After (mentally) banging my head at the wall wondering why my own circuit is glitchy after the PC communicates with it, I read some posts, and noticed others had very similar problems. All because the charge-pump action of the Max232 IC causes noise in the high millivolt range.
My source of info is here: https://e2e.ti.com/support/interface/f/138/t/151543#pi320995=1
I already considered the two-transistor circuits in place of Max232 but they aren't meant to be very reliable.
The speed I'm trying to work with is 38400bps and I want to be able to send and receive any set of characters over the serial port.
I thought of a replacement circuit but I don't know how well it works, but if it has hope, I'll add capacitors at the input and output of each regulator and between each VCC and GND pin.
Since much of my circuitry runs on 5V, the idea here is to use 2 pairs of inverters with 12VDC for VCC (might have to try CMOS IC) then the conversion takes place in the inverters themselves, then after conversion, the signals are sent to and from microcontroller (AT89C2051 in this example) via 5VDC only. The "SerialIn" and "SerialOut" are connected directly to the PC's serial port's TX and RX respectively.
Would such an idea work? If not, then what is a better alternative to Max232 that's commonly available and that doesn't make noise?