The AS-i system operates with a symmetrically modulated communications system superimposed on a 30 VDC supply. This is a floating system with non-intuitive requirements for troubleshooting. This engineering brief provides an introduction to the signal, demonstrates the nature of the floating system, and concludes with recommendations for troubleshooting. Figure 1 provides a workbench view of the equipment used in this demonstration.
No! You cannot replace the 30 VDC AS-i bus power supply with a conventional power supply adjusted to 29 VDC. The zero-Ohm Thévenin output impedance will short the modulation which originates in the AS-i communications master.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Understand It → Network Fundamentals
Difficulty: 
Technician — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 06 Mar 2026
Figure 1: Image of the AS-i equipment on the author’s workbench.
What does the AS-i waveform look like?
The answer depends on how the signal is viewed.
DC Coupled Oscilloscope View
Figure 2 presents a near-ideal oscilloscope screen capture. In this example,
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Channel 1 is connected to the positive (brown) side of the AS-i pair
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Channel 2 is connected to the negative (blue) side of the AS-i pair
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Probe grounds are connected to the output-side GND (shield) terminal of the AS-i power supply
We can see the 30 VDC power supply effectively split with positive 15 VDC on channel 1 and negative 15 VDC on channel 2. The AS-i signal intelligence appears as burst-like modulation on each channel. The intelligence is superimposed on the 30 VDC power supply.
The waveforms were captured using a Digilent Analog Discover 3.
You are correct to question the placement of the probe grounds as the oscilloscope is internally ground referenced. Please read on to see that this AS-i bus GND (shield) is floating with zero reference to earth ground or to 24 VDC return.
Figure 2: Oscilloscope screen capture of the DC coupled AS-i signals.
AC Coupled Oscilloscope View
The AC coupled screen capture in Figure 3 reveals the nature of the AS-i signal. Here, we retain the probe connections to channels 1 and 2. The added red trace shows the difference between the signal (channel 1 minus channel 2).
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The signals are mirror images of each other.
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The signaling period is approximately 6 µs providing the 167 kbps (baud) rate.
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The red reconstructed difference signal is cleaner than either channel.
AS-i is a Balanced Signal
The AS-i signal is similar to the audio signal in a professional (balanced) microphone. Recall that we “transmit” signal and signal_not. The receiver reconstructs the signal by taking the difference between the two signals. This would seem unnecessary until we consider that most noise is common to both wires; a noise source injects an equal signal onto both signal wires.
The receiver’s reconstruction effectively removes common noise. This mechanism is suggested in Figure 3 as the reconstructed difference (red) signal is cleaner than the input channels. This is CMRR in action.
Figure 3: Oscilloscope screen capture of the AC coupled AS-i signals.
Multimeter Measurements as Measured Across the AS-i Signal Lines
A multimeter may be used to provide limited analysis of the AS-i signal.
DC Mode
When placed across the two AS-i terminals, a multimeter set to DC will see a nominal 30 VDC from the power supply. This will be steady, with no variations. If the system is operating normally the absolute voltage from each line to GND (shield) will be approximately 15 VDC as shown in Figure 2. See the next section for troubleshooting an abnormal (unbalanced) operation.
AC Mode
A multimeter set to measure AC will see approximately 0.8 VAC when placed across the AS-i terminals. This is an empirical measurement taken from the workbench using a Fluke type 87 V multimeter.
This is an out of specification measurement for the multimeter. Your results may vary. This non-sinusoidal, high frequency, burst signal is far away from the pedestrian 60 Hz signal we typically measure with the multimeter.
Internal Construction of the AS-i Bus Power Supply
This section addresses the AS-i power supply as shown in Figure 1. This is essential system-level knowledge required to troubleshoot AS-i ground faults.
AS-i Power Supplies
There are two power supplies associated with an AS-i system.
“Black cable” Auxiliary Bus
An example of an auxiliary load is a conveyor motor. For our purposes, the auxiliary supply has a 24 VDC power terminal and return terminal. This is a conventional low voltage supply that will not be addressed in this AS-i centric article. Not all AS-i devices require the auxiliary supply.
Yellow Cable AS-i Communication Bus
Instead of using the term “power supply,” let’s call this the power supply partner to the AS-i master. Together they form a data-modulated power rail. While a conventional (ideal) power supply has a near-zero-Ohm Thévenin output impedance, the AS-i bus power supply has a moderate impedance into which the AS-i master can modulate the AS-i intelligence. Figure 4 shows this decoupling network as a capacitor-inductor pair near the output of the power supply.
The conventional 24 VDC supply (set to 29 VDC) will not work as an AS-i bus supply. The conventional supply has a near zero Ohm impedance that would short out the modulation signal of the AS-i master. This is not the place to save money.
Figure 4: Block diagram of the Pepperl+Fuchs VAN-24DC-K28 showing the floating outputs.
Floating AS-i Bus
The AS-i bus power supply has a floating output.
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The shield terminal should not be confused with a nonexistent “return” for the AS-i signal.
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The shield terminal has a high resistance when measured by a multimeter with respect to the power supply’s metal chassis ground and to the 24 VDC return (input side).
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The datasheet states “The precise and transformer coupling permits the use of unshielded load lines.”
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The Siemens AS-i master shown in Figure 1 has no intrinsic connection to the power supply’s shield terminal.
Note that the Pepperl+Fuchs VAN-24DC-K28 AS-i power supply shown in Figure 1 has a output-side terminal labeled as GND. This “GND” terminal is called “SHIELD” in the datasheet.
Deliberate Imbalance
Two simple experiments were conducted to determine the nature of the floating power supply.
Soft Short
A 1 kΩ resistor was connected between the blue AS-i wire and earth ground. The results are shown in Figure 5.
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This is almost identical to Figure 2 except that the DC levels have been shifted.
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The PLC and AS-i system continued to operate flawlessly with no visible errors. We will disregard the alarm capabilities of the Siemens AS-i master to keep this article short.
This provides strong evidence that the supply is truly floating as a 1 kΩ resistor is inconsequential to a 30 VDC 4 A power supply.
Hard Short
For the next test, the brown AS-i wire was shorted directly to earth ground. In practical terms this is like driving a screw through the yellow AS-i wire, thereby connecting one of the conductors to the grounded chassis of the conveyor.
The system continued to run flawlessly with no visible alarms!
This is a latent fault. A second fault will drop the system.
Figure 5: DC imbalance caused by a 1 kΩ resistor.
Recommended Periodic Maintenance
Before joining the DigiKey team I served as a sailor. Most marine vessels operate with a ungrounded 460 V three-phase system. Like the AS-i bus, the entire three-phase system floats. This is a fault-tolerant system. Without exaggeration, the system would continue to run if you accidentally ran a screw through a single power conductor.
We use a floating system as a form of redundancy, providing a single-fault-tolerant system. In sailor’s slang, we say that the first short is free, the second takes down the system. On the ship the second fault forms a hard phase-to-phase short that trips the circuit breaker.
My sailor’s three-phase story carries the same truth as the AS-i network. A short circuit of a single conductor to ground may lay hidden in your system. The second short will take the system down. With preventive maintenance we can find the latent short on our own time. Without maintenance, the second short will happen on its own time—the worst possible time—when downtime will damage your reputation and finances. In the AS-i example, the excessive current fault is power supply dependent. The failure can be handled gracefully with overcurrent protection or like a hammer on upstream circuit protection. Either way communications is lost when the power supply output collapses.
Let’s be honest, a single-conductor short is a 3 AM recall waiting to happen.
As a side note, you do not want circulating current in the hull of a ship. They tend to literally eat the hull (electrolysis) especially if you are running on a shore tie.
Procedure
As part of your routine maintenance, measure the DC voltage on each AS-i line.
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The brown line should read about +15 VDC and the blue about -15 VDC relative to the AS-i bus power supply’s GND (shield) terminal.
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There should be about 30 VDC between the conductors.
Classification of AS-i Faults
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Hard Fault: If you discover a line at zero volts, you have a “screw in the wire”.
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Leakage Fault: If you discover an imbalance, there may be leakage somewhere in the system. For example, conductive liquid may have worked its way into one of the AS-i vampire taps.
Next Steps
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Explore the datasheet and operating instructions for the AS-i master. Determine what faults are detected.
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Log AS-i faults as part of a robust auto-detection mechanism. This error count could identify a degraded system to be repaired on your time schedule not when the equipment fails.
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Audit your AS-i-related maintenance. Ask if your technicians can discover a degraded network before a hard failure sets in.
Sincerely,
Aaron
Continue Exploring Industrial Control Systems
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About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.