Dan Truitt

Frozen Analog Input on Compact Logix

3 posts in this topic

This is a bizarre one...

CompactLogix, 1769-IF8 with 1492-AIFM8-F-5 wiring system.

My client has reported several instances where one of his flow rate displays is frozen at its maximum value. Each time, I have gone to the site, found that the value in the analog input tag is at full scale, and measured more than 20mA at the input - as much as 28mA! I measure it using a clamp-on meter and I measure it at the input module (1769-IF8) and the external, pre-wired terminal block (1492-AIFM8-F-5). I cycle power to the PLC. No change. I cycle power on the transmitting device. No change. I disconnect and reconnect the input (signal wire at the module or the terminal block) and everything goes back to normal. This has happened on multiple channels on multiple input modules and with two different types of transmitters.

I didn't install this system, but it seems to have been done properly and it had been running fine for many years. These problems just started happening out of the blue.

Has anyone out there had any similar experience? This is a new one for me after more than 25 years in the business.

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Did you鸿运彩票手机app disconnect the signal wire at the transmitter and measure the output signal there?

Double check the configuration of the IF8?

Verify the scaling either in the block or in the code?

Any new equipment been installed near the transmitter? Check for proper shielding of the signal cable?

Edited by pcmccartney1

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sounds like the v+ line dropped

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