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Basic concepts of computer to computer communications. Models for communications links and problems with long distance communications. Cross-talk in parallel conductors. Extension of internal communications concepts to external communications (parallel communications). The Centronics and IEEE-488 (GPIB) communications ports.
3.1 Eight Fundamentals of Computer to Computer Communication In order for two, computer-based devices to communicate with one another, even at the lowest level, a number of fundamental criteria must be satisfied: (i) A "physical" link between the two devices must be established. This is referred to as the transmission "medium" and may be realised through conducting cable, optic fibre cable or through electromagnetic wave propagation at radio, micro- wave or millimetre wave frequencies. (ii) Both devices must use the same representation for binary data on the external communications link. For example, both devices may need to accept that a binary "1" is represented by voltage or current level "A" and binary "0" is represented by voltage or current level "B" on the link. (iii) Both devices must use the same physical reference level with which to interpret data on the communication link. For example, if voltage is used for signal representation then a receiving device "X" must measure voltage on the link with respect to the same reference voltage that a device "Y" uses to transmit. (iv) If transmitted bit streams represent alpha-numeric characters, then both devices must interpret the characters in the same way. For example, if one device transmits bits representing ASCII characters, then the receiver must decode the bits as ASCII characters and not as EBCDIC characters. (v) The transmitting device must actively run software that sends data through its communications port and the receiving device must actively run software that reads in data through its communications port. The software must be co- ordinated so that transmission does not occur until the receiving device is ready to handle incoming data. (vi) The transmitting device must not send out data at a rate which is too high for the receiving device to handle.
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