Help with Connecting Two EndPoints to an Arduino (Unable to use Qwiic Connectors)

Qwiic System
1

Hi SparkFun community,

I’m integrating two QwiicBus EndPoints into a custom I²C system but not using the Qwiic connectors. I’m wiring through the header pins only due to sourcing limitations and need help confirming the correct configuration. Here’s my setup:

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System Overview

•	Controller: Arduino UNO R4 WiFi (RA4M1-based)
•	Power Supply: 3.3V and 5V available from internal regulators
•	Wiring: CAT6 Ethernet cable (15–20 ft), not using Qwiic cables
•	Local EndPoint (controller box):
•	Connected to UNO R4 via SDA/SCL lines and header pins only
•	Powered by 3.3V or 5V (selectable)
•	A DS3231 RTC module is also connected to the alternate I²C pins on the UNO R4 (not the Qwiic bus)
•	Remote EndPoint (sensor box):
•	Connected via RJ-45 to the Local EndPoint
•	Connected to a TCA9548A I²C multiplexer
•	4x VEML7700 sensors connected to the mux
•	All devices powered from the Remote EndPoint via header pins

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Why I’m Not Using Qwiic Connectors

I originally tried powering the Local EndPoint through the Qwiic connector from the UNO R4. However, neither EndPoint lit up, and I measured only ~0.5V on the 3.3V pin at the Remote EndPoint. Due to this and part availability, I’m now wiring power and signal through the headers only.

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What I Need Help With

  1. Header Pin Usage:
    • What is the correct use of VCC1, GND, GRN, and GRNW pins?
    • Which of these pass through the RJ-45 cable?
    • Can I safely input 3.3V or 5V into VCC1?
    • Are GRN/GRNW connected to RJ-45 pairs or just breakout pins?

  2. Power Strategy:
    • If I supply 3.3V or 5V to VCC1 on the Local EndPoint, will that power the Remote EndPoint and its I²C devices through the cable?
    • Or should I inject power again at the remote side?

  3. Enable Pin Behavior:
    • Is the EN pin active-HIGH or active-LOW?
    • Can I leave it floating, or should I pull it high?

  4. Remote Power Load:
    • Remote sensor box will draw ~20–25mA total:
    • TCA9548A: ~2.5mA
    • Each VEML7700: ~1.8mA
    • Will this current be safely delivered via VCC1 over a 15–20 ft CAT6 cable?

  5. Multi-Bus Conflict:
    • I’m using the secondary I²C pins on the UNO R4 for a DS3231 RTC (not on the Qwiic bus).
    • Any risk of bus contention or conflict with the QwiicBus EndPoint running on the primary I²C lines?

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Thank you in advance — this differential I²C setup is ideal for my use case, and I’d like to finalize a reliable configuration with your input.