Svet Črnil

Description

Siemens SIMATIC S7-200 SMART CPU ST20 for automation

Twelve digital inputs, eight transistor outputs and one goal: clear control of the operation of the smaller machine. When the sensor needs to detect the product, the conveyor belt waits and the valve switches at the right time, we need coordinated control. Siemens SIMATIC S7-200 SMART CPU ST20 for automation is a compact programmable controller or PLC for such tasks.

We offer it for engineering projects, upgrade of control and integration of industrial processes. Before selecting, we check together the number of signals, the type of exits and communication with other equipment. I recommend that we start with the actual operation of the machine: what we perceive, what we switch on and what must happen if the product stops. This helps us to build a solution that you will also use and maintain!

PLC for smaller machines

ST20 should be considered in projects with a limited number of digital signals. The input receives information from the appropriate sensor or switch, the program processes it, and the output in the intended order of control of the connected element or interface. The specific usability is always determined according to the requirements of the device.

• Smaller manufacturing companies: automation of repeated steps in construction and assembly.
• Maintainers: renewal of existing control where documentation, programme and electrical design are properly prepared.
• Machine builders: control of compact dedicated devices and connection of individual stations.
• System integrators: integration of sensors, control screens and other compatible equipment into the whole.
• Packers and workshops: planning sequences for transport, positioning and simple processing operations.

Imagine the packing station. The sensor detects the packaging, the program checks the readiness of the next step, then triggers the displacement and valve via the appropriate interfaces. Such a sequence must also be specified in the case of a missing product or a process interruption. It is precisely these details that show the value of a well - thought - out program.

ST20: 24 V DC and 20 signals

The basic characteristics of the ST20 family are clearly defined:

• Power supply 24 V DC.
• 12 digital inputs.
• 8 digital transistor outputs.
• Implementation of DC/DC/DC with Ethernet network connection installed.

The DC/DC/DC code describes DC power supply, DC digital inputs and transistor DC outputs. Together you get 20 digital signal positions installed. This is useful information from the first inventory of the machine: count the sensors, switches and commands, then check the users on the output page.

Planning example: if the project requires 10 digital inputs and 8 digital outputs, the number of basic ST20 connections is sufficient and two input connections remain. The exit reserve is no longer available in this case. Electric compatibility, current constraints and possible extensions are checked separately.

Transistor control exits

When choosing the exits, I look primarily at the connected load and the required switching. Transitory exits do not have mechanical contacts, so they are interesting when changing the corresponding one-way loads frequently. The exit marking itself does not mean that any consumer can be connected to it.

• In the case of an electromagnetic valve, check the voltage of the coil, current and adequate protection when off.
• In the case of an engine, determine the appropriate drive, contactor or other interface; the power engine is not directly connected to the PLC digital output.
• For signal lights and intermediate relays, check consumption and correct binding.
• For fast signals, we check the limitations of the selected connection and the hardware version.

In practice, an appropriate choice of exits may reduce the need for additional interfaces. The wrong choice quickly means processing the locker and extra work. So before ordering, I ask for a load list, not just for the number of exits.

Ethernet and connecting machine

The built-in network connection allows networking and the family uses the STEP 7-Micro/WIN SMART software environment. When connecting to a control screen, another controller or computer, we check the supported communication protocol, settings and software version.

The same connection on two devices does not in itself guarantee the exchange of information. Send the display, drive or other device code you want to connect. So we can check how communication will take place and whether an additional interface will be needed before choosing.

Comparison ST20 and Schneider M221

I chose a competitive comparison Schneider Electric Modecon M221 TM221CE16T. Both models have transistor digital outputs and Ethernet. The table below shows in particular the difference in the number of digital connections installed and the significant difference in analogue inputs.

The specific advantage of ST20 is three additional digital inputs and one additional digital output according to selected M221. In a project that fits this scale, we can only avoid expanding because of the missing digital connections. If the project needs analogue measurements, the comparison also includes the installed analogue inputs of the competitor and the cost of the corresponding ST20 equipment.

The comparison refers to those models and selected properties, not the whole family. Source for the competitive model: official website Schneider Electric TM221CE16T. The final choice is determined according to the entire application and offer.

Cheaper, less costly and less loss-making

In his economic advice, Benjamin Franklin warned: Beware of small expenses; a small crack sinks a large ship. In the case of a machine, such cracks can be repeated waiting, unnecessary manual intervention and poorly planned upgrade.

Source of thought, Slovenian translation: Benjamin Franklin, Poor Richard's Almanac, 1758.

To perform automation cheaperLet's write down the whole project first. The purchase is favourable set when considering the controller, necessary accessories, programming, installation and subsequent maintenance. Only this calculation shows where it can be formed savings.

Simple calculation example: 12 minutes of lost time per working day at 220 working days means 44 hours per year. This is an illustration, not a promise of saving by buying PLC. What part of these losses can be eliminated, measured by the process and tested by the proposed solution.

In the case of renovation, we should also check which parts we can reasonably keep. If sensors, cabinets and mechanical assemblies are adequate, it may be possible to improve control without replacing the whole device. Such an approach can reduce costs and the amount of equipment discarded.

Automatisation through Reuters view

On May 22, 2025, Reuters reported an interest in specialised robots dedicated to clearly defined tasks. The focus of the article is the usability of a specific task and the path to profitability.

My connection to this project is simple: let us also start with a measurable problem when choosing PLC, such as waiting between two steps. The contribution deals with wider automation and does not assess the ST20 model.

Source: Reuters, contribution on specialised robots and effective performance of tasks.

Practical example of PLC selection

Illustrating example of planning: the maintainer is preparing the renovation of the smaller machine. First, it lists sensors, switches, valves, and other consumers. It then compares the required number of digital connections with the available 12 inputs and 8 transistor ST20 outputs.

If the number of connections is sufficient, check the power supply, load currents, protection and method of connection to the control screen. This review facilitates the preparation of the offer and reduces the possibility of discovering the missing interface only when editing. The example illustrates the process of selecting, not measuring the result or testimony of the client.

ST20 or SR20: FAQ

What's the difference between ST20 and SR20?

ST20 uses transistor outputs and power supply 24 V DC. SR20 uses relay outputs; standard SR20 version is intended for network AC power supply. Check the label and specific performance data before connecting. The models shall not be interchangeable without verification of wiring and load.

Which model to choose?

The decision depends on the load, speed of switching and power. Send a list of entrances and exits and we will help you choose the appropriate execution. In the selection we also check the necessary reserve for future changes.

Is it enough just to buy a controller?

We also need an appropriate program, power supply, electrical design and testing on the machine to operate automation. The range of additional equipment is determined according to the sensors, consumers and the mode of operation. The advantage of preparing is that we know these needs as we assemble the offer.

Local advice for PLC: Ljubljana

At the company Svet Črnil in Ljubljana we combine the offer of a controller with an understanding of machine building and process integration. If advice is important to you local, is Ljubljana the starting point for discussing your project, its scope and the next steps.

For checking, prepare us:

1. Brief description of the machine and operating sequence.
2. A list of digital inputs and outputs.
3. The tensions and currents of the loads you want to control.
4. Screen tags, drives and other communication equipment.
5. Photograph of the existing cabinet and mark of the current PLC if it is an upgrade.

Contact us to check compatibility with your machine! Together we check if Siemens SIMATIC S7-200 SMART CPU ST20 is the right basis for your automation and what must be included in the offer.

Technical source: Siemens S7-200 SMART catalogueAdditional documentation: Siemens Technical Data Catalogue ST20 and SR20Finally, the catalogue number and machine version are confirmed before delivery.

Is it really economical to put off the reconditioning of the controls if, in the meantime, you pay employees every day to wait by the machine?