Sigma – Analogue Input Channels Display Incorrect Values

[Product_Support]

Issue

Analogue input (AI) channels are not displaying the correct values.

Product Line

Satchwell BAS & Sigma

Environment

Sigma Analogue Input

Cause

The issue is observed across multiple analogue input channels, cards, and controllers. This widespread behavior suggests the root cause is likely external to the individual AI objects or hardware components.

Resolution

Follow the steps below to isolate and resolve the issue:

Scenario 1: Issue Occurs on a Single Controller

1. Isolate Inputs:

   • Disconnect all inputs to the AI card except one.
   • Use a known resistive input for testing.

2. Test Input Stability:

   • Replace the connected analogue input with a fixed resistor.
   • Observe the reading—if it stabilizes, investigate the other inputs for faults.

3. Verify Voltage Reading:

   • Change the object lookup table entry to 2000.
   • Measure the voltage across the resistor.
   • Compare the measured voltage with the maximum input voltage (2V).
     • Example: If measured voltage = 1.5V
       Calculation: 1.52.0=0.75×100=75\frac{1.5}{2.0} = 0.75 \times 100 = 752.01.5​=0.75×100=75
       The AI object should display 75.

4. Check Earthing:

   • Ensure the controller’s earth bar is properly connected to a good earth.

5. Hardware Checks:

   • If the issue persists, consider replacing the AI card.
   • If unresolved, consider replacing the CPU card.

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Scenario 2: Issue Occurs on Multiple AI Cards in One Controller

1. Repeat Isolation Steps:

   • Disconnect all AI inputs on all cards.
   • Connect only one resistive input to a single card and observe behavior.

2. Repeat Steps 3–5 from Scenario 1.

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Scenario 3: Issue Occurs Across Multiple Controllers

1. Check Earthing:

   • Confirm all controller earth bars are securely connected to a good earth.

2. Inspect Power Supply Boards:

   • Older power supply boards (especially in BAS-to-Sigma upgrades) may cause various issues.
   • Overheating of voltage regulator ICs due to loose securing bolts can lead to ‘current limit’ mode.
   • Action: Retighten bolts securing the regulator tabs and TO3-type regulators. Allow the heatsink to cool down.