3-3
Section
Sharing the Communications Power Supply
34
each node using the formula below. If the best location for each node can be de-
termined using the formula, the specifications for the power supply to each node
can also be met. Do not exceed the maximum current capacity of the cable
(Thick cable: 8 A and thin cable: 3 A).
Condition Formula 1 (For the Voltage Drop of the Trunk Line)
(Ln x Rc = Nt x 0.005) x In
4.65 V
Ln:
The distance between the power supply and the nodes (not includ-
ing the lengths of the drop lines)
Rc:
Maximum cable resistance
(Thick cable: 0.015
Ω
/m, thin cable: 0.069
Ω
/m])
Nt:
The number of taps between each node and the power supply
In:
The consumption current required for the communications power
supply for each node
0.005
Ω
= The contact resistance of the taps
Supplying Shared Power for Communications and Internal Circuit
The permissible ambient power supplies of the communications and the internal
circuit are as shown below.
Note Make every effort to avoid sharing the communications power supply and the
internal circuit power supply. (Refer to 3-7 Sharing the Same Power Supply for
details.)
•
Permissible ambient voltage of the communications power supply:
11 to 25 VDC
•
Permissible ambient voltage of the internal circuit power supply:
24 VDC from +10% to –15%
The lowest permissible ambient voltage is 11 VDC for the communications pow-
er supply, whereas it is 21 VDC including the allowance for the internal circuit
power supply. Due to this difference in lowest permissible ambient voltages,
when the internal circuit power is supplied by the communications power supply,
the permissible ambient voltage drop at is lower than if the power supplies were
separate. When the output voltage ambient fluctuation of the communications
power supply has a lower limit of 23 V, the permissible maximum voltage drop of
a single power supply line is (23 V – 21 V) /2 = 1 V. Of the permissible maximum
voltage drop of a single power supply line (1 V), the permissible maximum volt-
age drop of trunk lines and drop lines are 0.65 V and 0.35 V respectively.
Calculate the distance between the power supply and each node and the total
current consumption of each node in the communications and the internal cir-
cuit. (Refer to the DeviceNet (CompoBus/D) Operation Manual (W267).)
Try to calculate the best location for each node using the formula below. If the
best location for each node can be determined using the formula, the specifica-
tions for the power supply to each node can also be met. Do not exceed the max-
imum current capacity of the cable (Thick cable: 8 A and thin cable: 3 A).
Condition Formula 2 (For the Voltage Drop of the Trunk Line)
[(Ln x Rc = Nt x 0.005) x In]
0.65 V
Ln:
The distance between the power supply and the nodes (not includ-
ing the lengths of the drop lines)
Rc:
Maximum cable resistance
(Thick cable: 0.015
Ω
/m, thin cable: 0.069
Ω
/m])
Nt:
The number of taps between each node and the power supply
In:
The consumption current required for the communications power
supply and the internal circuit power supply for each node
0.005
Ω
= The contact resistance of the taps
