it is a fact, that windows sometimes sends pulses on TX and DTR even if no software using the port is running!
You can see it on every windows-machine with a simple LED-Tester.
It occurs on a normal “old” hardware-comm and even on “modern” USB-Serial-Ports!
I see in this moment on my tester: ( I made a video! )
1: sporadic pulsing DTR high and going low again
2: Rising DTR, sending some pulses on TX and afterwards DTR goes low again
3: Sporadic pulses on TX without rising any statuslines.
These effects make many trouble since I work with serial equipments. (more than 20 years)!
Case 1 normaly has no negative effect.
Case 3 normaly has no negative effect. Without DTR=high most intelligent instruments ignore TX-Datas. But simple 3-wire-connected (TX-RX-GND) devices may be disturbed!
And case 2 brings errors to nearly all devices! The device receives TX-datas and can not interpret or does not understand. -> Protocoll-Error!
This happens on my maestro. I close the connection after resetting errostatus. No RED-LED! In a time of 1 to 10 Minutes the green LED-flickers sometimes. In most cases the red error-LED goes on!
I have set alls ports to “ignore” and my equipment works. But this is a workaround.
If somebody is interessted in how I work with Excel, finding the COM-Port-Adress via VBA and controlling my maestro via Excel I can post information.