PoE I/O modules for buildings: power and data over one cable

In ceilings, risers and small technical rooms, the difficult part of adding a few I/O points is often the power supply, not the signal. PoE I/O modules take their power from the Ethernet cable, so one cable from a PoE switch replaces both a local 24 V supply and a separate data connection. This article covers PoE classes and power budgets, switches and injectors, the ICP DAS PoE I/O series, and the limits of PoE in buildings.

How Power over Ethernet works

The power sourcing equipment (PSE), usually a PoE switch or an injector, first detects a compliant powered device (PD) and only then switches on the nominal 48 V. Power runs either on the data pairs or on the spare pairs, and the PD has to accept both. The NS-205PSE switch, for example, feeds power on pins 1, 2 and 3, 6, while the tNS-200IN injector uses the spare pairs on pins 4, 5 and 7, 8. During detection the PD signals a class that tells the PSE how much power it may draw.

The difference between PSE and PD power covers the loss in up to 100 m of cable. IEEE 802.3bt adds higher classes that I/O modules do not need.

Worked example: PoE budget for a technical room

A technical room on one floor gets an 8-port NSM-208PSE switch. It supplies these devices, with class and maximum consumption taken from their data sheets:

The actual maximum load is about 21 W. A budget based on class limits, which is what a PSE may reserve per port, comes to 7 x 4.0 W + 15.4 W = 43.4 W. Every device stays within the 15.4 W per port that the NSM-208PSE delivers. For the 48 V supply of the switch, the data sheet gives 2.8 A at 48 VDC with all PDs at full load. Sizing for that value covers any 802.3af device on any port.

PoE switches and injectors

• NS-205PSE: 5 ports, 4 of them PSE to IEEE 802.3af with up to 15.4 W each, 46–55 VDC input, -40 to +75 °C.
• NSM-208PSE: 8 PSE ports with up to 15.4 W each in a metal housing.
• NS-205PSE-24V and NSM-208PSE-24V: versions for panels that only have 24 V; they boost the input to the 48 V needed for PoE.
• NSM-205GP and NSM-208GP: Gigabit switches with IEEE 802.3at ports of up to 30 W, redundant power inputs and a relay alarm output.
• iNS-306: IoT switch with 4 PoE+ ports, a web interface that shows PoE voltage, current and consumption, and per-port power on/off and reset schedules.
• tNS-200IN and tNS-200GIN: single-port injectors with 15.4 W and 30 W that add PoE to a port of a non-PoE switch. The -24V variants run from 24 VDC.
• NS-200PS: PoE splitter that provides 24 VDC at up to 10 W for a device without a PoE input.

ICP DAS PoE I/O modules

• PET-7000: PoE versions of the ET-7000 modules with Modbus TCP and UDP, a built-in web server and Web HMI, for example PET-7015 (RTD), PET-7005 (thermistor), PET-7017 (analog input) and PET-7060 (6 DI, 6 relays). They are IEEE 802.3af devices, most of them Class 1, and also run from 12–48 VDC on the terminal block.
• PET-7200: like PET-7000, with a 2-port Ethernet switch and LAN bypass for daisy-chain wiring, for example PET-7215 (RTD) and PET-7260 (6 DI, 6 relays).
• PET-2200: the ET-2200 family with a 2-port switch and Modbus TCP/UDP, MQTT and SNMP V2c. Depending on the I/O, they are Class 1, 2 or 3.
• tPET: tiny Class 1 modules with Modbus TCP/UDP and MQTT, powered from PoE or a DC input. Examples are the tPET-P6 and tPET-PD6 with 6 wet or dry contact inputs, and the tPET-P2R2 with 2 inputs and 2 power relays.
• WISE-7100: PoE modules that run IF-THEN-ELSE rules set up in a web browser, such as the WISE-7115 with 7 RTD inputs.

Other building devices with PoE input include the DL-302 and CL-202-E air quality data loggers, the PIR-230-E motion, temperature and humidity sensor, the ACS-10VP-MF and ACS-11P-MF card readers and the ALM-Horn-RGB siren.

Advantages and limits in building installations

Advantages

• No local power supply in ceilings, floor boxes or small enclosures, and one cable type for the installer
• UPS backup for all connected devices through the switch supply
• Remote power reset of a hung device from a managed switch such as the iNS-306
• The PET and tPET modules also run from a DC supply on the terminal block where no PoE port is available

Limits

• The cable between switch port and device can be at most 100 m long, as for any copper Ethernet link.
• PoE powers the module electronics, not the field wiring. Wet-contact inputs and open-collector outputs need their own field voltage, and relay contacts switch an external supply. Dry-contact inputs such as those on the tPET-PD6 read volt-free contacts.
• Modules with a 2-port switch pass data along a daisy chain, but plan the power supply of each module separately.
• Relays that switch mains voltage need the usual separation from data cabling inside the enclosure.
• The power budget of the switch and its supply, not the port count, limits the number of devices.

Send the points per location and a description of the network to our technical sales team for a proposal.