RS-232 rounding

Should I round serial communication cables (when use it COM1 in laptop) pin 5 to UNO:s GND (same side as tx and rx)?

What is rounding?

You can not connect an RS232 com port directly to an Arduion, the voltages are too high and the logic is inverted. ORyou need an Rs232 to TTL converter.

I think we figured he means grounding in this context, but it is a concern, and noting his singular previous question, your comment Mike, is very apt.

Unless otherwise stated - and RS-232 is no exception - you require a ground in all the serial communications interfaces - RS-232, RS-485, RS-422, USB.

Ethernet is in fact different and should normally be isolated at each interface with a pulse transformer, usually incorporated into the board connector.

As Mike says, the concern is that the interface on an Arduino when you are not using the USB, is not RS-232 but a serial interface at TTL levels. Most laptops nowadays have USB in preference to RS-232 and so most Arduinos either require or contain a USB to TTL protocol translator which not only is grounded to the Arduino board, but commonly used as a source of its 5V power.

If you really want to connect to a genuine DE-9 connector RS-232 port, there are boards readily and cheaply available to interface to the TTL levels.

Thank you for your replies.

Can you name some of those boards (interface to TTL levels) that I could buy it?

I am very thankful for your helps!

How about this one