How do parallel and serial interfaces differ?


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Serial and parallel transmission

A basic distinction is made between the serial and the parallel principle when transferring data. Serial means that information is sent bit for bit one after the other over a data channel. Parallel means that a certain number of data channels are used simultaneously for transmission.

When characters, e.g. ASCII, are transmitted, a serial transmission means that the individual bits of the character are sent bit-serially over a data channel. If whole bytes of characters are transmitted in parallel over 8 lines, but the characters are sent one after the other, one speaks of a byte serial transmission. Byteserial transmission is a special case of serial and parallel transmission; it is used for the IEC bus (see Section 8.4).

Serial transmission is widespread, since in principle only one pair of lines (signal and return or ground line) is required, because only one data bit is transmitted at a time. Large distances can thus be bridged inexpensively, whereas with parallel transmission the costs for the cable increase with each additional data channel. Since the individual bits are sent one after the other over the line, the serial transmission is relatively slow.

Serial data format: The interpretation of the received data represents a further problem in data communication. In order to receive and interpret the characters correctly, synchronization methods for timing, which are based on a certain data format, were developed. The data format specifies the form in which characters are sent. There is an asynchronous and a synchronous data format. The data is determined on the line by a defined valid voltage value (level), the duration of which determines the transmission speed. This must be the same on the recipient and sender side.

Asynchronous data format: A character that is to be transmitted is embedded in a start bit and one or two stop bits. The start bit indicates the beginning of a character; the stop bits indicate the end of a character and give the recipient the opportunity to adjust to the next character. This format supports sending character by character (see also Section 8.3).

Synchronous data format: This format combines several characters into a block. A block always begins with a synchronization byte and ends with a special end character. The individual bytes are transmitted without any further identification. For synchronization, a clock pulse is sent parallel to the block, which specifies a time grid that identifies the data as valid.

Control of data transfer: In addition to the synchronization of the transmission of characters, protocols are also required that control the status and the sequence of the transmission process. The protocols start and end a transmission, determine the direction and report errors. Control characters are required for a transmission protocol.

ASCII control characters: Certain ASCII characters are used in transmission protocols. The control characters of the ASCII character set can be divided into the following classes:

  • Transmission: ACK (Acknowledge) confirms error-free receipt. EOT (End Of Transmission) indicates the end of the transmission.
  • Formatting: You control the form of presentation at the recipient. LF (Linefeed) causes a line feed. CR (Carriage Return) resets to the beginning of the line. HT (Horizontal Tabulator) moves to the next tabulator mark, if set.
  • Device control: Control characters DC1, ..., DC4 are interpreted differently depending on the end device.
  • Code extension: Creation of additional characters and control commands. SO (Shift Out) and SI (Shift In) for upper and lower case, ESC (Escape) to define control characters (Escape sequences).
  • Other: Acoustic signal BEL (Bell) and DEL (Delete) to delete the previous character.

Handshake procedure: Handshake procedures are used to control the data transfer between two devices. The main reason for such handshakes is the often different processing speed of e.g. computer and device. This allows the devices to signal their readiness to exchange data.

  • Hardware handshake: Electrical signals between the devices control the flow of data. The signals are specified for interfaces, with certain signal lines being used.
  • Software handshake: A program on both interfaces controls the data transfer. One example is the XON / XOFF protocol of a V.24 interface.


Next page:The Centronics interface Upwards:Transmission of data Previous page:Operating modes & nbsp content Lars Tornow 2003-04-02