Schneider Electric VW3 A3 303 用户手册
45
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).
If Re is too high, adapt the master’s polarization (if possible) or add polarization resistors (Rp).
Example 1
If the master has 470
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 k
Ω
A/Re = 1/470 + 18 x 1/4700
i.e., Re = 168
i.e., Re = 168
Ω
Example 2
If the bus polarization Rp is 470
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
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
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