ESP8266 Community Forum

I think RS232 is allowed to be +3 -> +15V and -3->-15V. Levels actually seen vary from driver to driver. The cable converter as linked has typical +/- 6.5V

Series resistors are always a good idea if there is any uncertainty about levels and will normally be prevent damage if current is less than a mA or so.

Without trying to nit-pick, real RS232C is/was commonly 12 Volts, sometimes a bit higher to span longer distances. So that's why I like to keep the nomenclature clear; separate between RS232C and serial TTL. It also has a completely different purpose; serial TTL seldomly spans more than a few feet, mostly even within a device, while RS232C is inter-device. For real RS232C generally TTL->RS232C converters are used, so don't use them when talking serial TTL

And then some nit-picking on myself: TTL is officially 0 V vs. 5 V, so the 3.3 V variant really should be called CMOS serial or something like that .

also w/o nitpicking: according to wikipedia the standard (ANSI/EIA/TIA-232) says: +/- 3...15V
https://en.wikipedia.org/wiki/RS-232

the difference between TTL vs. CMOS is technology (not the supply voltage) and thus the technology dependend signal level thresholds for H/L.

I wrote: "in practice"...

Also the name TTL or CMOS "levels" are questionable, but in wide use, so I can't hardly oppose.