Sentinel architecture: single-cable decision

One of the more debated decisions in the Sentinel HMD-1 architecture is the single-cable approach: power and data on a single USB-C connection, no separate power rail, no dedicated data bus. This note documents the reasoning and the trade-offs accepted.

The conventional approach

Most HMD systems at this form factor use separate power and data connections. The argument is straightforward: isolation reduces noise coupling, allows independent power management, and means a data cable failure doesn't also kill the display. This logic is sound, and for higher-power or higher-precision systems, it's correct.

For the Sentinel HMD-1 operational brief — low-signature, mobile, field-serviceable — the two-cable approach introduces costs that aren't offset by the benefits.

What two cables actually costs

In the field, a second cable is not just a cable. It's:

• A second connector on the head unit (weight, waterproofing, mechanical failure point)
• A second cable route to manage (routing, retention, chafe)
• A second spare to carry in the service kit
• A second failure mode to diagnose when something goes wrong

The numbers that settled it

Maximum measured head unit power draw at B01: 3.8W at peak display brightness with all sensors active. USB-C PD at 9V/2A headroom gives us 18W available — 4.7× margin. Noise coupling at these signal levels is below the display driver's rejection threshold by approximately 34dB; the isolation argument doesn't apply.

Data bandwidth requirement for the configuration interface and diagnostic telemetry combined: under 2Mbps sustained. USB 2.0 headroom is 480Mbps. The cable isn't a bottleneck.

The USB-C connector specified (Molex 2172021001) is rated for 10,000 mating cycles. That's the single-cable failure mode. At one connect/disconnect per operational day, it's a 27-year part. We'll wear out the rest of the unit first.

What we accepted

• Dependency on USB-C PD negotiation — tested to full compliance in firmware rev B01.3, including the fallback state that drops to 5V/1.5A if PD negotiation fails (which reduces display brightness and disables one sensor rail, not a hard failure)
• Single point of failure at the cable — mitigated by the field-swappable display module
• Reduced maximum power delivery compared to a dedicated rail — acceptable at current loads; will be revisited if the B02 sensor suite increases draw above 6W

Result

The B01 architecture uses a single USB-C connection. PD negotiation adds approximately 180ms to boot time — measured, documented, within brief. Field tests to date have not surfaced cable reliability as a concern.

This will be revisited at B02 if field data suggests otherwise.