Siemens S7 PLC Analog I/O Modules, Power Supply & Communication Modules

Siemens S7 PLC Analog I/O Modules, Power Supply & Communication Modules – Complete Overview

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Introduction

In this article, we provide a detailed overview of Siemens S7 PLC analog I/O devices, including how analog signals are processed, the role of signal module status indicators, and the importance of system and load power supplies. We also discuss communication modules and special-purpose modules used in advanced control applications such as PID control.

This guide is useful for engineers, technicians, and students who want to understand how Siemens S7 PLC systems interact with sensors, actuators, and industrial networks.

Siemens S7 PLC Analog I/O Modules, Power Supply & Communication Modules

Programmable Logic Controllers (PLCs) play a critical role in modern industrial automation systems. Among the many components of a PLC system, analog input and analog output modules, signal modules, power supplies, and communication modules are essential for interfacing field devices with the control system.

ANALOG INPUTS

An analog input signal is generated by a sensor that provides voltage or current that can vary within a range such as 0 to 10 volts or 4 to 20 mA.

Siemens SM 1231 Analog Input Module

This image shows a 4-channel analog input expansion module (Siemens SM 1231 Analog Input Module) used in the Siemens S7-1200 PLC system to read analog signals from field instruments.
The Analog Input Module is used to receive analog signals from field sensors and convert them into digital values that the PLC CPU can process.

Common input signal types include:

0–10 V voltage signals
±10 V signals
4–20 mA current signals
0–20 mA signals

An analog input signal module converts the current or voltage to a binary value that the CPU stores for use in the user program.
The number of bits in the binary value is referred to as the channel resolution.
The more bits the greater the number of values that can be represented.
The resolution available depends on the module.
For example, the accompanying graphic shows a signal module with a channel resolution of either 13 bits or 16 bits depending on the module version.
Temperature measurement is an example of an application that requires special signal processing.
Two types of sensors commonly used to measure temperature are Thermocouples and Resistance temperature detectors (RTD's).

A Thermocouples is a Temperature sensor made from two dissimilar metals joined at a point called a junction.
Thermocouple produces a small voltage that is Temperature dependent.
An RTD also called a Resistance Thermometer is a Temperature sensor made from a metal such as platinum, nickel, or copper that varies in resistance in a predictable manner as temperature varies.
There are various types of Thermocouples and RTD's, and each type has different characteristics that determine its application range.
Because Thermocouple and RTD signals are different and require special processing some Siemens PLC'S have thermocouple and RTD signal modules.
However the S7- 1500 PLC analog input signal module shown in the accompany graphic can be used with a variety of analog signals including RTD and thermocouple signals.

ANALOG OUTPUTS

When a CPU sends a numerical value to an analog output channel, the analog module converts the value to a voltage or current within the range set up for the output channel.
The channel is typically connected to an analog output device that is controlling something in a machine or process, such as motor speed, valve position, or temperature.

2-Channel Analog Output (AO) Signal Module.

This module is the Siemens SM 1232 Analog Output Module, which is part of the Siemens SIMATIC S7-1200 PLC family manufactured by Siemens.
The Analog Output module converts digital values from the PLC CPU into analog electrical signals to control field devices.

The module can generate standard industrial analog signals such as:

0 – 10 V (Voltage output)
±10 V (Bipolar voltage output)
0 – 20 mA (Current output)
4 – 20 mA (Industrial current loop)

PLC analog output signal modules vary in the number of output channels, channel resolution, and output signal types.
The module shown in the accompanying graphic has two analog output channels.
These channels can be set up for either the -10 to +10 volts range or the 0 to 20 milliamps range.
The output resolution is 14 bits for the voltage range and 13 bits for the current range.

Signal Module Status Indicator

Signal modules have LED status indicators that are useful for system Installation and Troubleshooting.
For example the accompanying graphic shows two S7-1200 signal modules a combination digital input/output signal module and a combination analog input/output signal module.

The modules shown in the image are Siemens SIMATIC S7‑1200 PLC series, specifically:
1. Siemens SM 1223 Digital I/O Module (Combination Discrete Input/Output Signal Module)
2. Siemens SM 1234 Analog I/O Module (Combination Analog Input/Output Signal Module)
Both modules have a status LED for each channel.
The digital channel status LED is green and turns on when the input or output is on.
The analog channel LED is green when the channel has been configured and is active and red when there is a channel error.
Both modules also have a diagnostic LED that is green when the module is operational and red when the module is defective or non-operational.

POWER SUPPLIES

Multiple system power supply types are typically available for a PLC model to match the system voltage.
System power supplies also specify a power budget which is the available power that a supply can provide to interconnected modules.
A load power supply provides power to input and output devices and channel circuits.
In some cases, the load power supply also provides power to other PLC modules including a system power supply module.

Siemens PM 1507 24V/3A Power Supply
Siemens PM 1507 24V/8A Power Supply

Because input and output devices vary considerably in terms of their current voltage and power requirements load power supplies come in a variety of types and capabilities.
For example, the accompanying graphic shows a load power supply module for an S7-1500 PLC that mounts on the mounting rail with other modules.
Siemens also manufactures a range of SITOP power supplies for use with many types of equipment including PLC'S.

Programmable Logic Controller Power Supply Modules and Communication Modules

Siemens SITOP power supplies

Power Supply Modules

Some CPU modules have an integrated power supply for powering on-board circuits.
However, some PLC models or configurations require a separate system power supply for this purpose.
This system power supply converts the incoming power to filtered, regulated, low-voltage DC power.
The system power is provided to a back plane that is common to multiple modules.
Multiple system power supply types are typically available for a PLC model to match the input voltage, which may be DC or AC.
System power supplies also have a power budget that defines the amount of power available for interconnected modules.
A load power supply, also called a load current power supply, provides power to input and output devices as well as the related input and output circuits, In some cases, the load power supply also provides power to other PLC modules, including a system power supply module.
The load power supply module (PM) for a SIMATIC S7 1500 PLC mount for a SIMATIC S7-1500 PLC mounts on the same mounting rail as other S7-1500 modules.

SIMATIC S7-1500 PLC

The image shows a modular Siemens SIMATIC S7-1500 PLC system. The main devices/components visible are:

CPU Module (CPU 1516-3 PN/DP),
Integrated Display (HMI on CPU),
I/O Modules (Digital / Analog Modules),
Power Supply Module,
Mounting Rail (DIN Rail)

Siemens also offers a variety of SITOP power supplies for use with a wide range of equipment.

SIGNAL MODULES

PLC'S have a variety of signal module types. Each module has multiple I/O channels, and each I/O channel connects to one I/O device.
The number of I/O channels on a module varies with the module type. Some modules also have both input and output channels. Each input or output channel on a module has an LED that shows the status of that channel.
One or more additional LED'S show the status of that module. Signal modules of various types are interspersed as needed throughout a system.
Each module is configured at installation, and this configuration includes the assignment of a module address.
This address together with the module type also identifies the location of image table status bits for the connected inputs and outputs.
As a result of this configuration, a discrete input (DI) or a discrete output (DQ) is associated with a specific image table status bit, and this input and output status information is available to the user program.

Because each analog input or output must represent any value in a range of values, each analog input (AI) or analog output (AQ) is associated with a word, 16 bits, of memory.

COMMUNICATION MODULES

At one time, PLC communications primarily used networks designed by equipment manufacturers for their own equipment.

Siemens PLC communication Module

As more control devices with built-in computers were developed, the need for common approaches for communications emerged.
This lead to development of a variety of field bus by industry associations. Afield bus is an industrial network used for distributed control.
One example of a fieldbus is PROFIBUS DP the leading fieldbus standard, which was developed by PROFIBUS & PROFINET International (Pl).

SIMATIC S7-300 CPU 315-2 PN/DP

The image shows a Siemens SIMATIC S7-300 CPU 315-2 PN/DP with its communication and storage interfaces. The main devices/components visible are:

CPU Module (CPU 315-2 PN/DP),
Micro Memory Card (MMC Slot),
PROFINET Port (Ethernet Communication),
PROFIBUS Port (DP Communication),
Mounting Rail (DIN Rail),
Front Connector / Terminal Area

With the growth in information technology (IT), the need for control systems to communicate with IT systems increased. IT systems often communicate via Ethernet local area networks (LAN'S), however, the original form of Ethernet was not adequate for factory communication so Industrial Ethernet was developed. PROFINET, which was also developed by PI, is the leading Industrial Ethernet standard.
A PLC CPU often has one or more ports for a specific type of network communications.

network communications

For example, S7 1200 and S7-1500 CPU'S all have at least one PROFINET port.
When additional network ports are required or when a different type of connection is needed, communication modules are added to the configuration.

Special Purpose Modules

PLC'S are very fast and have many types of signal modules, but some tasks require modules.
Many of these modules are designed for closed-loop control applications, which involve controlling a process or machine in response to a feedback signal from a sensor.
A control loop may require a special purpose module because of the type of sensors, control devices or algorithm used.
A control algorithm is a set of rules or calculations that define the control process For example, the proportional integral- derivative (PID) control used in many applications uses a control algorithm.
PLC'S are very fast and have many types of signal modules, but some tasks require modules.
Many of these modules are designed for closed loop control applications, which involve controlling a process or machine in response to a feedback signal from a sensor.
A control loop may require a special purpose module because of the type of sensor, control device, or algorithm used. A control algorithm is a set of rules or calculations that define the control Process.

For example, the proportional integral- derivative (PID) control used in many applications uses a control algorithm.

Conclusion

Siemens S7 PLC systems provide a flexible and powerful platform for industrial automation. Analog input and output modules allow the PLC to interact with sensors and control devices, while signal modules and status indicators simplify system monitoring and troubleshooting.

Power supply modules ensure reliable operation by providing regulated power to the PLC system, and communication modules enable integration with modern industrial networks such as PROFINET and PROFIBUS. Understanding these components helps engineers design efficient and reliable automation systems for a wide range of industrial applications.

Frequently Asked Questions (FAQs)

1. What is an analog input module in a PLC?

An analog input module receives analog signals from sensors such as temperature, pressure, or flow transmitters and converts them into digital values that the PLC CPU can process.

2. What is the purpose of an analog output module in a PLC?

An analog output module converts digital values from the PLC CPU into analog signals such as 0–10 V or 4–20 mA to control devices like drives, valves, or temperature controllers.

3. Why are communication modules used in Siemens PLC systems?

Communication modules allow PLCs to connect with industrial networks such as PROFIBUS, PROFINET, and Industrial Ethernet, enabling data exchange between controllers, field devices, and monitoring systems.

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