Siemens S7-300, EM331 AI card; multiple instruments show full scale
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These instruments are located on the same card; the AI card does not supply power. The instruments on site use a four-wire system. This card is connected to seven instruments, two of which have safety barriers, while the rest have their signals connected directly to the card. The current in the four-wire signal circuits is normal, and the voltage in the signal circuits is 22V. When the negative terminal of one of the wires that connect directly to the card, without going through a safety barrier, was removed (only the positive terminal remained connected), all the other instruments that also do not have safety barriers (including those with their wires removed) showed a full-scale reading. Does anyone know why this happens?
Short-circuit the negative terminals of the channels together, and short-circuit them to the MANA terminal along with M-!
Well, you’re right; it’s simple to shorten MANA to M
For Siemens AI cards, those with a four-wire system need to have their potential equalized; otherwise, phenomena like the one you’re experiencing will occur