When data is transmitted serially over distances greater than a few metres the 5v TTL logic system is found to be inadequate, and typically a line driver is needed.
Line driver ICs improve data transmission reliability and noise immunity.
Some common physical layers used for serial communications are EIA-232 and EIA-485.
EIA-232 sends signals using a voltage level often larger than the IC supply voltage, this requires the line voltages to be generated by an internal charge pump. Receivers for unbalanced transmission set a receive threshold that is relative to a common reference (ground)
For balanced transmission, such as EIA-485, smaller vltages are often used on the line (e.g. 0/5V). Receivers for balanced transmission, use a threshold measured as the difference between the two receive lines.
A line receiver often implement hystersis to reduce the effect of varying signal levels. For example a thresholds of 200mV hysteresis establishes two thresholds 200mV apart:
In simplex communications links a driver operates either as an output at the sender or as an input at a receiver. When half-duplex mode is used, the driver needs to alternate between input and output modes.
Suppose we wish to transmit the byte "10110010" over a communications link, then we may perhaps expect the signal corresponding to the byte to be distorted a little as it travels along the cable.
In fact, there are many different ways a byte may be distorted, some of which are represented below at three points along the cable. The crucial question is: "Will the receiver be able to recognise the signal it receives and associate it with the transmitted bit sequence?"
If the byte is correctly interpreted by the receiver at the end of the cable, we say that we have achieved error-free communication. If however, the receiver sometimes makes mistakes in interpreting the bits received, we say that bit errors have been introduced by the link.
As it propagates along the cable, the signal will be subject to attenuation - due to signal loss (e.g. the resistance of the cable per metre); distortion - due to the capacitance of the cable (acting as a filet that attenuates higher frequencies) and noise/interference - this reduces the ability of the receiver to discriminate the baud symbols from other extraneous signals (the less energy per baud, the harder it is to discriminate).
The easiest effect to imagine is that as the signal travels along the cable, some of the signal fades away. In effect, the signal gets weaker, and weaker, the further it has to travel. This is known as attenuation.
Often the attenuation which is observed is not linear. If we view the frequency spectrum of the signal which was originally sent, we would see that some parts of the spectrum are attenuated more than other parts. This causes the shape of the received signal to change as it passes down the cable. In some cases, the level of distortion may be predicted (e.g. it may be a function of the type of cable used and proportional to the length of the cable). If the level of cable distortion is known, it cou;d perhaps be compensated for by applying non-linear amplification (this is known as signal equalisation).
Attenuation and (predictable) distortion alone would not be a significant problem for error-free communication. Expressed in decibels (dB), loss of signal happens along the length of any cable. It is a natural phenomenon that occurs for any type of transmission: electrical power or network data. The longer the cable, the greater the loss. Loss also occurs at any connection points along the cable such as connectors. To provide a successfull link, the received signal needs to be above the receiver threshold for reliable communications.
All communications systems also experience random noise which appears at the receiver mixed with the received signal. The use of amplifiers along the length of the cable can help. As the signal becomes attenuated, it approaches the level of noise (i.e. it becomes hard to differentiate the signal from the background random noise). Simply amplifying the signal will also amplify the noise, which also will grow with each stage of amplification.
The key to transmission is not the only to use regenerative digital repeaters. A repeater amplifies the received signal and then digitally samples the signal to decode the bits being sent (i.e. recognises each bit in turn). Once decoded, the digital bits may be sent a fresh down the cable to the next repeater, regenerating a signal at the output of the repeater which is as good as the original sent signal. By using repeaters at suitable points along the cable (before the signal deteriorates below the level at which it may be reliably decoded), arbitrary long distances may be reached with little probability of bit errors.
See also:
Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland. (2018)