Before starting the question, a reminder of the difference between on how a baud rate and a bit rate relate.
Two people are standing 1km apart and they want to exchange a 8 bits of
data. Beforehand they decided that each will be given flags that have
digits from 0-3 printed on them. They also agreed that they will change the
flag every 1 second. They also agreed on the following symbol to bit
mapping:0 – 00
1 – 01
2 – 10
3 – 11
They will be exchanging the information at a rate of 1 symbol per
second. The baud rate is 1 symbol per second. The bit rate is 2
because one symbol carries 2 bits of information.
To connect to a serial console of an embedded device we physically connect the device with a cable similar to this one. After specifying the correct baud rate we get a terminal and are able to communicate with the device through our PC.
My question is whether there is a standard UART mapping between the symbols and the bits?
Is the UART symbol to bit mapping a one on one relation? Voltage level low -> 0 Voltage level high -> 1?
If not, what kind of carrier for bits is used?
Best Answer
The UART used in a PC COM port doesn't carry out modulation as such, that would only happen when you send the serial data through something like a modem or radio link where it has to be converted into an audio signal in the range carried by a telephone line or used to modulate a carrier wave.
The "mapping" is defined by the interface standard; in the case of RS-232, anything less than -3V is considered a logic 1 and anything greater than +3V a logic 0.