high-frequency carrier wave and PWM modulation generated by the built-in oscillator, and transmitted by modulation. Low-pass filtering at the output removes high-frequency carriers. The high-frequency signal is not transmitted through modulation and coding, and is transmitted directly through the isolation layer after differential conversion. The output terminal makes logical decisions based on the time relationship, so as to control the output multiplexer to output correctly. ^[2]

Magnetic coupling isolation

Magnetic coupling isolation utilizes the principle of a transformer. Signals are transmitted through the magnetic coupling between the primary and secondary coils of the transformer to achieve the isolation effect. ADI's patented iCoupler technology is based on the magnetic isolation technology of the air core transformer in the chip. ADUM series is the application of typical magnetic coupling isolation technology. ^[2]

iCoupler magnetic isolation technology realizes signal isolation through the magnetic coupling between the primary and secondary coils of the air-core transformer realized in the chip's internal characteristic size. Signal transmission uses a specific combination of short pulses to indicate high and low levels. Two consecutive short pulses indicate a high level, and a single short pulse indicates a low level. The output terminal determines the output level state according to the number of detection pulses. The refresher circuit and the watchdog circuit provide protection for the input terminal's level state and the output terminal's fail-safe state. ^[2]

RF isolation

Radio frequency isolation uses the principle of radio frequency transmission. At the transmitting end, the original signal modulation based on the high-frequency signal is completed and transmitted through the transmitting antenna. At the receiving end, demodulation of the modulated signal is performed by a demodulator to restore the original signal. Through such modulation and demodulation, the effect of isolation is achieved. Silicon Labs' RF isolation is RF isolation, and the Si84xx series is the application of typical RF isolation technology. ^[2]

RF isolation uses the ISOpro RF RF isolation principle. The chip is composed of a semiconductor RF radio frequency transmitter, a receiver, and a differential capacitive isolation barrier between the two. During work, the basic ON / OFF button (OOK function) is used. When the input data is high voltage, the transmitter generates RF modulation signal. When the input data is low level, the generator has no RF modulation signal. The modulated signal is sent to the receiver through the isolation barrier. When the receiver detects a modulation signal in the same frequency band, it is demodulated by the demodulator and outputs a high level; when there is no modulation signal, it outputs a low level. ^[2]