Serial data transmissions transmit digital data autonomously on one line (or on one pair of lines). In contrast, in parallel data transfers, data is transmitted synchronously over multiple lines. The fundamental difference is that you don’t have to take into account runtime differences between different lines for serial transmissions, which allows for much higher clock speeds.
The name serial is associated with misconceptions, since in principle all data transmission works serially. A better name is bit-serial data transfer (as opposed to the byte-serial transmission), but this name also evokes false associations, since even serial data transfers can use several lines in parallel (e.g. PCI Express, Gigabit Ethernet, HDMI) and more complex modulations can be used that no longer know single bits (PCI Express, USB 3.0, USB 3.1, SATA, Ethernet from Fast Ethernet).
Various serial interfaces are defined for serial data transfers. These include connectors, voltages, modulations, protocols used, and software interfaces.
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Nowadays, with a few exceptions (DDR RAM connection to CPUs and legacy interfaces), only serial data transfers are used. Even ultra-fast ADCs use serial ports (with special output circuits) to output the converted data. In the past (until the 1990s), serial interfaces were used for slower data transfers (up to approx. 10 KByte/s) over longer distances (a few hundred meters), parallel interfaces for faster transmissions (up to 1 MByte/s) over shorter distances.
Serial Communication Protocols
There are several serial communication protocols, each with its own specifications and characteristics. Some of the most common ones include:
- RS-232: RS-232 is a standard for serial communication transmission of data between computers and peripheral devices. It uses voltage levels to represent binary data, with logic levels typically ranging from -15V to +15V.
- RS-485: RS-485 is a differential serial communication standard that allows multiple devices to communicate over long distances (up to 1200 meters) at high speeds. It uses differential signaling to transmit data, providing greater noise immunity and longer range compared to RS-232.
- UART (Universal Asynchronous Receiver-Transmitter): UART is a hardware component commonly used for asynchronous serial communication. It handles the conversion between parallel data from a microcontroller or CPU and serial data for transmission over a communication line.
- SPI (Serial Peripheral Interface): SPI is a synchronous serial communication protocol commonly used for communication between microcontrollers and peripheral devices such as sensors, displays, and memory chips. It utilizes four wires for communication: MOSI (Master Out Slave In), MISO (Master In Slave Out), SCK (Serial Clock), and SS (Slave Select).
- I2C (Inter-Integrated Circuit): I2C is a serial communication protocol developed by Philips (now NXP Semiconductors) for communication between integrated circuits. It uses two wires for communication: SDA (Serial Data Line) and SCL (Serial Clock Line).
Serial Communication Basics
Serial communication involves two main components: a transmitter and a receiver. The transmitter sends data, one bit at a time, over a communication line, while the receiver receives and interprets the transmitted data. The following are key concepts in serial communication:
- Baud Rate: Baud rate refers to the speed at which data is transmitted over a serial communication line, measured in bits per second (bps). Common baud rates include 9600, 19200, and 115200 bps.
- Data Bits: Data bits represent the number of bits used to transmit each character of data. Common data bit configurations include 7-bit, 8-bit, or 9-bit data.
- Parity: Parity is a simple error-checking mechanism used in serial communication to detect transmission errors. It involves adding an extra bit to each character of data to make the total number of bits either even (even parity) or odd (odd parity).
- Stop Bits: Stop bits indicate the end of a data frame and provide synchronization between the transmitter and receiver. Common stop bit configurations include one stop bit (1), one and a half stop bits (1.5), or two stop bits (2).
Features
The following is a list of some terms or characteristics that can basically be assigned to any serial transmission standard. Furthermore, a distinction is made between the properties of the physical interface, hardware and the protocols.
- Connector design, pin assignment
- Differential (balanced) transmission or non-differential transmission
- Voltages, currents, impedances, terminating resistors, wavelength (for optical transmissions)
- DC voltage component, galvanic isolation
- Unidirectional: Simplex, Bidirectional: Half-Duplex, Full-Duplex
- Line coding or modulation
- Self-clocking or with additional clock signal
- Number of transmission channels, per direction
- Hardware or software handshake
- Transmission error handling: parity, CRC, Hamming distance, etc. (see coding theory)
- Point-to-point (P2P) or multipoint (serial bus) connection
- Arbitration: Multimaster or Masterslave
- Real-time capability: e.g. required for fieldbuses
- Arbitration for data buses: priority control via tokens, CSMA, etc.
There are various serial port standards that can be used for serial transmission.
