No evidence establishes a fixed hardware or PME device-count limit for a dual-port Ethernet daisy chain. Determine the usable chain length from the confirmed device specifications, subnet capacity, upstream-switch restrictions, traffic load, cable-run limits, and the outage risk created by each inline device.
Separate hard limits from practical limits
The available evidence describes each dual-port device as behaving like a small embedded switch: one internal connection serves the device and two external ports extend the network. Treat this architecture as a hypothesis until the device manual confirms it. The evidence also cites a 100 m Ethernet wiring limit, but does not identify the cable category, link speed, measurement endpoints, or governing standard; verify the permitted link length against the product documentation and applicable Ethernet cabling rules.
| Constraint | Engineering decision |
|---|---|
| Device hardware | Check the manual for a maximum linear-node count or embedded-switch restriction. |
| Upstream switch | Confirm whether its port limits learned addresses or connected devices. |
| IP subnet | Verify that the subnet has enough usable addresses for every device. |
| Cabling | Validate every individual Ethernet link; the evidence mentions 100 m but does not establish the applicable specification. |
| PME | Confirm the supported device count and polling capacity for the actual server and project configuration. |
Calculate the traffic constraint
All downstream communications traverse the upstream portion of a linear chain, so that portion carries the aggregate traffic. The evidence does not provide a confirmed port speed, packet rate, PME polling interval, or per-meter traffic volume; therefore, it cannot support a numeric device limit.
Aggregate upstream load = sum of traffic from all downstream devices
Required margin = link capacity - aggregate upstream loadMeasure utilization during normal polling and during bursts such as simultaneous data retrieval. A low average does not prove that response time, retries, or peak utilization will remain acceptable.
Evaluate the availability tradeoff
An inline device that loses control power or fails may disconnect every device downstream. This is the strongest practical constraint in the evidence, especially when meters take control power from the monitored equipment rather than an independently backed source. A chain contained within one enclosure has a smaller physical fault domain than a chain spread across multiple locations, but it still retains the same downstream dependency.
Do not assume Ethernet behaves like an RS485 multidrop trunk during a device power loss. In the described topology, each device participates in forwarding traffic; test whether its ports pass traffic without device power before relying on that behavior.
Validate the proposed chain
- Confirm the dual-port forwarding architecture and any stated node-count limit in the exact device documentation.
- Verify each cable link against the documented Ethernet distance and media requirements.
- Check subnet capacity and upstream-switch address or device restrictions.
- Connect the planned device count and measure PME response, retries, missed data, and link utilization under representative traffic.
- Remove control power from a middle device and verify which downstream devices become unreachable.
Accept the topology only if both communication performance and the power-loss test meet the application's data-availability requirements. If losing several downstream devices is unacceptable, divide the installation into shorter chains or connect devices through a topology that removes the inline power dependency.
FAQ
How many dual-port Ethernet devices can I daisy chain?
The evidence provides no fixed count. Establish the limit from the exact device documentation, upstream-switch restrictions, subnet capacity, measured traffic, PME performance, and acceptable outage scope.
Is the Ethernet daisy-chain cable limit 100 meters?
The evidence mentions 100 m as an expected Ethernet limit but does not define the media or link endpoints. Verify every individual link against the device manual and the applicable cabling rules.
What happens if a meter in the middle loses power?
The evidence indicates that downstream devices may lose their network path. Prove the behavior by removing control power from a middle device and checking downstream connectivity and PME data collection.