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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteS7comm is Siemens S7 Communication: the PLC-oriented application protocol used above ISO-on-TCP and COTP in a common Ethernet connection. In a packet capture, the layers matter: TCP carries ISO-on-TCP framing, COTP establishes and carries the connection, and S7 communication handles PLC-specific setup and requests. A request may identify a memory area, data block, and address, but the CPU family and configuration determine what can actually be accessed.
What is S7comm?
S7comm is an application-layer communication protocol associated with Siemens SIMATIC S7 PLCs. It is distinct from the transport and framing layers beneath it. The Wireshark Foundation’s S7Comm reference describes the traditional protocol and its packet placement, particularly in connection with S7-300/400 systems. It is a dissector-oriented overview, not a complete specification for every Siemens CPU or modern protocol variant.
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Siemens documentation uses the broader term “S7 communication” for functions such as PUT and GET on S7-1200 G2. That documentation and Wireshark’s traditional S7comm reference help explain the general layering and data concepts, but do not establish that all S7-1200, S7-1500, S7-300, or S7-400 models expose identical services or address PLC data in the same way.
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteWhat is the difference between ISO-on-TCP and S7comm?
They name different layers of the communication path. Siemens says its S7 protocol uses ISO-on-TCP according to RFC 1006 for PG/HMI communication. ISO-on-TCP lets ISO-style applications operate over TCP/IP; COTP provides connection-oriented transport services and data packets within that framing. S7 communication supplies the PLC-oriented setup and operations carried above them.
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| Layer or term | Role in the connection |
|---|---|
| TCP/IP | Network transport over which the connection is made. |
| ISO-on-TCP (RFC 1006, including TPKT framing) | Carries ISO-style transport data over TCP/IP. |
| COTP | Establishes the ISO transport connection and carries its data packets. |
| S7 communication (S7comm in traditional packet-analysis usage) | Provides PLC-oriented setup and requests, including data operations. |
For analysis, do not call the whole stack “TCP” or treat ISO-on-TCP as the application protocol. Each layer contributes different information to a capture.
What port does S7comm use?
Siemens’ Communication protocols and ports used by Ethernet communication documentation for S7-1200 V20 lists TCP port 102 for ISO-on-TCP. That is the documented port in that manual’s scope; check the relevant CPU and configuration documentation rather than assuming every deployment has identical communication behavior. Port 102 identifies the transport endpoint, not the S7 request type or a particular PLC memory location.
How does S7comm work?
A typical connection proceeds through TCP, ISO transport connection setup, and then S7 communication setup. The Wireshark S7Comm reference outlines these stages; Siemens’ S7-1200 System Manual V4.7 (December 2024) explains the roles of RFC 1006 and TSAPs.
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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →- Open TCP: The client connects to the PLC’s TCP endpoint, commonly port 102 in the S7-1200 V20 documentation.
- Establish COTP: The client sends a COTP Connect Request to set up the ISO transport connection.
- Set up S7 communication: The endpoints exchange S7 setup messages, including negotiation of S7-specific parameters such as PDU size.
- Exchange requests and responses: Once set up, the connection can carry PLC communication operations supported by the CPU and its configuration.
What TSAPs identify
TSAPs identify communication endpoints associated with an IP address in the ISO-on-TCP connection. Their exact values and the permitted connection behavior depend on the CPU and connection configuration; a TSAP is not a universal PLC address or a guarantee that a service is enabled.
How to recognize the layers in a capture
When inspecting an authorized capture in Wireshark, follow the packet from Ethernet/IP/TCP to ISO-on-TCP framing, then COTP connection or data packets, and finally S7 setup or request/response content. Wireshark’s S7 Communication display-filter reference lists parser fields for items such as function, memory area, DB number, and address. Those field names describe what Wireshark parses; they are not, by themselves, a normative specification of every PLC’s behavior.
How do I read Siemens PLC memory over Ethernet?
At a high level, a client must establish the supported connection, then issue a data request that the target CPU and its configuration permit. In a traditional packet view, a request may identify a memory area and include a DB number and address. Those fields are useful for understanding what a packet refers to, not a universal map of Siemens memory.
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Siemens’ S7-1200 G2 V20 manual documents PUT and GET instructions for writing to and reading from a remote CPU. For the documented configuration, the “Permit access with PUT/GET communication from remote partner” setting must be enabled. Consult the manual for the exact CPU, firmware, and project settings: supported operations and address interpretation can vary with PLC generation, memory layout, and access configuration.
- Confirm the exact CPU family and firmware, then check its current communication manual.
- Verify the required connection configuration and access settings for the intended communication function.
- Interpret memory-area, DB, and address fields in the context of that CPU’s data layout and the request type.
- Use a controlled, authorized lab or network; a successful packet exchange does not establish that broad access is safe.
Security: what S7 communication does not provide
Siemens warns in its S7-1200 G2 V20 PUT/GET documentation: “If an attacker can access your networks, the attacker can possibly read and write data.” The same documentation identifies PUT/GET among communication mechanisms with no security features. Do not assume S7comm supplies encryption or authentication, or that enabling a CPU function makes it safe to expose broadly.
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- Photoelectric isolator: The device is also called a photocoupler, or optocoupler for short. Optical couplers use light as a medium to transmit electrical signals. It has a good isolation effect on input and output electrical signals.The main advantages of optocouplers are: signal transmission in one direction, electrical isolation at the input end and output end, the output signal has no effect on the input end, strong anti-interference ability, and stable operation.
- Features and technical indicators: software version STEP7 V5.2 and above, STEP7 Micro /Win 4.0 and above. MPI baud rate 19.2Kbps, 187.5 Kbps. PPI baud rate 9.6Kbps, 19.2Kbps, 187.5Kbps. The MPI port automatically adapts to the communication rate of 19.2Kbps and 187.5Kbps, 500Kbps, 1.5M Kbps DP master communication.
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Keep PLC communication on protected, controlled networks. Segment industrial networks, restrict access to authorized engineering and control systems, and follow Siemens’ industrial security recommendations. Apply the security guidance relevant to the specific product and deployment.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What varies between Siemens PLCs?
Protocol concepts help read a capture, but they do not replace the manual for the target device. Before relying on a connection or memory request, check these differences:
- CPU and firmware: Features and supported communication behavior are model- and version-dependent.
- Connection and access configuration: A function may require particular settings, and the CPU may restrict permitted partners or operations.
- Addressing and memory layout: A DB number or address in a parsed request does not imply that the same address is valid or accessible on another CPU.
- Communication goal: Native S7 communication, open user communication, and other supported interfaces are different approaches. The cited Siemens material documents selected S7 communication functions, not a comprehensive comparison of alternatives.
For model-specific behavior, use the documentation for the exact CPU family and firmware. The Siemens S7-1200 overview documentation provides product-family context, while the relevant CPU manual supplies implementation details.
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