Siemens S7-200 for analog signal processing and communication with DCS
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In our torch area, we use Siemens S7-200, which communicates with Zhejiang University’s ZK300XP via 485; the signals are displayed on the DCS. There is an EJA pressure transmitter on site; when the DCS displays a value of 4.4, the local gauge shows 1.2%, with a range of 300 kPa. Both the DCS and the Siemens laptop show values around 4.4, so the communication is working properly. In the afternoon, DCS began to rise gradually, reaching 14.9 in the end, while the on-site gauge showed little change. Is this a problem with the PLC’s accuracy in handling analog signals, or is there interference with the 485 signals?
When the DCS showed an increase, did you check whether there was any change in the values sent by the S7-200 MODBUS?
The on-site meter doesn’t move much; it’s also possible that there is an issue with the 200 AI channel at the site. It is recommended to use Siemens’ micro/win to connect to the 200CPU and check what the values are in the program.
Check the hardware first, then the software. Select the current to check and see if it matches after conversion.
The on-site gauge has been replaced, but the effect remains the same
The Modbus values also change, to match the values at the DCS; with the field instruments remaining unchanged, the values at the DCS change
Current conversion is the same as that of the on-site meter. It differs from PLC-based online monitoring
Unified response: Today I directed the pressure directly to the DCS, and the pressure value dropped immediately from 4.2 to 3.6. I monitored it throughout the morning, and the signal fluctuated only between 3.6 and 3.8; it did not rise from 3.6 to levels in the teens. Also, I checked it online using the S7-200 software yesterday, and both the PLC’s monitoring values and those of the DCS were higher than the actual values. I think there’s no issue with the on-site equipment, and the 485 communication lines are fine; the problem lies with Siemens’ handling of analog signals
You can first check the original values entering the AI channel to see if there are similar variations like this. If such variations exist and the table remains stable, then there might be an issue with that particular AI channel; try using a different channel
Are you saying that the current conversion is the same as that of the EJA transmitter? Is that correct? In other words, the current is correct, but there’s an issue with the output of your PLC. Let’s try it with a different AI channel. If there are no other problems, then it can be said that there’s an issue with the program.