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Higher Computing. Computer Systems Interfaces. Introduction . All peripherals connect to the input/output bus via slots in the back of the computer. All peripherals work at different operating speeds, have different languages and work with different amounts of data at a time.

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higher computing

Higher Computing

Computer Systems


  • All peripherals connect to the input/output bus via slots in the back of the computer.
  • All peripherals work at different operating speeds, have different languages and work with different amounts of data at a time.
  • Interfaces are used to allow the computer system to communicate with the peripherals and compensate for the differences between them.
interface functions
Interface Functions
  • Voltage conversion
    • Often peripherals work at different voltages to the computer.
interface functions1
Interface Functions
  • Buffering
    • A buffer is a small amount of memory where data is stored while the peripheral deals with it.
    • Most peripherals cannot process data as quickly as the CPU so, to allow the CPU to get with something else, the data sits in the buffer until the peripheral can deal with it.
    • Printers in particular require buffers.
interface functions2
Interface Functions
  • Transmits and receives status/control signals.
    • For example, if a printer is out of ink or paper, the disk in an external disk drive is not being read,…
    • Telling the CPU whether or not the peripheral is read to send/receive information or not.
interface functions3
Interface Functions
  • Protocol conversion
    • A protocol is a standard that enables communication between computer systems.
    • The interface ensures that the protocol used by a peripheral can be understood by the computer system and vice-versa.
interface functions4
Interface Functions
  • Data format conversion
    • Serial/parallel data conversion.
    • Data is either sent in parallel (several bits at a time) or in series (one bit after another).
    • The interface may have to allow conversion from one to the other.
parallel data transmission
Parallel Data Transmission
  • Faster than serial transmission but only suitable over short distances as over long distances skewing can occur.
  • Example 1 - Communicating with a Printer
    • A printer is connected to a computer using a standard, Centronics parallel interface. What follows is the sequence of events to transfer data to the printer.
    • 1. The interface puts data on the parallel lines.2. Signals the byte is ready to be transmitted.3. If ready, the printer reads the byte transmitted.4. Printer sends back an acknowledge signal to the CPU.5. Interface prepares the next byte.
  • A parallel interface may also contain status wires used to signal events.
serial data transmission
Serial Data Transmission
  • Bits sent one at a time.
  • Start bit set first to let the system know that data is to be sent.
  • Parity bit sent at the end to ensure data is correct.
  • Finally a stop bit is sent.
  • Two types of transmission:
    • Asynchronous – bit sent as soon as a byte is available.
    • Synchronous – transmission occurs in rime with the clock pulse.
serial data transmission1
Serial Data Transmission
  • Example 1 - Communicating with a mouse
  • As there is no pressing need for speed, a mouse is connected to a computer using a serial DB9 or DB25 connector. A universal serial bus (USB) may also be used. A USB interface is a typical port used to connect many devices.
  • 1. A single data bit at a time sent to the CPU - typical rate of 1200 bits per second.2. The interface converts this stream of bits to parallel to send on system bus.3. Once the byte ready interface asserts an interrupt request.4. CPU acknowledges request and interface places the byte on the system bus.
  • Parity is a protocol used to ensure that data transmission is error free.
  • There are two types:
    • Even parity
    • Odd parity
  • The parity bit is such that there is an even (for even parity) number of 1’s in the data transmission.
  • (Odd parity works in the opposite way).
international standards
International Standards
  • A number of international standards have been developed for interfaces. These include:
    • RS 232 – Recommended Standard (serial)
    • SCSI – Small Computer System Interface (parallel)
    • USB 1 & 2
    • IEEE – Institute of Electrical and Electronic Engineers (Apple Firewire 400 & 800)
  • Interfaces worksheet.