A PLC communicates with field devices by reading and writing electrical signals through its input and output modules, either via direct hardwired connections or digital communication networks called fieldbuses. The PLC scans these signals continuously, interpreting sensor data and sending control commands to actuators in real time. The sections below unpack each layer of that communication process, from signal types to protocol differences and common failure points.
What types of signals do field devices send to a PLC?
Field devices send two fundamental categories of signals to a PLC: analog signals and digital signals. Digital signals are simple on/off states, such as a switch being open or closed. Analog signals carry continuous values, such as a temperature sensor transmitting a 4-20 mA current that represents a range of temperatures. The PLC interprets both types through its input modules.
Digital inputs are the most straightforward form of PLC field device communication. A pushbutton, proximity sensor, or limit switch sends either a 0 or a 1, representing two possible states. These signals are fast to process and require minimal wiring complexity.
Analog inputs are more nuanced. Common standards include 4-20 mA current loops, 0-10 V voltage signals, and thermocouple or RTD signals for temperature measurement. The 4-20 mA standard is particularly valued in industrial automation because the signal remains stable over long cable runs, and a reading of zero milliamps indicates a wiring fault rather than a valid zero measurement.
Beyond these two categories, some field devices also communicate using pulse signals, which count events like flow pulses, or frequency signals that encode speed data from rotating equipment. Each signal type requires the correct input module on the PLC side to be interpreted accurately.
How does a PLC read and write data from field devices?
A PLC reads and writes data from field devices through a continuous scan cycle. During each cycle, the PLC reads all input values into a memory area called the process image input table, executes its control logic, and then writes the results to a process image output table, which updates the physical outputs connected to actuators and other devices.
This scan cycle typically completes in milliseconds, making the process appear continuous from the perspective of the connected field devices. The speed of the cycle depends on the complexity of the control program and the number of I/O points being processed.
On the hardware side, I/O modules act as the interface between the PLC processor and the physical world. Discrete input modules convert incoming voltage levels to logic states. Analog input modules convert continuous electrical signals to digital values through an analog-to-digital converter. Output modules work in reverse, converting digital commands into voltage or current signals that drive actuators, valves, and motor starters.
Modern PLCs also support intelligent I/O modules that can perform local preprocessing, such as averaging, filtering, or alarm detection, before passing data to the main processor. This reduces the load on the central CPU and speeds up response times for critical control loops.
What is the difference between hardwired and fieldbus communication?
Hardwired communication connects each field device to the PLC with its own dedicated cable, while fieldbus communication uses a shared digital network where multiple devices communicate over a single cable using a defined protocol. Hardwired connections are simple and reliable but become costly and difficult to maintain as the number of devices grows. Fieldbus networks reduce wiring, enable diagnostics, and allow devices to share a single communication path.
In a hardwired setup, a sensor with a 4-20 mA output needs its own pair of wires running all the way back to an input module in the control cabinet. For a plant with hundreds of instruments, this means enormous cable runs, large marshalling cabinets, and significant installation time. Troubleshooting also requires physically tracing individual cables.
Fieldbus systems replace much of that wiring with a single cable segment that connects multiple devices in series or in a trunk-and-spur topology. Each device has a unique address on the network, and the PLC or distributed control system polls or receives data from all devices over that shared medium. This architecture also enables two-way communication, meaning the PLC can send configuration commands or retrieve diagnostic data from a field device, not just read its measured value.
The trade-off is complexity. Fieldbus networks require proper termination, address management, and network commissioning. However, for large installations in sectors like chemical processing or oil and gas, the reduction in wiring cost and the gain in diagnostic capability make fieldbus the preferred approach.
Which fieldbus protocols are most commonly used in industrial automation?
The most commonly used fieldbus protocols in industrial automation are PROFIBUS, PROFINET, Modbus, EtherNet/IP, and FOUNDATION Fieldbus. The right choice depends on the application, the existing infrastructure, the required speed, and the vendor ecosystem in use. PROFIBUS and PROFINET are dominant in Siemens-based environments, while Modbus remains widespread due to its simplicity and long history.
- PROFIBUS DP: A serial fieldbus designed for fast communication between PLCs and distributed I/O, drives, and sensors. Widely used in process and factory automation since the 1990s.
- PROFINET: The Ethernet-based successor to PROFIBUS, offering higher bandwidth, real-time communication, and integration with standard IT networks.
- Modbus RTU/TCP: One of the oldest and simplest protocols, still heavily used for connecting instruments, drives, and meters, particularly in energy and building automation.
- EtherNet/IP: Common in Rockwell Automation environments, using standard Ethernet hardware with the Common Industrial Protocol (CIP) on top.
- FOUNDATION Fieldbus: Used primarily in process industries for intrinsically safe environments where sensors and actuators require detailed diagnostic data.
- IO-Link: A point-to-point communication standard for smart sensors and actuators at the lowest level of the automation hierarchy, enabling detailed device parameterization.
In 2026, Ethernet-based protocols like PROFINET and EtherNet/IP continue to grow in adoption as plants modernize and integrate operational technology with IT systems for data analytics and remote monitoring.
How does PROFINET differ from PROFIBUS for PLC communication?
PROFINET uses standard Industrial Ethernet as its physical layer and supports significantly higher data rates and more flexible topologies than PROFIBUS, which relies on RS-485 serial communication. PROFIBUS operates at up to 12 Mbit/s on a linear bus topology, while PROFINET supports 100 Mbit/s and above with star, ring, and line topologies. PROFINET also enables seamless integration with corporate IT networks and supports real-time and isochronous real-time communication classes.
PROFIBUS was the industry standard for decades and remains in active use across thousands of installed plants. Its simplicity, robustness, and well-understood behavior make it a trusted choice for process automation environments where stability is prioritized over speed. Siemens PCS 7, for example, has deep roots in PROFIBUS DP for connecting remote I/O and field instruments.
PROFINET offers several advantages that become important in modern automation architectures. It supports device-level diagnostics through standard mechanisms, integrates with safety communication (PROFIsafe) and motion control (PROFIdrive), and allows the use of standard Ethernet switches and cabling infrastructure. This makes PROFINET easier to extend and monitor using familiar network management tools.
For plants running plant automation projects today, the choice between PROFIBUS and PROFINET often comes down to whether the installation is a greenfield project or an upgrade to an existing plant. New installations typically favor PROFINET, while existing PROFIBUS infrastructure is often retained and extended rather than replaced, sometimes using PROFIBUS-to-PROFINET gateways to bridge the two worlds.
What causes communication errors between a PLC and field devices?
Communication errors between a PLC and field devices are most commonly caused by wiring faults, incorrect termination, address conflicts, electromagnetic interference (EMI), and hardware failures in cables, connectors, or modules. In fieldbus networks, missing or incorrect bus termination resistors are among the most frequent sources of intermittent faults that are difficult to diagnose.
Physical layer issues
The physical layer is responsible for the majority of communication faults in both hardwired and fieldbus systems. Damaged cables, corroded connectors, and improper shielding all degrade signal quality. On PROFIBUS networks, the cable must be terminated at both ends of the segment with a 220-ohm resistor network. Missing termination causes signal reflections that corrupt data frames and produce sporadic communication errors.
Electromagnetic interference from variable frequency drives, large motors, or poorly grounded equipment can induce noise on signal cables. Proper cable routing, separation of signal cables from power cables, and correct shield grounding are essential preventive measures.
Configuration and software issues
Beyond physical problems, configuration errors are a common cause of PLC communication failures. Duplicate device addresses on a fieldbus segment prevent the network from functioning correctly. Mismatched baud rates between the PLC and a connected device cause the PLC to fail to recognize responses. Incorrect GSD or GSDML device description files result in the PLC expecting a different data structure than the device actually provides.
Firmware mismatches between a field device and the version expected by the PLC configuration can also cause faults, particularly after device replacements. Keeping device firmware, PLC firmware, and engineering project files synchronized is a discipline that prevents many avoidable communication errors in the field.
How CoNet supports your PLC communication challenges
Understanding how a PLC communicates with field devices is one thing. Getting it right in a live industrial environment, across hundreds of instruments, over aging infrastructure, and under production pressure, is where specialist knowledge makes the difference. At CoNet, we bring deep expertise in Siemens automation to every project, covering the full range of PLC signals, fieldbus architectures, and protocol integration.
We support our clients with:
- Engineering and commissioning of PROFIBUS and PROFINET networks in process and factory environments
- Diagnosis and resolution of persistent PLC I/O and fieldbus communication errors
- Migration projects from legacy PROFIBUS installations to modern PROFINET architectures
- Integration of field devices with Siemens PCS 7 and SIMATIC platforms
- Ongoing maintenance and optimization of industrial automation systems across chemical, food and beverage, oil and gas, and energy sectors
As the only company in the Netherlands certified as a PCS 7 Process Safety Specialist and a Siemens COMOS partner, we combine platform knowledge with hands-on field experience. If you are dealing with communication issues or planning a new automation project, contact us and we will help you find the right solution.
