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Summary of all analog module wiring problems of Siemens 300PLC
It is essential to identify the reference potential point when working with analog modules. Recently, a new colleague from the hotline team asked me to help troubleshoot an issue with an analog module. The user reported that the readings on the S7-300 analog module were consistently showing 32767, which indicates that the signal is out of range. Although analog modules are commonly used, such issues often lead to confusion and frustration for users. After reviewing the available information, I noticed that there was no comprehensive explanation of this problem. Therefore, I have compiled my experience to provide a clearer understanding.
When an analog module shows a constant value of 32767, it means the signal has exceeded the measurement range. If the reading is 0, it may indicate a problem with the analog input itself. However, these issues often arise due to differences in the reference potential points. For example, if the signal coming from the field is 5V, but the reference point is at 0V instead of 10–15V or -10–5V, the measurement will be incorrect. To avoid such problems, it's crucial to ensure equipotential bonding between both ends of the system. The reference potential point for the analog module is called MANA, and all wiring should be connected to it.
Another important consideration is the isolation of the analog module. Isolated modules have their reference potential (MANA) disconnected from the ground (M), allowing them to measure signals without interference. Non-isolated modules, on the other hand, must be connected to the ground M, making it the reference point. This distinction is critical because isolated modules can prevent common-mode interference, which can cause inaccurate readings. For instance, in the S7-300 system, only the SM334 (excluding SM355) and the integrated analog inputs of CPU31XC are non-isolated. These modules typically have a shared M terminal for input and output.
Sensors also play a role in isolation. Non-isolated sensors often have a common terminal that connects to the negative end of the signal. In contrast, isolated sensors do not connect the negative end to the ground. It’s always best to consult the sensor manual to determine whether it is isolated. When connecting sensors to analog modules, ensuring proper grounding and equipotential bonding is vital to avoid measurement errors.
To achieve correct measurements, it's important to understand the connection methods between the signal source and the measuring terminal. Key terms include:
- **M+**: Positive measuring wire
- **M-**: Negative measuring wire
- **MANA**: Reference potential point of the analog module
- **M**: Ground terminal
- **UCM**: Common-mode voltage, which refers to the potential difference between the signal source and the module’s reference point
- **UISO**: Potential difference between MANA and the ground terminal of the CPU
The UCM is a major factor in causing over-range readings. Different modules have varying limits for UCM. For example, the SM331 module has a maximum UCM of 2.5 VDC. If the signal’s negative terminal is not connected to MANA, the module may read 32767 due to exceeding the limit. Most over-range issues stem from improper connections between the signal’s negative terminal and MANA.
In some cases, modules like the 6ES7331-7NF10-0AB0 have isolated channel groups, where each group uses its own M- as the reference. In such cases, the UCM between channels can be as high as 75 VDC. It’s recommended to ground MANA in these configurations to minimize interference.
When using isolated sensors with non-isolated modules, the MANA must be connected to the ground M. This ensures that the reference point is consistent. For example, the SM334 module requires this connection, as noted in the manual. Failing to do so can result in incorrect readings or module malfunctions.
For non-isolated sensors connected to isolated modules, the signal’s negative terminal must be grounded at the source. If multiple sensors are involved and located in different areas, it’s important to bond their negative terminals together with thick wires to reduce common-mode voltage. Additionally, MANA should be bonded to the local ground to maintain consistency.
Finally, when both the sensor and module are non-isolated, all components must be properly grounded and bonded. This ensures that the reference potential is consistent across the system. If the plant has an equipotential grounding grid, this process is simplified. However, in most cases, poor grounding leads to measurement issues. Using isolated modules and sensors can significantly reduce these problems.
In summary, the key takeaway is that the signal source and measuring end must be at the same potential. This principle applies not only to analog systems but also to digital wiring. A real-world example involved a CPU separated from the I/O power supply. Despite having 24V on the terminal, the input lamp did not light up. By shorting the M terminals of the power supplies, the issue was resolved—again highlighting the importance of reference potential alignment.
By following these guidelines, you can ensure accurate and reliable analog measurements in your system.