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Asked:20 Oct 2025

SIMATIC S5 / STEP 5: Questions about S5 Networking

Dear Community,
I thought to seek help on a subject from this helpful community.
I have good experience with S7 (300 & 400) but I have a project setup with step 5 that I have some questions about.
The plant imported a line from Germany that was disassembled and then re-assembled.
Hardware setup:
I have an S5 PLC 115U CPU 945 (6ES5 945-7UA13) and 3 HMIs (COROS OP25 6AV3 525-1EA41-0AX1). After doing some research and understanding some S5 basics, I recognized an IM308B module (6ES5 308-3UB11). I understand that the IM308B will be networked with the HMIs using profibus to establish comm. between the "old" SINEC S5 and the "new" HMIs since the IM and the HMIs had profibus front connectors on them.
Software setup:
I created a virtual machine and setup (Win XP SP3 32-bit) windows OS and installed (Simatic Step 5 V7.2) and tried fiddling with it a little bit to understand its operation.
I faced a problem in the Step 5 when trying to open COMET200 from inside the Step 5 (Change-->Others) and browsing for the COMET200 folder, I get an error:
"16-bit MS-DOS Subsystem
The ntvdm cpu has encountered an illegal instruction"
I want to start the COMET200 to check the network of the IM308B as I know that the COMPROFIBUS is for IM308C only.
I understand that the configuration is "saved" on the EPROM of the IM308B.
My questions are as follows:
1. Is my assumption correct about the IM308B handling the profibus comm. with the HMIs ?
2. If not, how can the HMIs be connected to the S5 ?
3. Do I need the COMET200 to "review" the network or no ?
4. Is it safe to assume that the IM308B EPROM will have the network configuration ?
5. How to handle the network configuration in Step 5 ?
6. Can somebody provide a solution to that "ntvdm" problem ?
Sorry for the long post
Thank you all in advance.

#sosflex #SIMATIC S5 / STEP 5 #siemens
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Answers(1)
Siemens Test Added an Answer on20 Oct 2025

Understanding S5 PLC & Profibus Communication with HMIs

Dear Community Member,

You've encountered a common challenge when bridging older S5 systems with newer HMIs. Here's a breakdown of your questions, leveraging SiePortal knowledge:

  1. IM308B & Profibus: Your assumption is correct. The IM308B was designed to act as a Profibus gateway for S5 PLCs, enabling communication with Profibus-enabled HMIs like the COROS OP25.
  2. Alternative Connection: If the IM308B isn't functional, direct connection between S5 and older HMIs (non-Profibus) was possible via serial communication (MPI/PG). However, the IM308B is the preferred method for modern HMIs.
  3. COMET200: Yes, COMET200 is essential for configuring the IM308B's Profibus network, even though it's designed for the IM308C. It allows you to view and modify the network parameters.
  4. EPROM Configuration: Correct, the IM308B stores its network configuration in EPROM.
  5. Step 5 Network Configuration: Within Step 5, you'll configure the IM308B's communication parameters (station address, baud rate, etc.) using the 'PG/PC Interface' settings within the 'Parameters' table. This tells the S5 how to talk to the IM308B, which then handles the Profibus side.
  6. "ntvdm" Error: This is a known issue when running COMET200 on virtual machines. Here’s a Siemens support article outlining solutions, including compatibility settings and potential VM configuration adjustments. Try running COMET200 in compatibility mode for Windows 98/ME.

Important Resource: Siemens Industry Online Support - S5 with IM308B is an excellent resource for more detailed information.

Best regards,
Siemens Self Support

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