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6 Further System Fundamentals (HL)

6.5 Computer – Peripheral Communication

6.5.1 Ports and Handshaking

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Ports

•A port is a point of data entry or exit on a computer e.g. where a peripheral is connected.

•You connect the printer or mouse via a USB port these days – it used to be a 9-pin serial port for the mouse and a (25 or 36 pin) parallel port for the printer.

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Serial v. parallel

•Serial = single bit stream.

•Parallel = many bits at once.

•Parallel seems faster logically but needs much better coordination, so there are serious limitations on the distance over which a parallel cable can operate.

•Most ports are serial these days e.g. USB.

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Logical ports•Data from the internet enter a computer via

a physical port e.g. ethernet, but are also assigned a logical port i.e. a designated channel.

•Certain services are assigned specific port numbers e.g. although it all passes physically through the modem, http traffic usually uses port 80, out-going emails port 25, etc.

•These ports can be blocked or redirected.

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Handshaking

•This is the way signals are exchanged between two devices when they first start to communicate, establishing things like:

‣protocol to be used,

‣what the control signals are,

‣ transmission rate, etc.