IC693ALG223 Group 2 Zero Readings: MUX or Configuration?

IC693ALG223 Group 2 Zero Readings: MUX Failure or Configuration Oversight?

When all eight channels in the second group of a GE Fanuc IC693ALG223 analog input module return zero counts, maintenance teams often suspect a burned multiplexer. However, field experience tells a different story. Configuration gaps and wiring faults are far more likely culprits in industrial automation systems.

Incident Overview: A Clean Group-Wide Zero Pattern

A Series 90-30 PLC system recently reported a partial analog input failure. Channels 1 through 8 delivered valid process values. In contrast, every channel in the second group showed zero counts in the %AI registers. The fault boundary matched the module’s internal channel grouping exactly. As a result, the maintenance team initially blamed a damaged internal multiplexer (MUX).

Why the MUX Hypothesis Deserves Scrutiny

The IC693ALG223 provides 1,500 V isolation between the field side and the logic side. Cross-channel rejection exceeds 80 dB from DC to 1 kHz. A total failure of one MUX group without external transients is statistically uncommon. Therefore, engineers should prioritize alternative causes before condemning the hardware.

Configuration Defaults and Channel Activation

The module does not auto-configure its 16 channels. By default, it activates only a limited number of channels, often just channel 1, with a raw data format of 0–4095 counts. If the second group was never explicitly enabled in Proficy Machine Edition, the %AI registers for channels 9–16 stay at zero regardless of field current.

Storing Configuration to Non-Volatile Memory

Configuration data must be downloaded to the CPU and stored in flash memory. A power-cycle or stop-to-run transition is required before new channel activation settings take effect. In many reported cases, engineers configured channels online but failed to store the configuration. Consequently, the module reverted to its default single-channel state after the next power cycle.

Module Power Supply and Common Terminal Integrity

The IC693ALG223 requires external 24 VDC power for its field-side circuitry. This supply connects between terminal 1 (positive) and terminal 19 (common). If the 24 VDC supply is missing or the terminal 19 connection is loose, all channels report zero. Moreover, a marginal internal power distribution can produce a partial failure pattern.

Measuring Supply Voltage Under Load

The module draws 65 mA from the external 24 VDC supply in addition to the current loop currents. A degraded power supply with excessive ripple, specified at a maximum of 10%, can cause the second group to fail first. Technicians should verify 24 VDC between terminals 1 and 19 under load. A reading below 20 VDC points to a supply or wiring fault rather than a MUX failure.

Wiring Polarity and Signal Loop Verification

Current input modules require a closed signal loop through the correct terminals. For channels 9–16, the return path must connect to the module’s common terminal. A reversed polarity on a single channel within the group drives that channel to zero or a negative value, depending on the selected range. In addition, a loose shared common wire for the second group causes all eight channels to read zero simultaneously.

Using a Calibrator to Confirm the Fault Location

Apply a 12.00 mA calibrator signal to a properly configured channel in the 4–20 mA range. The %AI register should read 16000 ± 100 counts. If the register stays at zero while the calibrator confirms 12.00 mA at the terminal, the fault lies in the module configuration or internal hardware. If the calibrator cannot drive current through the loop, the field wiring is open.

Internal MUX Failure Probability and Isolation Testing

The possibility of a burned MUX cannot be dismissed entirely. The module uses a single-ended input architecture with a shared 250-ohm input impedance. A high-voltage transient on any channel in the second group could damage the internal multiplexer that scans those eight inputs. However, this failure mode typically affects only the specific channel exposed to the transient, not all eight simultaneously.

Performing a Conclusive Isolation Test

Force a known current into channel 9 while monitoring the %AI register in the programming software’s data view. If the raw count changes with the applied current, the MUX and analog-to-digital converter are functional. If the count remains exactly zero under a confirmed 20 mA input, and the configuration is verified, the module hardware is suspect and should be replaced.

Recommended Three-Step Diagnostic Sequence

A structured procedure is advised before condemning the module. First, verify that channels 9–16 are marked active in the hardware configuration and that the configuration is stored to flash. Second, measure 24 VDC between terminals 1 and 19 with the field wiring connected. Third, apply a 12.00 mA signal directly to terminal 9 and observe the %AI response.

Author’s Perspective on Grouped Zero Faults

Available evidence indicates that the suspected MUX failure is an unlikely explanation for a clean group-wide zero pattern. The IC693ALG223 has a mean time between failures that is substantially longer than typical configuration errors in the field. The probability that eight channels fail identically from a single MUX burnout is lower than the probability of a missing configuration download or a loose common terminal. In my view, a systematic verification of software and power supply parameters should always precede any module replacement in factory automation projects.

Application Case: Avoiding Unnecessary Module Replacement

Consider a water treatment plant running a Series 90-30 PLC with several IC693ALG223 modules. After a scheduled power shutdown, the second group of one module reported zero on all eight channels. The maintenance team prepared to order a replacement module. Instead, the technician checked the hardware configuration and found that the channel activation settings had never been stored to flash. After downloading and storing the configuration, all channels returned to normal operation. This case illustrates why a structured diagnostic sequence saves both time and spare-part costs in control systems.

Frequently Asked Questions

Why do all eight channels in group 2 read zero at the same time?

A shared cause is the most likely explanation. Check the common terminal wiring, the 24 VDC supply, and the channel activation settings before suspecting the MUX.

Does the IC693ALG223 enable all 16 channels by default?

No. The module activates only a limited number of channels by default, often just channel 1. You must explicitly enable channels 9–16 in Proficy Machine Edition.

Why does the module revert to default settings after a power cycle?

The configuration was probably not stored to flash memory. Download the configuration and store it to non-volatile memory before cycling power.

What voltage should I measure between terminals 1 and 19?

Measure 24 VDC under load with the field wiring connected. A reading below 20 VDC indicates a supply or wiring fault rather than a hardware failure.

How can I confirm whether the MUX is actually damaged?

Apply a known current, such as 12.00 mA, directly to terminal 9 and watch the %AI register. A changing count confirms the MUX and ADC are working.