Dry Contact & Binary Input Sensors — PoE, Ethernet & RS232 Status Monitoring

What is a dry contact sensor?

A dry contact – also called a volt-free or potential-free contact – is a switch that carries no voltage of its own. It simply opens or closes a circuit, and the monitoring device supplies the sensing voltage. That electrical isolation is the point: a door switch, float switch, pump relay or alarm panel can report its state to your monitoring system without any shared power between the two circuits.

A dry contact is one of three signal types these inputs accept. Every COMET device on this page has one to three binary inputs, and each will take:

• A dry (volt-free) contact – a relay, reed switch or mechanical switch with no voltage of its own.
• An open-collector output – as found on many PLCs, counters and controllers.
• A two-state voltage signal – logic low 0 to +0.5 V, logic high +3.0 to +30 V, so a 5 V, 12 V or 24 V status line connects directly.

So if your equipment already provides a run, fault or alarm signal – as a contact or as a voltage – it can be wired straight in. Rather than buying a separate contact monitor, you connect it to a sensor you were already installing, and read temperature, humidity and equipment state on the same web page, SNMP trap or Modbus TCP register.

PoE dry contact sensors

The COMET P8652 and P8653 are powered and networked over a single Ethernet cable. Each carries three dry contact inputs alongside its temperature and humidity probes, so a comms rack, server room or plant room gets climate monitoring and door, flood or fault status from one PoE drop and one IP address.

Most competing PoE dry contact sensors monitor contacts and nothing else. These measure as well – which usually removes a second device, a second cable run and a second switch port from the job.

What you can monitor with a dry contact input

Door and access status. Pair any sensor here with the COMET SA200A magnetic door contact to log and alarm on cabinet, cold room or comms room door openings.

Flood and leak detection. The COMET P8653 ships with the LD-81 flood detector wired to its first input, with two inputs left free. This is water presence detection for plant rooms, raised floors and risers — not domestic smart-home leak alarms.

Power presence and mains failure. A UPS, distribution board or generator changeover panel almost always provides a volt-free contact for mains-present or on-battery status. Wiring it to an input means a power event and the temperature rise that follows it appear on the same trend — which is usually how you find out a comms room lost cooling rather than lost power.

Equipment run and fault signals. Most plant already offers a volt-free contact for run, fault or alarm status. Wiring it to a spare input puts machine state on the same trend as the environment it sits in.

Beyond those, an input will accept the contact from almost any third-party device you already have on site — a smoke or fire panel’s auxiliary relay, an airflow or pressure switch, a float switch, a glass break or intruder detector, a filter differential-pressure switch, or a gate or barrier position switch. COMET sensors monitor and report these contacts; the detectors themselves are supplied by their own manufacturers.

Dry contact inputs or relay outputs – which do you need?

These are opposite directions and it is worth being certain which way your signal flows.

A dry contact input lets the sensor read the state of something else – a door, a float switch, a pump’s fault contact. That is what the products on this page do.

A relay output lets the sensor switch something else – a fan, a heater, a dehumidifier, an alarm beacon – when a reading crosses a threshold. For that, see our sensors with relay outputs.

Most COMET Ethernet websensors on this page do both: three dry contact inputs for reading, two relay outputs for switching.

Choose-by-connectivity

Every binary input above accepts three signal types: a dry (volt‑free) contact, an open‑collector output, or a two‑state voltage signal — logic low 0 to +0.5 V, logic high +3.0 to +30 V. The inputs are not galvanically isolated and the minimum pulse duration is 500 ms, so they monitor status rather than count fast pulses; for pulse counting use the U7844 or U7844G.