Internet PLC Forum
General => Technical support => Topic started by: BC SYSTEMS on September 22, 2010, 02:45:26 PM
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Hi,
I beleive I have a damaged AI.
After correcting some wiring in a control panel I noticed one of the AI's wasn't reading the input signal. The AI was subject to 24VDC for the best part of a weeK!
Looking at the PCB, after the input terminal there is a capacitor and a resistor bank "RA4"?, then off the to main processor.
Are there any checks I can do or components that I can replace?
Cheers
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There is a 2.2K resistors (RA4) in series with the analog input. This is to limit the current flow into the CPU's ADC pin if the ADC input is subject to higher than 5V momentarily. But long term exposure may damage the CPU pin and in that case, a brain transplant (CPU swap :) ) may be required.
But before you send in for repair you may want to verify to see if the AI is indeed damaged and not because a bad connection at the screw terminal? Apply a voltage to the AI and then use the ADC(n) function to read the ADC.
If you are using T100MD888+ please beware that the order of the AI is different from the tiny marking on the DB15 connector. Please refer to the installation guide to ensure that you are connecting the voltage to the correct AI and read from the correct channel when using the ADC() function.
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Sorry hi-jack this post for a while. If I have a device that output 0-10vdc analogue output, how do I prevent it from killing the plc while at same time able to read the analogue signal?
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You can use two 100K resistors to divide the voltage by 2 so that it becomes 0-5V which can be connected to the PLC's analog input.