Siemens PLC Distributed I/O, System Configuration, PLC Tags & Monitoring in TIA Portal – Complete Guide
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- Programmable Logic Controllers (PLCs) are widely used in industrial automation to control machines and processes efficiently. In a Siemens PLC system, the Input/Output (I/O) system, distributed I/O modules, system configuration, PLC tags, and monitoring tools play an important role in connecting field devices and managing process data.
- This article provides a detailed overview of Siemens PLC I/O systems, including digital inputs and outputs, distributed I/O systems such as SIMATIC ET200 series, system configuration using STEP 7 in TIA Portal, PLC tag management, and monitoring values in the CPU. Understanding these concepts helps engineers and technicians design, configure, and troubleshoot PLC-based automation systems effectively.
PLC (I/0) System
- The role of the PLC's input/output (I/O) system is to translate the signals received from input devices into low-voltage DC signals that the CPU can use and to perform the reverse operation when the CPU sends signals to output devices.
- In addition, the I/O system protects internal circuits from harmful voltage and current variations. A small PLC may have some or all of its I/O channels in the same housing with the CPU.
- For example, the accompanying graphic shows a CPU 1211C with 6 on board connections for inputs and 4 on-board connections for outputs.
- Top section: Connections for 6 Inputs → Digital input terminals (for sensors, switches)
- Bottom section: Connections for 4 Outputs → Digital output terminals (for relays, motors, actuators)
- This S7-1200 CPU model also permits a small signal board to be added internally for some I/O expansion but does not permit the addition of external signal modules.
- Most Siemens PLC'S however including the ones shown in the accompanying graphic allow signal modules to be added.
- This image shows a Siemens SIMATIC S7-300, SIMATIC S7-400, SIMATIC S7-1200, SIMATIC S7-1500.
- Some or all of these modules may be close to the CPU but signal modules can also be remote from the CPU.
- In addition for large or complex applications the number and types of signal modules is typically greater because the number and types of I/O devices required are greater.
Digital Inputs
- Examples of Input devices used with digital signal modules are (push buttons, limit switches, temperature switches, pressure switches, level switches, flow switches, photoelectric switches, and proximity switches).
- Some input devices use direct current (DC) and other input devices use Alternating current (AC).
- The specifications for each device type also identify the level of voltage and current required by the device. An additional consideration when matching a digital input signal module An additional consideration when matching a digital input signal module to a DC input device is the polarity of current flow.
- This image explains the wiring and terminal connections of a PLC output signal module, specifically from Siemens SIMATIC S7-1200 relay output module, likely SM 1222 DC.
- Also, this image shows how to wire a PLC relay output module with external AC/DC loads using proper supply connections.
- L+ terminal: +24 VDC (positive supply)
- M terminal: 0V (common/negative)
- Terminals: .0, .1, .2, etc. = Output points (Q0.0, Q0.1, Q0.2…)
- These outputs control external devices: Lamps, Relays, Contactors, Solenoid valves.
- Some DC devices are electromechanical so the polarity of current flow does not matter but other devices have built- in electronics that require a specific polarity of connection.
- For example, consider the S7-1200 PLC signal module shown in the accompanying graphic.
- This signal module is designed to work with 24V DC inputs and has, this signal module is designed to work with 24V DC inputs and has connections for 8 DC input channels four on top and 4 on the bottom.
- The accompanying graphic for simplicity shows only the top connections for this signal module.
- Note: that this module allows the load power supply which powers the input devices and circuits to be connected with either polarity.
- The connection diagram shown on the left has Sinking connections and the connection diagram on the right has Sourcing connections.
- The terms sinking and sourcing are designed to identify the direction of current flow but because these terms are confusing it is best to follow connection diagrams provide with PLC documentation connection diagrams provide with PLC documentation.
- This diagram illustrates the working of a PLC digital input system for both DC and AC field devices. In the DC section, a DC device such as a sensor or switch is powered by a DC supply, and its ON/OFF signal is sent to the digital input signal module, where it is conditioned and then transmitted to the Central Processing Unit (CPU).
- Similarly, in the AC section, an AC device powered by an AC supply sends its signal to a digital input module, which converts and isolates the signal before forwarding it to the CPU.
- In both cases, the input module ensures safe signal processing and allows the CPU to interpret the device status as a digital ON (1) or OFF (0) signal for control and decision-making.
Digital Outputs
- Examples of output devices used with digital signal modules are (lights, Relays, Solenoids, Motor Starters, Valves, and Heating elements). Some output devices use DC and other output devices use AC.
- Three categories of digital output signal modules are AC modules, DC modules, and relay modules. AC output signal modules typically operate with a load power supply of 120 or 230 VAC or may be capable of operating with either source.
- These modules use a solid state switching device, usually a triac, to control each output.
- This type of output works reliably with many AC output devices, but because a Triac allows a small leakage current to flow when the output is off, it is not suitable for some AC devices.
- The accompanying graphic shows a DC output signal module designed to work with 24V DC outputs.
- This signal module has connections for eight DC output channels, four on top and four on the bottom, and it is important to observe proper power supply polarity when making connections.
- The connection diagram shows the correct connections for the top terminals of the module. You may see the terms source and sink as it relates to DC output signal modules, but because these terms can be confusing, it is best to refer to the connection diagrams.
- The PLC’s brain. It sends a control signal when an output needs to turn ON or OFF.
- This module receives the signal from the CPU and processes it safely for field devices.
Inside the module, this channel contains electronic components:
- An opto-isolator (for electrical isolation)
- A TRIAC (used to switch AC loads)
- Protection components (resistors, capacitors)
- When the CPU sends a signal - TRIAC turns ON
- When OFF - TRIAC stops current flow
This is the actual equipment being controlled, such as: (Fan, Motor, Valve, Heater).
Load Power Supply (AC Source):
External AC supply provides power to the device.
- Relay output signal modules offer the advantage of being able to work with either AC or DC devices as long as the devices operate within the module's specifications and the switching speed required is not too fast. Each output on a relay output module is a set of relay contacts that close when the output turns on and open when the output turns off.
- This diagram illustrates how a PLC digital output system controls both DC and AC field devices. The Central Processing Unit (CPU) sends control signals to the digital output signal modules, where each module processes the signal through its output channel.
- In the DC section, the module switches a DC device using a DC power supply, while in the AC section, the module controls an AC device using an AC power source.
- The output module acts as an interface between the CPU and field devices, ensuring proper signal conversion and safe operation, allowing the PLC to turn devices ON or OFF based on program logic.
- The accompanying graphic shows a relay output signal module with eight output channels, three in top and five on the bottom.
- The connection diagram is for the top connections on the module. The markings above and below the load power supply symbol identify the connections required.
- For a single-phase AC supply, L stands for line and N stands for neutral. The + and connections apply if a DC supply is used.
Distributed I/O
- A PLC I/O system can include components mounted near a CPU and components dispersed throughout a facility. PLC I/O system components mounted remote from the CPU are referred to as distributed I/O.
- Many Siemens PLC models are designed to use SIMATIC ET200 distributed I/O systems with network connections.
- These systems include some components designed for control cabinet mounting and other components designed for use without control cabinets.
- ET200S
- ET200MP
- ET200SP and ET200 ISP
- ET200M
- ET200 Pro & ET200 Pro WLAN
- ET200 eco
- ET 200 eco PN
- ET200S is a Multi-functional highly modular I/O system. Interface modules with a PROFINET or PROFIBUS connection and with or without an integrated CPU and are available.
- ET200MP is a scalable I/O system used both as centralized I/O and in distributed configurations with PROFINET ET200SP is a scalable and highly flexible distributed I/O system with a PROFIBUS or PROFINET interface module.
- Interface modules are also available with CPU functionality.
- The version of this system designed for areas subject to explosion hazards is ET 200ISP ET200M is a modular I/O station for the control cabinet in high-density I/O channel applications. Interface modules are available for PROFIBUS and PROFINET ET 200pro is an especially small, extremely rugged, and high-performance I/O system.
- Interface modules are available for PROFIBUS and PROFINET an ET200pro IWLAN version is available for wireless connection to a PROFINET 10 controller. Interface modules are also available with CPU functionality ET 200eco is a compact block I/0 for cabinet free installation directly on the machine and it connects via PROFIBUS.
- ET 200eco PN is a compact block I/O for cabinet free installation directly on the machine and it connects via PROFINET.
System Configuration
- Siemens SIMATIC PLC'S configuration and parameter assignment is accomplished using STEP 7 engineering software which was originally developed before TIA Portal and is now also available in TIA Portal.
- STEP 7 Professional TIA Portal works with most modular SİMATIC S7 PLC'S.
- S7-1200 PLC'S can also use STEP 7 Basic TIA Portal.
- Part of this configuration involves assigning unique identifiers to system components. There are various types of identifiers needed for different uses.
- For example device addresses identify programmable modules with interfaces to a network. A type of addressing that is critical for signal modules is I/O addressing.
- I/O addresses are assigned automatically when signal modules are added in STEP 7 and can be changed later if needed.
- The addresses of a digital signal module's inputs (I) or outputs (Q) are assigned in groups A, even if the module does not use all 8 channels.
- This gives every channel on a digital signal module a unique address that identifies the bits in the CPU's process image that store the channel statuses.
- This graphic also shows the input address assignments for each channel if the module is configured as described. If this digital input signal module is assigned a starting byte address of 3 the channels will be assigned the input addresses shown below and will control the corresponding input status bits in the process image.
- Because an analog signal module has a resolution of up to 16 bits, 16 bits are required in the CPU's process image to store an analog input or output channel value. As an analog signal module is added in STEP 7, I/O channels are automatically assigned addresses.
- If the starting byte address of this analog input signal module is 256 the analog channel will use the following input bytes.
- For example if an S7-1500 8-channel analog input signal module is assigned input byte 256 as the starting address the channels will use the input bytes shown in the accompanying graphic.
PLC Tags
- STEP 7 allows the use of symbolic names, referred to as tags, associated with memory addresses. PLC tags are stored in the CPU's PLC tag table. Tag names can contain letters, numbers, and some special characters, and each tag name must be unique for a CPU.
- Use of tags helps when debugging a program or troubleshooting a system. The accompanying graphic shows an example of a PLC program network with the following tags assigned:
- Tags are shown in quotation marks in the STEP 7 (TIA Portal) program editor.
- Instruction operands can also be displayed.
- Absolute operands (such as 11.0,11.1, and Q3.0 in the accompanying graphic) are preceded by %. Absolute operands are available for use in any program block.
- Operands preceded by # are assigned to a specific program block
Monitoring Values in the CPU
- STEP 7 (TIA Portal) software provides a variety of ways to monitor system operation if the programming device is connected to a CPU. This is useful for debugging a program or troubleshooting a system.
- For example the accompanying graphic shows how a program network appears while being monitored in the STEP 7 TIA Portal program editor appears while being monitored in the STEP 7 TIA Portal program editor with the push buttons in their normal state and the motor stopped.
- Note that the Start and Motor normally open contact instructions are shown in blue to indicate that they are open.
- The Motor output coil instruction is also shown in blue to indicate that it is off.
- The accompanying graphic shows how that program network appears in the STEP 7 (TIA Portal) program editor while the start push button is being pressed.
- that the three normally open contact instructions are shown in green to indicate that they are closed and the motor output coil instruction is also shown in green to indicate that it is on.
- Finally the accompanying graphic shows how that program network appears in the STEP 7 TIA Portal program editor after the start push button is released.
- that the Start normally open contact instruction is once again shown in blue to indicate that it is open.
Conclusion
Siemens PLC systems provide flexible automation solutions through advanced features such as distributed I/O systems, modular signal modules, and powerful configuration tools in TIA Portal. By understanding digital inputs and outputs, ET200 distributed I/O systems, system configuration methods, PLC tags, and real-time monitoring, engineers can design efficient control systems and quickly diagnose operational issues. These capabilities make Siemens PLCs a reliable platform for modern industrial automation applications.
Frequently Asked Questions (FAQs)
1. What is distributed I/O in a PLC system?
Distributed I/O refers to input and output modules installed remotely from the PLC CPU and connected through industrial networks such as PROFINET or PROFIBUS.
2. What are PLC tags in Siemens TIA Portal?
PLC tags are symbolic names assigned to memory addresses that represent inputs, outputs, or variables, making PLC programs easier to read and troubleshoot.
3. Why is monitoring values in the CPU important?
Monitoring values in the CPU helps engineers observe real-time program operation, diagnose faults, and troubleshoot automation systems during commissioning and maintenance.
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