Wire lengths of AC and DC discrete signals on field side of
I/O modules.
Purpose
To explain what determines the length of wire that can be used on the field
side of I/O modules.
Question
How long can the wire be from an I/O module to a field device? Why doesn’t
Modicon specify that length of wire?
Units affected
All discrete I/O modules for Modicon’s 200,500,800 series I/O platforms as
well as Quantum’s I/O.
IR drop
How does IR drop affect the length of wiring?
 IR drop is the product of the resistance of the wire (depends on wire
gauge size) and the current drawn by the load. (IR = volts)
 After calculating the IR drop of the field wire what you have left is
available at the module input.
IR drop
calculation
example
Here is an example of calculating the IR drop to see if enough is left over to
turn on an I/O module/s input point.
 Assume a B805 which needs 80 VAC min turn on volts and assume a
field source of 120 VAC.
 The current drawn by the B805 is 6ma.
 Consult the vendor of the wire to get the resistance of the wire (usually as
ohms per 1000 feet and this depends on gauge and length of wire. In this
example assume the total resistance of the wire length is 1000 ohms.
 .006 (ma) X 1000 (ohms res of the field wire) = 6 (VAC)
 120 VAC minus 6 VAC = 114 VAC (plenty left over to turn on B805
inputs as minimum needed is 80 VAC.)
Additional
precautions
The IR drop can only be a rough estimate along with empirical testing to fine
tune the wiring length. Neither Modicon nor any one else can declare what
the length of wiring can be due to the following unknown variables.
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Shielded verses non-shielded wire.
Single verses wiring pairs.
Wire impedance.
Electrical noise.
Routing of wiring such as running in parallel with hi voltage that can
induce capacitive and inductive coupling of noise spikes.