We are evaluating the MEGA4 with a Raspberry Pi 4B and a Waveshare RM520N-GL 5G DONGLE (B), whose documented USB input is 5 V / 2 A. We have read MEGA4 User Manual revision 1.01, including No Back-Power and About Output Current. Please confirm:
1. If the supplemental 5 V input disappears while the Pi stays powered and the modem is active, does MEGA4 immediately draw the downstream load from the Pi? What happens to port power when that load exceeds the Pi's total 1.2 A USB budget?
2. Is there a supported hardware configuration that prevents power draw from the host and keeps downstream ports off when supplemental power is absent, without modifying USB cables?
3. For a 2 A USB data device, what supplemental source and cable specification do you recommend? Your forum states that the USB-C input does not negotiate power; please distinguish the connector's 5 A rating from the current a standard USB source will actually provide.
We are not asking for uninterrupted operation or a modem shutdown controller. We need to establish the supported electrical behaviour before choosing the power arrangement.
I try to answer your questions as below:
1. What happens when supplemental power is lost?
Yes. MEGA4 allows power to flow from the USB host when supplemental power is unavailable. The ideal diode circuit prevents supplemental power from back-feeding the Raspberry Pi, but does not prevent power from flowing in the opposite direction.
MEGA4 has its own per-port overcurrent protection, but it does not know or enforce the Raspberry Pi's total USB current limit of 1.2A.
If the downstream load exceeds what the Raspberry Pi can supply, the voltage may drop or the host's protection circuitry may activate. This could cause USB devices to disconnect, reset, or behave unpredictably. In some circumstances, the Raspberry Pi itself may also become unstable or reboot.
MEGA4 cannot guarantee stable operation under these conditions.
2. Can host-side power draw be disabled?
There is no supported hardware configuration that disables power draw from the USB host or automatically turns off downstream ports when supplemental power is lost.
Modifying the upstream USB connection or MEGA4 hardware might make this possible, but such modifications are not officially supported and may affect USB functionality.
An external circuit could potentially detect the loss of supplemental power and use PPPS commands to turn off the ports. However, this approach depends on detection and software communication, so it cannot guarantee a sufficiently fast or reliable response.
If preventing power draw from the host is a strict requirement, the standard MEGA4 configuration does not meet that requirement.
3. Recommended power supply and cable
For a USB device requiring up to 5V/2A, we recommend a stable, regulated 5V power supply capable of delivering at least 3A, assuming no other high-current USB devices are connected.
A conventional fixed 5V power supply is preferable. MEGA4 does not implement USB Power Delivery (PD) negotiation, so it cannot request higher power from a PD charger. The 5A rating of its USB-C input connector describes the connector's current-carrying capability, not the amount of current that any connected USB power source will necessarily provide.
The power cable should be rated for the required current and have sufficiently low resistance. A short, good-quality cable rated for at least 3A is recommended, since excessive voltage drop could cause problems under heavy load.
Please also allow sufficient power capacity for the MEGA4 itself and any other connected USB devices.