Siemens SCALANCE XM-400 Configuration Manual page 19

Cable used for the power supply
● Alternative A (redundant wires)
In Fast Ethernet, the wire pairs 1, 2 and 3, 6 are used to transfer data. Pairs 4, 5 and 7, 8
are then used to supply power. If there are only four wires available, the voltage is
modulated onto the wires 1, 2 and 3, 6 (see variant 2). This alternative is suitable for a data
transmission rate of 10/100 Mbps. This type of power supply is not suitable for 1 Gbps since
with gigabit all eight wires are used for data transfer.
● Alternative B (phantom power)
With phantom power, the power is supplied over the pairs that are used for data transfer,
in other words, all eight (1 Gbps) or four (10/100 Mbps) wires are used both for the data
transfer and the power supply.
A PoE-compliant end device must support both alternative A and alternative B over redundant
wires.
A switch with PoE capability can supply the end device either using
● alternative A or
● Alternative B or
● alternative A and alternative B.
Note
The SCALANCE PE408PoE extender supports alternative B.
Endspan
With endspan, the power is supplied via a switch that can reach a device over an Ethernet
cable. The switch must be capable of PoE, for example a SCALANCE X108PoE, SCALANCE
X308-2M PoE, all SCALANCE XM400 switches with PE408PoE, SCALANCE XR552‑12M.
Midspan
Midspan is used when the switch is not PoE-compliant. The power is supplied by an additional
device between the switch and end device. In this case, only data rates of 10/100 Mbps can
be achieved because the power is supplied on redundant wires.
A Siemens power insert can also be used as the interface for the power input. Since a power
insert supports a power supply of 24 VDC, it does not conform with IEEE 802.3af or IEEE
802.3at. The following restrictions relating to the use of power inserts should be noted:
Operate the power insert only when the following conditions apply:
● with extra low voltages SELV, PELV complying with IEC 60364-4-41
● in USA/CAN with power supplies complying with NEC class 2
● in USA/CAN, the cabling must meet the requirements of NEC/CEC
● Current load maximum 0.5 A
SCALANCE XM-400/XR-500 Web Based Management (WBM)
Configuration Manual, 05/2017, C79000-G8976-C248-12
WARNING
Description
2.4 Power over Ethernet (PoE)
19