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Technical Note
00840-1600-4420, Rev AA
Smart Wireless Gateway 1420 with Power over Ethernet (PoE)
May 2015
Smart Wireless Gateway 1420 with Power over Ethernet (PoE)
Note
When using a Gateway as IEEE 802.3 PSE device, check the total power levels of all the PD
equipment connected (including the Gateway itself 3.6 Watts) to make sure the power supply
to the gateway can source enough power. It is always a good design practice to make sure the
power supply has more than enough power capability to handle startup loads and future
expansion.
5 IEEE 802.3 PoE gives protection from damaging a
computer or another piece of equipment
When using IEEE 802.3 PoE, one of the important new features of this standard is that PSE
devices have a test mechanism to protect connected incompatible devices from being
damaged. Only devices which have an authenticating characteristic based on the IEEE 802.3
standard, receive power via the ethernet cable. To determine whether a PD is connected, the
input parameters are checked by the PSE. This method is called “Resistive Power Discovery”.
During the discovery process resistance, capacitance, and current are checked.
If the PSE detects a PD it starts classification, i.e. determination of the power requirement of the
connected device. For this the PSE applies a small defined voltage to the power input of the PD's
and measures the resulting current. The PD is assigned to a power class based on the value of
the current. Only now the total voltage is supplied to the power input.
This sophisticated system prevents computers and other devices from being damaged when
connected to these cables.
6 Proper PoE installation considerations
In all electrical installations, local codes and prevailing regulations must be observed. Only use
properly trained/licensed installers, approved materials, have installations inspected as required
and if in doubt seek help from a qualified person. PoE+ and the load of the Gateway
(approximately 3 to 4 Watts) can add up to 30 Watts of power; because of this the proper
ethernet cable must be selected depending on the length of the cable run. Please check with the
manufacturer for the specifications of the cable being used to determine the power versus
length requirements. Multiple powered ethernet cables running in the same location must be
considered for total temperature rise. Most ethernet cable suppliers have charts for PoE usage
on their websites.
Typically, Cat 5 should handle most installations with runs up to 100 meters (approximately 300
feet). The use of Cat 3 is not recommended in any installation PoE or non-PoE, Cat 3 may work
for some lower power short run applications, but overall it has poorer data handling and lower
power capability. Cat 6 and Cat 7 are respectively better than Cat 5.
Older non-IEEE standard PoE offerings may not have this protection and could damage
computers and other devices.
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