4'4 Optical Media

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4.4 Optical Media

• Optical fibres support the CAN bus. The dominant
  bit is characterized by the presence of light
  emission and the recessive state is characterized
  by its absence.
• As regards the propagation time of an optical
  fibre, the calculation process for speeds,
  distances, propagation time, nominal bit time,
  etc. is identical to that described above.
• The main advantage of optical fibres is to resist
  electromagnetic parasitic signals.
• the problems arising with optical fibres are
  mainly due to their insertion losses as a
  function of distance and their poor capacity to
  support the 'bus' topological configuration.
• 'point-to-point' connections provides a CAN
  optical fibre gateway between two CAN wire
  systems

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a 'tractor and trailer' unit
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the implementation of a CAN network on optical
fibres
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the implementation of a CAN network on optical
fibres
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4.5.1 Radio-frequency waves

• transport CAN frames by a radio-frequency wave
  medium
• amplitude modulation with total carrier
  suppression (ASK), where the dominant level is
  considered to be the physical state of the
  presence of the HF carrier and the recessive
  level is considered to be its absence
• frequency modulation with frequency hopping, in
  such a way that, for example, Fl is considered
  recessive and detected as such and F2 is called
  dominant
• The system constructed in this way must be both a
  transmitter and a receiver, and the delay times
  used to control reception, demodulation and
  detection (for the receiver) and modulation and
  transmission (for the transmitter) must be
  compatible with the bit time

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4.6 Pollution and EMC Conformity

• the EMC problems caused by wire links, in the
  form of differential pairs (parallel or twisted)
  dedicated to CAN applications.
• The problems of ensuring EMC mainly arise when
  high-speed CAN is used
• basic diagram of the standard CAN application in
  simplified form, two CAN nodes with their line
  drivers, together with line termination resistors
  (120O) at each end of an unscreened line, not
  forgetting the essential ground return, without
  which nothing would work between the two nodes.

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basic diagram of the standard CAN application
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4.6.1 The standards

• The standards are complex (IEC 806-6) but, the
  requirements are
• for transmission the system must not radiate, to
  prevent interference with radio or mobile
  telephone reception, etc.
• for susceptibility the system must resist
  disturbance by the proximity of electrical fields
  of the order of 100-200 V m-1 in a frequency band
  ranging from continuous current to several GHz.

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The theory

• The choice of a differential pair which is
  symmetrical and also twisted provides important
  structural advantages
• - It provides freedom from numerous constraints
  concerning the presence of parasitic or transient
  signals occurring on adjacent power supply line
  as these affect both wires of the bus
  simultaneously, they will not degrade the data
  signal.
• - The active signal (the transmitted data)
  develops on either side of the constant mean
  level with equal and opposite values at all
  times.
• the immunity of this transport medium is
  essentially determined by what is known as its
  'common-mode' performance.

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The reality

• in fact, signals have rise times which are never
  exactly equal and complementary to the signal
  developed on the other wire. Consequently, there
  is a difference signal which may produce
  electromagnetic pollution which must be allowed
  for and/or cancelled if possible.

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Description of the equivalent diagram

• Each node contributes to the radiation with two
  emission sources. The diagram shows
• Vemel, which is the voltage source equivalent to
  the pollution contributions due to the additional
  components of this node. This source is included
  in the ground connection of the node in question
• Vceml, which represents the voltage equivalent to
  the emission which is produced by poor
  compensation between the voltages developed on
  the CAN_L and CAN_H wires, with respect to the
  reference potential (ground) of the data
• Vdist, which represents the voltage equivalent to
  that due to the effect of the power of the
  electromagnetic interference captured by the
  CAN_L and CAN_H wires.

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4.6.2 Measurements and results of measurements

• In Common mode
• The common mode signal rejection is by far the
  most important criterion.
• 82C250
• This circuit, normally supplied at 5 V, has a
  common mode rejection voltage range from -7 to
  12 V, corresponding to the equivalent injection
  of a sinusoidal signal of 19 peak to peak, i.e. a
  sinusoidal voltage of approximately 7 V effective
  on the bus!

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Symmetry of the output signals

• The output stages of the transmission interface
  must be as symmetrical and simultaneous as
  possible, in order to reduce to a minimum the
  voltages present during the phases of switching
  from recessive to dominant and vice versa.
• In practice, this perfect complementarity is
  almost achieved because the output stages consist
  of complementary transistors whose performance is
  adjusted by means of carefully applied physical
  differences (optimal choice of conductivity and
  mobility of the type of free minority and
  majority carriers of the semiconductors used).

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Rise times of signals

• excessively high speed of the edges of the
  signals used can be resolved by optimizing the
  slew rate as a function of the loop and the
  chosen speeds.
• provision is made for the continuous adjustment
  (calculation below) of the signal slope (by
  increasing the rise time), while maintaining the
  speed, the precision of the sampling point, the
  length and the type of cable used.

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Example of calculation of the slew rate (dV/dt)

• the value of the resistor Rs to be connected
  externally between this pin and the ground. The
  relation between these parameters is then
• Sr is the slew rate to be obtained.
• As the manufacturer states that a slew rate of 14
  µs V-1 is obtained for a resistance Rs of 47kO,
• After calculation, the value of this constant k
  is 3.8 x 10-3 µs kO-1.

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4.6.3 Numerous consequences and conclusions if
problems arise

• In general, all these recommendations enable us
  to use parallel pairs with a ground return
  forming part of the connecting cable.
• For specific problems, in order to reduce even
  further the effect of undesired signals in common
  mode, it may be necessary to use twisted pairs
  plus a separate ground wire or to twist all three
  together.

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Making the output signals symmetrical

• connecting a balancing network for the CAN_H and
  CAN_L lines at the output of the driver stages.
• For this purpose, each of the adaptation
  resistors at the line ends (both 120 O) can be
  divided into two series resistors of 60 O, and
  their midpoints are connected to the AC ground by
  means of a capacitor

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Smoothing inductance

• two (very small) surge inductances, coupled
  together as required, can be used to eliminate
  the flow of current in the same direction on both
  outputs of the bus.

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a single diagram the main solutions used
conventionally to resolve most cases
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4.6.4 Results

• For immunity
• The graph shows how this network withstands an
  injection of parasitic signals on the bus at
  levels up to 20 V effective

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For radiation

• The three graphs in Figure 4.54 show the effect
  of the different components on the non-radiating
  quality of the network
• without slope control, without coil
• with slope control
• - without coil
• - with coil (Figure 4.55).
• One last comment on high speeds, the slopes used
  to reduce dV/dt, dI/dt, etc. of the signal and to
  reduce the EMC levels.

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4.6.5 Conclusion

• what is expected of a normally constructed CAN
  line driver, in terms of managing line short
  circuits, bus line faults, partial networks, low
  and very low power consumption, local and remote
  wake-up, recognition of wake-up modes, management
  of diagnostics, good EMC performance (in terms of
  immunity and susceptibility) and ESD resistance,
  compatibility with all the 3.3 V, 5 V and other
  microcontrollers in the market and resistance to
  heat stress.