Step 2: Evaluating The Configuration With Calculations - Omron DEVICENET Operation Manual

Step 2: Evaluating the Configuration with Calculations

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Step 2: Evaluating the Configuration with Calculations

96
Proceed with this step if the best location for the power supply cannot be
determined from the graphs. Unlike the graph method used in step 1, the cal-
culation method described in this section does not assume a worst-case
power supply configuration.
In DeviceNet, the maximum permissible voltage drop in one power supply line
(+V or − V) is specified as 5 V based on the specified communications power
supply voltage (24 V DC) and the input voltage of the communications power
supply at each device (11 to 25 V DC). The evaluation in this step is based on
the maximum 5 V voltage drop.
The maximum permissible voltage drop in the whole line is 5 V, with the maxi-
mum permissible voltage drop in the trunk line specified as 4.65 V and the
maximum voltage drop in a branch line specified as 0.35 V.
Understanding the Voltage Drop
The following diagram shows the voltage drop effect due to the cable. In the
DeviceNet network, the voltage drop over a communications cable is 5 V or
less (allowing some extra margin), based on the specifications for the
communications power supply device (24 V DC) and communications power
supply at each node (11 to 25 V DC).
V1
Voltage output at the
communications
power supply device.
V : Voltage supplied at the communications power supply device. Allow for
1
ambient variations of the power supply voltage and take V
V : Voltage supplied at each node. Allow an extra margin and take V
2
13 V or greater.
V : Voltage drop at the power supply cable (+V).
A
: Voltage drop at the power supply cable ( − V).
V
B
In the DeviceNet network, V
The voltage drop in the communications cables is specified as 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 drop in the trunk line is 4.65 V and
the permissible voltage drop in a branch line is 0.35 V.
Note The explanation above is for the communications power supply only. When a
single power supply must be used to supply both the communications power
supply and the internal circuit power supply, the maximum voltage drop for a
single power supply line (+V or − V) is just 1 V because the specifications for
the internal circuit power supply are much stricter.
Of the permissible maximum voltage drop for a single power supply line (1 V),
the permissible voltage drop in the trunk line is 0.65 V and the permissible
voltage drop in a branch line is 0.35 V. (Refer to page 97 for details.)
Voltage drop of V in the V+ line
A
in the V − line
Voltage drop of V
B
≤ 5 V and V ≤ 5 V.
A B
Section 3-5
Voltage supplied
V2
at each node.
to be 23 V.
1
2
to be