RS-232: Is the TX & RX line suppose to be isolated from one another?

Using the continuity functionality on a multimeter (the symbol for this function on the FLUKE 77 IV Multimeter looks like 5 parenthesis put side by side, same orientation (cup to the left), increasing in size), I get a "beep" when placing the multim's probe leads across the TX & RX line.

I concluded that they are shorted.

My better (supervising coworker) told me to not rely on this function to determine a short and instead use resistance measurements (which works).

I'm just curious what then the continuity function serves.

I would expect the continuity function to identify a short circuit - but it may be triggered at a low level of resistance (i.e. higher than zero) that, in some cases, is acceptable.

The resistance measurement gives you a bit more information.

The Rx and Tx wires in a cable should be isolated. But if the cable is connected to something you will be measuring the resistance within the something.


A discharged cap may also look like a short, hence the short beep as the cap starts charging up.

The pins are internally isolated by their circuitry, but they likely both have diodes to Vcc and Gnd for ESD protection.