3-3
Section
Sharing the Communications Power Supply
33
3-3-5 Step 2: Calculating the Best Location of the Actual Nodes
Go to Step 2 if the best location for the power supply cannot be determined from
the graphs. The second method calculates the best location for each actual
node and does not estimate the worst possible configuration for the power sup-
ply.
Basically, in the DeviceNet network the permissible maximum voltage drop with-
in the system can be specified at 5 V for a power supply line (+V or –V), by calcu-
lating the specifications for the voltage of the communications power supply (24
VDC) and the input voltage of the communications power supply of each device
(11 to 25 VDC).
Of the permissible maximum voltage drop within the system (5 V), the permissi-
ble voltage drop in the trunk lines and drop lines are 4.65 V and 0.35 V respec-
tively.
Voltage Drop
In the DeviceNet network the voltage drop at the communications cables,
taking the allowance into consideration, is 5 V or less, according to the spec-
ifications for the communications power supply device (24 VDC) and com-
munications power supply for each node(11 to 25 VDC).
+V Voltage drop V
A
Voltage supplied to
communications power
supply device
Voltage supplied
to each node
–V Voltage drop V
B
V
1
V
2
V
1:
Voltage supplied to the communications power supply device. Con-
sider the ambient variations of the power supply voltage and take
V
1
to be 23 V.
V
2:
Voltage supplied to each node. Consider the allowance and take V
2
to be 13 V or greater.
V
A:
Voltage drop at the power supply cables (+V).
V
B:
Voltage drop at the power supply cables (–V).
In the DeviceNet network V
A
and V
B
is taken as 5 V or less.
The voltage drop at the communications cables is specified at 5 V for a
single power supply line (+V or –V). Of the permissible maximum voltage
drop within the system (5 V), the permissible voltage drops in the trunk lines
and drop lines are 4.65 V and 0.35 V respectively.
Note Note that when the communications power supply for each node and the internal
circuit power supply must be shared, the permissible ambient voltage of the in-
ternal circuit is lower than the permissible maximum ambient voltage of the com-
munications power supply. Also, the maximum voltage drop for a single power
supply line (+V or –V) must be taken as 1 V. Of the permissible maximum voltage
drop for a single power supply line (1 V), the permissible voltage drop in the trunk
lines and drop lines are 0.65 V and 0.35 V respectively.
Formulas
Supplying Power Independently for Communications and Internal Circuit
Calculate the distance between the power supply and each node, and the cur-
rent consumption of each node’s communications. (Refer to the DeviceNet
(CompoBus/D) Operation Manual (W267).) Try to calculate the best location for
