To clarify:

1. Embedded development boards and some other devices/sensors intended to be connected to such boards use UART protocol with TTL (transistor-transistor logic) interface .

Serial communication at a TTL level will always remain between the limits of 0V and Vcc, which is often 5V or 3.3V. A logic high ('1') is represented by Vcc, while a logic low ('0') is 0V.

2. The serial port on computer and at PLCs complies with the RS-232 (Recommended Standard 232) telecommunications standard.

RS-232 signals are similar to microcontroller's serial signals in that they transmit one bit at a time, at a specific baud rate, with or without parity and/or stop bits.

TTL and RS232 signals differ solely at a hardware level.

By the RS-232 standard a logic high ('1') is represented by a negative voltage – anywhere from -3 to -25V – while a logic low ('0') transmits a positive voltage that can be anywhere from +3 to +25V.

On most PCs these signals swing from -13 to +13V or even from -7 to +7V.

The more extreme voltages of an RS-232 signal help to make it less susceptible to noise, interference, and degradation.

This means that an RS-232 signal can generally travel longer physical distances than their TTL counterparts, while still providing a reliable data transmission.

Following is RS232 and TTL signals diagram:

Summary:

PC/PLC RS232 interface and Embedded development boards/devices/sensors use the same UART protocol, but different physical interface and voltage levels.

This require to use Signal Level Converter between TTL and RS232 interfaces.

Such Signal Level Converters are supplied by different companies - DigiKey, Sparkfun, Element14, etc.

Hope this information helps.