Introduction: Connecting Sensors Is Easier With the Right Integration Plan
Can a pressure sensor connect to an existing PLC? How can a water level sensor send data to a cloud platform? Is it possible to monitor flow remotely without installing a PLC?
These are common questions when selecting industrial sensors for factories, water treatment plants, storage tanks, and remote monitoring projects.
The good news is that many industrial sensor integration projects can be completed using standard electrical interfaces, communication protocols, and data acquisition devices. The key is to match the sensor output, controller or gateway, network, and target platform.
This guide explains how to connect pressure sensors, level sensors, and flow meters to PLCs and cloud platforms, how to choose between a DTU, RTU, and industrial IoT gateway, and what to check before commissioning a system.
Quick Answer: How Do You Connect a Sensor to a PLC or Cloud Platform?
An industrial sensor can connect to a PLC or cloud platform through several common methods:
To connect a sensor to a PLC: Use a compatible analog input for 4–20 mA or 0–10 V signals, a digital input for switching signals, or an appropriate communication interface for RS485 and supported protocols such as Modbus RTU.
To connect a sensor to a cloud platform: Use a PLC with an industrial IoT gateway, or connect the sensor to a compatible DTU or RTU that collects and transmits data through Ethernet or a cellular network.
For remote monitoring without a PLC: Use a DTU or RTU with the required sensor input and communication functions.
For distributed, low-power applications: Consider LoRaWAN or NB-IoT solutions based on network coverage, power availability, and reporting frequency.
The right solution depends on the sensor output, electrical interface, communication protocol, number of measurement points, power supply, network conditions, and cloud platform requirements.
1.How to Connect an Industrial Sensor to a PLC
1.1 4–20 mA Analog Signal: The Most Common and Reliable Option
The sensor outputs a current proportional to the measured value: 4 mA corresponds to the lower range limit, and 20 mA corresponds to the upper range limit.
Connect it to the PLC’s analog input (AI) module.
It offers strong noise immunity and supports long-distance signal transmission. Most mainstream PLCs can receive 4–20 mA signals through compatible analog input modules.
The conversion formula is simple:
For example, for a level sensor with a 0–5 m measuring range, a 12 mA output corresponds to a liquid level of 2.5 m.
1.2. RS485 / Modbus RTU: More Data, More Possibilities
Connect the sensor to the PLC’s serial interface through the A and B wires. The PLC acts as the master, and the sensor acts as the slave.
In addition to the measured value, the sensor may provide temperature readings and status information, and support remote calibration and address configuration.
Multiple devices can share a single bus, helping reduce wiring costs.
1.3. HART: Analog Signals Plus Digital Communication
HART overlays digital communication on a 4–20 mA signal. It is suitable for applications requiring remote diagnostics and parameter configuration, and is commonly used in petrochemical, chemical, water treatment, pharmaceutical, and food processing projects.
1.4. Switching and Pulse Signals
Level switches and pressure switches provide on/off signals and connect to the PLC’s digital input (DI). Some flow meters provide pulse outputs and require a compatible high-speed counter or pulse input channel.
1.5 Do Pressure, Level, and Flow Sensors Use the Same Wiring Methods?
In many cases, yes. The basic connection method depends on the electrical output rather than the physical quantity being measured.
| Sensor type | Common outputs | Key integration considerations |
|---|---|---|
| Pressure transmitter | 4–20 mA, 0–10 V, RS485 | Measuring range, loop power, media compatibility, and overpressure protection |
| Submersible level sensor | 4–20 mA, 0–10 V, RS485, depending on model | Measuring range, liquid compatibility & density, vented cable requirements, and cable protection |
| Radar or ultrasonic level sensor | 4–20 mA, RS485, HART, depending on model | Installation position, measuring range, blind zone, and communication interface |
| Flow meter | 4–20 mA, pulse, RS485, depending on model | Flow range, installation conditions, output scaling, and pulse configuration |
A pressure transmitter and a level sensor with the same 4–20 mA output may use similar PLC wiring. However, their engineering units, measuring ranges, installation requirements, and configuration parameters will differ.
2. How Does Industrial Sensor Data Reach the Cloud?
A cloud monitoring system typically includes three functional layers.
| Layer | Main function | Typical equipment |
|---|---|---|
| Sensing layer | Measures pressure, level, flow, or another physical quantity | Pressure sensors, level sensors, flow meters |
| Data acquisition and communication layer | Collects, processes, and transmits measurement data | PLCs, RTUs, DTUs, industrial gateways, wireless devices |
| Platform and application layer | Stores, displays, analyzes, and uses the data | SCADA, HMI, IoT platforms, cloud dashboards, mobile applications |
The sensor measures the physical quantity. A PLC, RTU, DTU, or gateway collects or forwards the data, depending on its capabilities. The platform then makes the data available for monitoring, analysis, and alarms.
Not every system requires all these devices. A simple remote monitoring application may use a sensor and a suitable RTU, while a factory with an existing PLC network may only need an additional gateway.
2.1 Path A: Connecting an Existing PLC System to the Cloud
Typical architecture:
This approach is suitable for factories, water treatment plants, pumping stations, and other sites that already have a PLC-based control system.
The gateway can collect data from the PLC and forward it to a remote platform using a supported protocol, such as MQTT, OPC UA, or Modbus TCP, depending on the gateway and platform.
Advantages include:
- Reusing existing sensors and control logic
- Centralizing data from multiple PLCs or devices
- Supporting remote monitoring and reporting
- Reducing the need to replace existing automation equipment
Before implementation, check the PLC communication interface, gateway drivers, data mapping, network architecture, and cloud platform requirements.
Where remote access to a control network is involved, use appropriate network segmentation, access control, and secure communication methods.
2.2 Path B: Connecting Sensors Directly to a DTU or RTU
Typical architecture:
For sites without a PLC, a compatible data acquisition device can collect sensor data and transmit it to a remote platform.
For example, an RTU with a suitable analog input may collect a 4–20 mA level signal. A device with an RS485 interface and the required Modbus master functionality may collect data from compatible digital sensors.
This approach can be suitable for:
- Water tanks and reservoirs
- Rivers and remote water level monitoring stations
- Pumping stations and wells
- Industrial storage tanks
- Agricultural irrigation systems
- Unattended monitoring sites
Depending on the device, functions may include data acquisition, local storage, alarms, remote configuration, and communication recovery.
Important: A DTU or RTU does not necessarily support every sensor input or cloud protocol. Confirm the available interfaces and data handling functions before selecting the device.
2.3 Path C: Using Wireless Solutions for Distributed Monitoring
Wireless connectivity can reduce the need for long signal cables, particularly when measurement points are widely distributed or difficult to access. However, different wireless technologies have different network architectures.
LoRa and LoRaWAN
LoRa is a low-power wireless communication technology. LoRaWAN defines a network protocol commonly used to connect low-power end devices through LoRaWAN gateways and network servers.
A typical LoRaWAN architecture is:
The exact architecture depends on the network implementation.
NB-IoT
NB-IoT is a cellular IoT technology. A compatible NB-IoT device can typically transmit data through an operator’s network to an application platform without requiring a separate local LoRaWAN-style gateway.
A typical architecture is:
Network coverage, operator support, SIM or subscription requirements, and platform integration must be verified.
Battery-powered monitoring
For battery-powered installations, evaluate the total power consumption, reporting frequency, transmission conditions, and expected battery life. Solar power may be an option where battery-only operation is insufficient.
Wireless connectivity is not automatically the best choice for every site. Existing cabling, required response time, data volume, network coverage, and maintenance requirements should all be considered.
3. DTU vs. RTU vs. Industrial IoT Gateway: Which Should You Choose?
These devices may overlap in functionality. Their actual capabilities depend on the product design, so selection should be based on specifications rather than the device name alone.
| Device | Main Function | Suitable Applications |
|---|---|---|
| DTU (Data Transfer Unit) | Transparently transmits serial or analog data to a network | Remote data transmission from a single sensor or a small number of sensors, with a simple structure and low cost |
| RTU (Remote Terminal Unit) | Collects data and provides logic processing, data storage, and alarm functions | Sites requiring multi-channel signal acquisition, local logic processing, and data buffering during network outages |
| Wireless Gateway | Collects data from wireless nodes such as LoRa and NB-IoT devices and uploads it to the network | Projects with scattered measurement points where wiring is difficult |
| Industrial Gateway / Edge Gateway | Performs protocol conversion, aggregates data from multiple devices, and supports edge computing | Projects with multiple existing systems and mixed communication protocols that need to be integrated into a unified cloud platform |
A device marketed as a DTU may also include analog inputs or other data acquisition functions. Likewise, some gateways offer local storage and logic functions commonly associated with RTUs. Always check the actual product specifications.
3.1 Five Questions to Ask Before Selecting a Device
What output does the sensor provide?
Determine whether the sensor uses 4–20 mA, 0–10 V, RS485, pulse output, or another interface. The selected device must support that interface.
How many measurement points are required?
A single sensor may only need one input channel. A site with dozens of sensors may require a multi-channel RTU, a gateway, or several distributed acquisition devices.
What communication network is available?
Check whether the site has Ethernet, cellular coverage, a suitable LoRaWAN network, or another supported communication method.
What power supply is available?
Mains power, solar power, and batteries have different implications for device selection, operating costs, and reporting frequency.
Does the application require local functions?
If the site needs local alarms, logic control, offline storage, or data retransmission after a network outage, select a device that explicitly supports those functions.
4. Which Communication Protocols Do You Need?
Communication compatibility is one of the most important factors in sensor integration. A device may have the correct physical interface but still be incompatible with the intended controller or platform because it uses a different protocol, register map, or data format.
| Integration Target | Common Methods |
|---|---|
| PLCs (Siemens, Mitsubishi, Omron, Schneider, etc.) | Connect via 4–20 mA analog input modules, or communicate using Modbus RTU / TCP |
| SCADA / HMI / Configuration Software | Standard protocols such as Modbus and OPC UA |
| Cloud Platforms (various IoT platforms and self-built platforms) | MQTT, HTTP, Modbus TCP, etc. |
| Third-Party Systems | Integrate through a gateway for protocol conversion |
Modbus is one of the most widely used communication protocols in the industrial sector. Most PLCs, configuration software, and gateways support it. Choosing sensors and gateways that support standard protocols can help minimize compatibility issues.
What if the on-site protocol is non-standard? Using a gateway for protocol conversion is a common industry solution and usually does not require modifying the sensor or the existing system.
5. Sensor-to-Cloud Solutions for Different Applications
The most suitable architecture depends on the site’s existing equipment, communication conditions, and monitoring objectives.
| Application Scenario | Recommended Solution |
|---|---|
| Factories and water treatment plants with existing PLCs | Connect sensors via 4–20 mA / Modbus to the PLC, then upload data to the cloud through a gateway |
| Remote, unattended sites such as reservoirs, rivers, and well stations | Connect sensors directly to the cloud platform via a 4G DTU/RTU |
| Storage tanks and water tanks with multiple, scattered measurement points | Wireless sensor nodes + wireless gateway |
| Remote diagnostics and parameter configuration required | RS485 digital communication or HART |
| Hazardous areas | Explosion-proof sensors + safety barriers |
| Multiple types of sensors at one site (pressure + level + flow) | Use an RTU or edge gateway for unified data acquisition |
6. Important Considerations During Integration
Wiring and Power Supply
First, confirm the output type and power supply requirements on the nameplate (2-wire, 3-wire, or 4-wire).
Follow the wiring instructions in the manual strictly. Do not determine polarity based on wire color conventions, as wire color definitions may vary between products.
Ensure that the supply voltage is within the rated range and that the power supply is stable.
Signals and Interference
Use shielded twisted-pair cables for signal transmission, with the shield grounded at one end only.
Keep signal cables away from variable frequency drives (VFDs), motors, and high-power cables.
Termination resistors are recommended at both ends of an RS485 bus. Do not reverse the A and B connections.
Parameter Configuration
The sensor measuring range must match the range settings in the PLC or gateway. Otherwise, the readings may show a consistent deviation.
For RS485 communication, ensure that the slave address, baud rate, and parity settings are consistent.
When multiple devices share the same bus, each device must have a unique address.
On-Site Installation
The vented cable of a submersible level sensor must not allow water to enter or have its vent blocked.
Radar and ultrasonic level sensors should be positioned away from inlets and blind zones.
Flow meters must meet straight-pipe run and full-pipe requirements.
Outdoor equipment should have waterproofing, lightning protection, and surge protection. Pay attention to the IP protection rating.
Network and Security
For 4G devices, confirm network coverage, the SIM card plan, and available data allowance.
For cloud data transmission, encryption and device authentication are recommended to prevent devices from being directly exposed to the public internet.
For critical sites, data buffering during network outages is recommended so that data can be uploaded automatically when the connection is restored.
Power Supply and Power Consumption
For sites powered by solar panels or batteries, estimate power consumption based on the upload frequency and allow sufficient capacity for cloudy or rainy days.
7.Frequently Asked Questions
Q1: Can we connect to the cloud without a software development team?
Yes. Configure the protocol and cloud platform address through a gateway or DTU, and the data can be uploaded. In most cases, you do not need to write your own code.
Q2: Can we use devices from different brands in the same system?
Yes. As long as the communication protocols are consistent (such as Modbus), or protocol conversion is performed through a gateway, devices from different brands can be connected to the same system.
Q3: What if there is no Ethernet connection or mains power on site?
You can use a 4G DTU/RTU with solar power, or adopt a low-power wireless solution such as LoRa or NB-IoT.
Q4: Can multiple types of sensors, such as pressure, level, and flow sensors, be connected at the same site?
Yes. Select a multi-channel RTU or edge gateway to collect data from all sensors and upload it centrally.
Q5: Will data be lost if the network connection is interrupted?
Choose an RTU or gateway with local data buffering and data retransmission capabilities. It can store data during network outages and upload the stored data when the connection is restored.
Q6: What integration information do we need to provide if we already have a cloud platform?
Typically, you need to provide the platform’s communication protocol (such as MQTT), server address and port, device authentication information, and data format (including topic and data point definitions).
8. Need Help Connecting Your Sensors to a PLC or Cloud Platform?
Holykell develops and manufactures industrial measurement instruments, including pressure sensors, level sensors, flow meters, and related measurement solutions. Depending on the application and available product configuration, suitable data acquisition and wireless communication equipment can also be considered for system integration.
Whether you are upgrading an existing PLC system or planning a new remote monitoring project, selecting compatible measurement and communication equipment can simplify commissioning and reduce integration risks.
Share Your Project Requirements
To help identify a suitable solution, please provide:
- Measurement type and range: Pressure, level, or flow, including the medium being measured.
- Sensor output: 4–20 mA, 0–10 V, RS485/Modbus, pulse, or another interface.
- Existing control system: PLC brand and model, or details of the current monitoring system.
- Communication requirements: Ethernet, 4G, LoRaWAN, NB-IoT, or another available network.
- Power supply: Mains power, solar power, or battery.
- Cloud platform: Platform name, supported protocols, and data format, if available.
- Installation environment: Indoor or outdoor installation, hazardous-area requirements, and expected reporting frequency.
From Parameters to a Practical Integration Plan
Step 1 — Submit your requirements. Share the sensor specifications and site conditions.
Step 2 — Technical selection. Evaluate compatible sensors, interfaces, data acquisition devices, and communication options.
Step 3 — Solution consultation. Confirm the system architecture, protocol compatibility, wiring requirements, and commissioning plan.
Contact us to discuss your sensor integration project.
Conclusion
Connecting industrial sensors to a PLC or cloud platform does not have to be complicated. The key is to select the correct electrical interface, verify communication compatibility, choose suitable data acquisition and networking equipment, and configure the system according to the application requirements.
For projects with an existing PLC, an industrial gateway may provide a practical route to cloud connectivity. For remote sites without a PLC, a compatible DTU or RTU may provide direct data acquisition and transmission. Wireless technologies can offer additional options where cabling or power availability is limited.
By defining your measurement requirements, interfaces, network conditions, and platform requirements before purchasing equipment, you can make better integration decisions and build a more reliable monitoring system.
If you are planning a new project or upgrading an existing installation, share your technical requirements to discuss suitable sensor configurations and integration options.
