3-3
Section
Sharing the Communications Power Supply
30
Note If power supply specifications cannot be met with a single power supply when
the current capacity of the thick cable exceeds 8 A even after the power supply
location is modified, use more than one communications power supply. In con-
figuration 1. the power can be supplied bi-directionally to the trunk line as long as
each current capacity is 8 A or less when using thick cable. Accordingly, it is pos-
sible to have a configuration with a total maximum current capacity of up to 16 A.
Consider changing to thick cable to meet specifications if the current capacity of
the thin cable exceeds 3 A when using thin cable for the trunk line.
Determine whether or not the current can be supplied normally by finding the
current capacity required by each node and the voltage drop in the cables to be
used to provide power. Calculate the values below in advance.
•
The current capacity required by each node
•
The distance between the power supply and each node
The current capacity of the DeviceNet Interface Unit is 45 mA.
There are two methods to find the best location of the communications power
supply on the trunk line.
•
Simple calculation from a graph
•
Calculation by formula (Calculating the voltage drop from resistance and cur-
rent consumption of the communications cables).
Each drop line must satisfy the equation on page 27, which represents the rela-
tionship between the drop line length and the current capacity for the drop line.
•
From the graph, a hypothetical power supply location can be determined if
the conditions calculated in the graph are met by estimating the worst con-
figuration (that has the maximum voltage drop as shown in the diagram
below).
Communications
power supply
Node
Node
Node
Node
•
Even if the power supply specifications cannot be met using the graph, the
conditions can be met and a hypothetical power supply location deter-
mined by using the formula.
Note When the communications power supply and the internal circuit sup-
ply are the same, use the formula to calculate a hypothetical power
supply location because it cannot be determined by using the graph.
3-3-4 Step 1: Determining the Best Location for the Power Supply from
a Graph
A voltage drop occurs when a current flows through a communications cable.
The longer the communications cable, the greater the voltage drop. The com-
munications power supply at each node must be 11 VDC or more. To ensure this,
the relationship is plotted as shown in the graph below to find the maximum cur-
rent that satisfies the voltage of the communications power supply at different
trunk line lengths even if there is a voltage drop due to cable resistance.
Setting the Location of
the Power Supply
Calculating the Location
of the Power Supply
