1 6 Further System Fundamentals (HL) 6.5 Computer – Peripheral Communication

6.5.1 Ports and Handshaking

3 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.

4 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.

5 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 s port 25, etc. These ports can be blocked or redirected.

6 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.