PROFIBUS Manual
Electrical signals pass along the length of the lines at an ultimate speed of approx. 2/3 the speed of light. Per metre, therefore, they take approx. 5 ns. A cable measuring 1200 metres in length will therefore be travelled in approx. 6 µs.
Electrical energy cannot simply disappear at one end of the cable, but comes back again. This effect is called reflection. The reflected signal overlies the outgoing signal and can cause interference to the original signal. Reflections also occur for part of the signal output if there are changes to the properties of the line, e.g. if two different cables are switched together. These reflections in principle occur with all bit rates. However, they differ in effect. At high bit rates, i.e. for short bit times, the effect is more serious.
Effect of reflections for low and high bitrate
Signal with almost no reflections
Signal with too many reflections
As a counter measure, line termination resistors are used. The job of line termination resistors is to receive the energy of an electrical signal and so prevent reflections at the end of a cable. The best results are obtained when the termination resistor is equivalent in size to the cable's own wave impedance. Such termination must be applied to both ends of the cable.
The bus line can be modelled with inductance per unit length L' and capacitance per unit length C'. From these two values it is easy to determine the wave impedance, if frequencies are sufficiently high:
Every connector, such as a plug or terminal, represents a capacity and so increases the capacitance per unit length on the line. This changes the wave impedance and so leads to greater or smaller reflections. For this reason, PROFIBUS specifies maximum connector capacities and maximum lengths for stub lines.
