Filter
Top Products
45
9. Appendix
b Calculating the polarization
M Principle
You must ensure that the equivalent bus resistance is between 162
Ω and 650 Ω.
The equivalent bus resistance (Re) depends on the polarization resistance of the slaves (Rs) and the master (Rm):
If Re is too low, reduce the number of slaves.
If Re is too high, adapt the master’s polarization (if possible) or add polarization resistors (Rp).
Example 1
If the master has 470 Ω polarization and all the slaves have 4.7 kΩ polarization, a maximum of 18 slaves can be connected.
Rm = 470 Ω
Rs = 4.7 kΩ
A/Re = 1/470 + 18 x 1/4700
i.e., Re = 168 Ω
Example 2
If the bus polarization Rp is 470 Ω (installed in the master) and 2 slaves have 4.7 Ω polarization, the equivalent polarization is:
1/Re = 1/470 + 1/4700 + 1/4700
i.e., Re = 1/ (1/470 + 1/4700 + 1/4700)
and therefore Re = 390 Ω
390 Ω is between 162
Ω and 650 Ω, and the schematic is correct.
For an ideal equivalent polarization (650 Ω), the master’s polarization can be adapted so that:
1/650 = 1/Rm + 1/4700 + 1/4700
i.e., Rm = 1/(1/650 - 1/4700 - 1/4700)
and therefore Rm = 587 Ω
1
Re
--------
1
Rm
---------
1
Rs
1
----------
1
Rs
2
---------- …+++=
1
Re
--------
1
Rp
--------
1
Rm
---------
1
Rs
1
----------
1
Rs
2
---------- …++++=
1 nF
Rs
1
Rs
1
Rm
Rm
120 Ω
5 V
0 V
R
G
R
G
5 V
Rp
Rp
5 V
0 V
0 V
R
G
D1
D0
Master
Slave 1
Slave n
Common