TTL, RS232 and RS485 are the converters. UART is hardware module in microcontroller.

Can UART directly transmit and receive data without a converter? if it can't then why can't it ?

 

Micros can typically put out TTL compatible levels directly, no converter needed. RS232 and RS485 both require positive and negative voltages, which micros do not do, so a converter is necessary.

 

TTL, RS232 and RS485 are the converters. UART is hardware module in microcontroller.

Can UART directly transmit and receive data without a converter? if it can't then why can't it ?

Three important reasons:
1. A microcontroller output can only drive about 4mA. For RS232 it might need to drive 1km of cable, and that's a lot of capacitance. 4mA into that amount of capacitance would severely limit the maximum frequency (hence maximum data rate) that you can transmit
2. RS485 is balanced line - you would need two outputs from the UART one inverting and one non-inverting, and I have never found a microcontroller that can do that; and the same arguments about capacitance apply. It's balanced line so that it can accommodate different 0V levels at each end and eliminate interference.
3. A microcontroller input can easily be damaged by interference and other voltages that can exist on long cables. RS232/485 transceivers are designed to withstand them.
If you are transmitting data from one microcontroller to another on the same board, you don't need a transceiver.

 

This means that if we have to send or receive data over long distances, then we should use RS485 converter. And at less than that distance RS232 should be used. And less than that distance TTL converter should be used.

The conclusion is that the selection of the converter depends on the transmission distance.

 

This means that if we have to send or receive data over long distances, then we should use RS485 converter. And at less than that distance RS232 should be used. And less than that distance TTL converter should be used.

The conclusion is that the selection of the converter depends on the transmission distance.

That's a pretty good generalisation. RS232 is going out of fashion because it needs ±12V power supplies, and because it is limited to one source and one destination. RS485 can have multiple transmitters and receivers, but you have to design the software so that only one can transmit at once.
If you want multiple transmitters and receivers but don't want the trouble of anti-collision software use CAN.
If you need multiple transmitters and receivers on the same pcb, then use I2C.
You could also use fibre-optics.

 

There are some RS232 transceivers that have internal power supplies that make +/-10V supplies from 5V or 3.3V.

 

There are a number of reasons which all come down to distance.

The original requirement was to communicate with external devices that can be as much as 25 feet or 8m apart. This invites issues such as signal reflections, degradation, noise pickup, damage from high voltage EMI.

Converters are much more that voltage level shifters. They provide bandwidth limitations, line impedance matching, high voltage suppression, greater noise immunity.

If your connection is under 1 foot or 10cm it is safe to go direct UART-to-UART at low transmission rates.

 

I'm trying to understand how a one character is transferred from the microcontroller to the laptop over a USB to RS232 converter using the UART of microcontroller.

Both the microcontroller and the laptop's processor work on a 5 volt DC supply.
As mentioned above, RS232 require 12V power supply

What will be the voltage level between the microcontroller and the laptop when the microcontroller transfers a character to the laptop over usb to rs232 converter

 

A common RS-232 level interface chip like the MAX232 operates on 5 volts and has the internal circuits needed to eliminate extra external power supplies.

Saves Board Space
Integrated Charge Pump Circuitry
Eliminates the Need for a Bipolar ±12V Supply
Enables Single Supply Operation from +5V Supply
Integrated Capacitors (MAX223, MAX233, MAX235, MAX245–MAX247)
Saves Power for Reduced Power Requirements
5µW Shutdown Mode

Only the physical RS-232 line circuit (RS-232 inputs/outputs) will have +- voltage, everything thing else will be at the logic level voltages (5V, 3.3V, etc ...) of the micro-controller, USB or laptop.

 

The micro talks to the converter using TTL levels. A CMOS micro at 3.3V or 5V puts out acceptable TTL levels.

 

What will be the voltage level between the microcontroller and the laptop when the microcontroller transfers a character to the laptop over usb to rs232 converter

The microcontroller sends either 0V (for Logic 0) and 3.3V or 5V (for logic 1) (these are generally known as "TTL Levels") to the transceiver. If the input to the transceiver is 0V, the output on the RS232 line is +12V and if the input is 3.3V or 5V the output is -12V.
At the other end, the RS232 receiver reverses the process so the microprocessor in the laptop gets 0V and 3.3V, the same as the microcontroller sent to its transceiver.

 

As others have said, when two devices are connected via RS-232 they need RS-232 transceivers at both ends of the cabling.
Typically, the voltages used range from ±5V to ±15V. Normally, we don't worry too much about what these voltages are.

USB is a different protocol altogether, not with respect to voltage but how the data is transmitted.

UART is a single bit protocol called NRZ. You send a high voltage for a 1 and a low voltage for a 0.

USB protocol is a packaged system. Data is encapsulated in a package of many pieces of information.
In order to send a UART signal from a USB port an embedded microcontroller is required in order to decode the USB protocol and extract the single byte of data. Similarly, the reverse process has to happen when converting from a UART byte to the USB protocol.

In other words, USB ⇔ UART is a hack for compatibility with legacy devices.

 

As others have said, when two devices are connected via RS-232 they need RS-232 transceivers at both ends of the cabling.

I have downloaded this picture from internet please have a look at it. It uses only one trans receiver between two devices


Because one end of it connects to the USB port of the laptop, that's why I was thinking that it would also use the USB protocol.

 

Get one of these CP2102 USB-to-UART bridge.
You don't need RS-232 to connect to your PIC16F877A.

 

I have downloaded this picture from internet please have a look at it. It uses only one trans receiver between two devices
View attachment 283999

Because one end of it connects to the USB port of the laptop, that's why I was thinking that it would also use the USB protocol.

It doesn't show the other one that is in the USB to DB9 converter.