Prevent USB serial connection from restarting MEGA

[kootzie]

I have developed an application using Feather Mo LoRa + DHT20 + RTD sensors
to wirelessly measure humidity and temperatures.
The "server" or aggregator is a MEGA + LoRa module/chip

These have been powered and connected via USB and I use two concurrent IDEs
to program/debug both client and server nodes
I have now developed USB-free operation...
The MEGA is powered by a 9V adapter into its power plug
The Feather M0 (SAMD21) and powered by a Lipo battery
I have incorporated low power "sleeping" between hourly readings to maximize battery life.

I notice now that the MEGA server is running and providing data to the telemetry system.
When I connect the USB in otder to monitor/interact with the server application,
the system appears to restart/reboot - all data is cleared and there is a period of
re-normalization...
What gives with that ??
I expect that since server power is being supplied by a power supply,
the USB connection should have no impact on the running server application.
I should be able to connect via USB serial, examine the operating status or
perform some adjustments and then disconnect
Some ideas as to the cause...
The transition between power-supply power and USB power could be causing a restart ?
Some quirky setting "must be happening"

[J-M-L]

It is by design that the Arduino resets when the PC opens the serial port

A possible solution is to use a USB to serial cable (FTDI cable - USB-ttl cable) connected directly to pins 0 and 1

There are other options documented on the internet like

ARDUINO MEGA 2560 REV3 Disable auto-reset

Hello everyone, surfing the net I read some articles that say that you can disable the auto reset of the arduino. I have not found anything about the ARDUINO MEGA 2560 REV3, can someone tell me if it is enough to put a 120 ohm resistor between the Reset and 5V or for this model there are other things to do? I'd like a confirmation before I go and ruin my card... Goodbye, everyone

(First random Google hit)

[sterretje]

3^rd party terminal program often allow you to disable DTR and RTS when the port is opened; those signals cause the reset of the Arduino.

[blh64]

The most flexible way of solving this is to cut the jumper on the board "Reset EN" and then it will no longer reset when you connect. If you attach a switch to each side of the jumper after cutting it. You can put the switch in two modes: 1 - connection is made and the board will reset - use this for programming your board. 2 - connection is broken and the board will not reset - use this for normal operation when you want to connect and view data.

[kootzie]

Kewl... Thank you very much for that precise concise explanation.