Serial port

The serial interface is a colloquial term for an interface for data transmission between two devices in which individual bits are transmitted one after the other in time (serial data transmission). The term is used in distinction to a parallel interface, where several bits are transmitted simultaneously on several circuits. Both terms are imprecise and are used for a large number of different interface definitions.

Compared to parallel interfaces, the serial interface requires fewer circuits and consequently less cabling, which is particularly significant for longer distances. Parallel interfaces historically offered greater transmission speeds and were therefore preferred for applications where large amounts of data had to be transmitted over short distances, where cable costs did not dominate. However, this argument is now obsolete; the technical development of serial interfaces has overtaken that of parallel interfaces.

In the narrower environment of personal computers, the serial interface usually means an interface according to EIA-RS-232, usually using a 9-pin D-Sub connector. It is traditionally used there to connect peripheral devices to the PC that manage with comparatively moderate transmission speeds, such as keyboards, mice, printers, dial-up devices, etc., but has now been largely replaced by the more universal USB interface. Although the USB interface also works serially, it is usually not meant colloquially when one speaks of "the serial interface". There are inexpensive adapters between USB and RS-232, so that most PC manufacturers have started to do without the RS-232 interface in the basic device.

Serial interfaces in the broader sense are very widespread and exist in many forms and variants. In addition to point-to-point connections such as RS-232 and RS-422, they also include network and bus interfaces such as Ethernet, CAN bus or RS-485. Data rates, possible transmission distances and other properties differ considerably in some cases, so there is a suitable interface definition for almost every area of application. See the list below for more information.

Serial interfaces differ not only in the connector used and the electrical transmission parameters, they also use different methods for transmission control, data flow control and synchronization (see communication protocol). They can operate in one direction ("simplex") or in both directions ("duplex"), the latter either alternately ("half-duplex") or simultaneously ("full-duplex") (see Duplex (communications technology)).

D-Sub connector of a serial interface (RS232, 9-pin)Zoom
D-Sub connector of a serial interface (RS232, 9-pin)

Standards

There are several standards that specify serial transmission modes. These include:

  1. Original interface standards:
    • EIA-232 (RS-232, V.10) - serial interface introduced in 1962
    • EIA-422 (RS-422, V.11) - differential transmitters and receivers
    • EIA-423 (RS-423, V.10) - asymmetric interface similar to RS232
    • EIA-485 (RS-485, V.11) - differential transceivers (also incorrectly referred to as partyline)
    • Partyline - differential transmitters and receivers each with additional ground wire (6-wire)
    • Low Voltage Differential Signaling (LVDS, EIA-644) - see RS-422
    • TTY interface (current-loop)
  2. Communication mainly at board level:
    • I²C-Bus - For communication within a device, with a data and a clock signal, e.g. for telephone cards.
    • I²S bus - For the exchange of audio data (Inter-IC Sound interface) e.g. ADC → signal processor → DAC
    • Serial Peripheral Interface (SPI) - For communication within a device, with data and clock signal e.g. serial flash memory
    • 1-Wire bus - For communication within one device or between several proprietary devices via one data line (e.g. MDE, PDA and battery)
  3. Communication within a device:
    • Serial ATA (S-ATA) - for connection to PC hard drive
    • Serial Attached SCSI (SAS) - for connection to SAS hard drives
    • M.2 connection for mini-SSDs on motherboards
    • PCI-Express - new PC standard, replaces the parallel data buses AGP and PCI
  4. Communication in the range of 1 to 10 meters:
    • FireWire (i.LINK or IEEE 1394) - e.g. for hard disk, audio and video data
    • Universal Serial Bus (USB)
    • PS/2 - Mini-Din connector (formerly DIN) for mouse and keyboard on PCs
    • CBM bus - a bus adapted bit-serially by Commodore Business Machines on the basis of the IEC 625 bus.
    • Digital Visual Interface (DVI) - Transmission of video data
    • High Definition Multimedia Interface (HDMI) - Digital transmission of audio and video data
    • RC5 code - Infrared transmission standard for remote controls
    • Infrared Data Association (IrDA) - Standard for IR communication between laptops, mobile phones, etc.
    • Bluetooth - radio network standard for smaller distances for headset, mobile phone, laptop
    • Unilink - proprietary vehicle infotainment interface developed by Sony
    • MIDI - for musical instruments
  5. Distance up to 100 meters:
    • S/PDIF - for audio and musical instruments
    • To Host - see also MIDI, MP3
    • DMX - stage technology, lighting
    • Sony/Philips Digital Interface (S/PDIF, TOSLINK) - for consumer audio data
    • AES/EBU (AES3) - for audio data in the studio area, see also S/PDIF
    • LIN bus (Local Interconnect Network) for networking sensors and actuators
  6. More robust bus systems:
    • Controller Area Network (CAN bus) - Control technology, automotive engineering
    • Profibus - Industrial bus for automation technology from Siemens
    • Profinet - Ethernet-based industrial bus for automation technology from Siemens
    • DIN measuring bus - field bus for automation technology
    • Multi Point Interface (MPI) - for automation technology from Siemens
    • Interbus-S - Fieldbus for automation technology from Phoenix Contact
    • BITBUS - Fieldbus IEEE 1118
    • Modbus - Fieldbus between devices
    • Partyline - Fieldbus for terminals for production data acquisition from INCA
    • Local Operating Network (LON) - also automation technology
    • S0 bus - terminal interface for ISDN (digital wide area network; telephony)
    • Ethernet - Networks, Internet
    • Wireless LAN (WLAN) - local radio network IEEE 802.11
    • FlexRay - Automotive engineering
    • AS-Interface - Fieldbus for connecting actuators and sensors
    • EtherCAT - Fieldbus for automation technology (initiated by Beckhoff)
  7. Data buses in the field of avionics:
    • ARINC 429 (Aeronautical Radio Incorporated) - Classic data bus for commercial aircraft
    • ARINC 629: data bus for civil aircraft (Boeing 777), further development of ARINC 429
    • ARINC 664: Ethernet protocol AFDX for aircraft
    • ARINC 825: CAN-based data bus for Airbus and Boeing
    • AS6003: Time-Triggered Protocol implements fault-tolerant time-triggered communication
    • AS6802: TTEthernet (Time-Triggered Ethernet) time-controlled communication based on Ethernet

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