Issue
Analogue input channels do not display the correct value.
Environment
Sigma
Analogue Input
AI
Channel
Cause
Analogue Input values do not display the correct value. This issue is not confined to one card, or even one controller.
Resolution
The fact that the problem is spread across several analogue input channels, cards and controllers, suggests that the problem is not due to the specific objects themselves, and that some external influence is having an affect.
Carry out the following checks;
The issue is only seen on one controller.
- Disconnect all inputs to the card except for one. This should be a resistive input. If this has helped, then investigate the other inputs.
- Disconnect the wiring to the connected analogue input and replace it with a fixed resistor. The reading should be stable.
- If this input still reads incorrectly then change the object lookup table entry to 2000, measure the voltage across the resistor and compare it with the maximum input voltage of 2 volts.
- Example measured voltage of 1.5 volts. - 1.5/2.0 = 0,75 * 100 = 75. AI object should display 75
- Confirm that the controller earth bar is connected to a good earth.
- Consider replacing the AI card.
- Consider replacing the CPU card.
The issue is only seen on one controller, but with multiple analogue input cards.
- On all of the cards disconnect all of the analogue inputs, except for one input on one card. This should be a resistive input. If this has helped, then investigate the other inputs.
- Confirm that the controller earth bar is connected to a good earth.
- Consider replacing the AI card.
- Consider replacing the CPU card.
The issue is seen on multiple controllers.
- It is very likely that there is an earth issue, confirm that the controller earth bar is connected to a good earth on all controllers.
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Older type Power Supply boards (Bas-to-Sigma upgrades) can also cause many different (seemingly unrelated) problems, especially when the voltage
regulator IC's start to overheat due to the bolt securing the tab of the regulator to the heatsink becoming slightly loose: the regulator IC's go into 'current limit' mode.
Re-tightening these (and also the bolts securing the TO3 type regulator) and allowing the heatsink to cool-down will often cure problems.
