Civil Engineering Reference
In-Depth Information
Fig. 2.27
Simplified equivalent circuit for calculating the maximum allowed cable length at a
point-to-point connection
with NBT = nominal bit-time, t
RD
= propagation delay recessive-to-dominant,
NBT
(
)
(
)
1 ∆
= “fast” clock and
NBT
= “slow” clock.
⋅ −
f
⋅ +
1 ∆
f
Furthermore, it shall be considered that the evoked time gap, caused by the prop-
agation delays, and the different fast-running clocks shall be smaller than the SJW
to avoid a drifting of the clocks over longer time spaces.
2.2.3.4
Limitations of the Cable Length in Consideration of the Line Losses
In the following, static influences are analysed which impact the cable length and
signal quality. The maximum possible cable length is limited mainly by the resis-
tance per unit length and capacitance per unit length and the number of implement-
ed nodes. Through an implementation of too many nodes containing very high dif-
ferential capacitances, bit deformation may occur. The dominant bit shape becomes
close to a capacitive charging shape (slope to dominant) respectively discharging
shape (slope to recessive).
Latest, at the sample point position of a dominant bit, the differential voltage shall
have (even in worst case) a voltage of 0.9 V or more to guarantee the communica-
tion. However, the recessive bit shall have at the same position a differential voltage
of less or equal to 0.5 V. The worst-case consideration with the maximum/minimum
thresholds of 0.9 respectively 0.5 helps here again in analysing the influences of
resistances of cable, transceiver and bus connecting circuitry. Abstractions to simple
equivalent circuits using resistances help the right dimensioning of the network.
At a simple point-to-point connection, the differential voltage of the sender acts
as a simple voltage source. Parallel to this voltage source is located the termination
resistance. The receiver is to substitute by the second termination resistance and the
differential resistance of the receiver stage. The cable length-dependent cable resis-
tances connect the sender and receiver equivalent circuit. As mentioned above, the
basic requirement making a communication possible is that at the receiver side the
differential voltage shall be equal to or higher than 0.9 V. Therefore, the equivalent
circuit shown in Fig.
2.27
can be used for calculations.
