Serial protocols

WHAT IS PROTOCOL?

             An agreed-upon format for transmitting data between two devices. The protocol determines the following:

1. The type of error checking to be used.

2. Data compression method, if any.

3. How the sending device will indicate that it has finished sending a message.

4. How the receiving device will indicate that it has received a message.

5. It also uses (saves) following (master and slaves) data i.e.,

          Slave ID
          Function code
          Starting address
          Read count

RS232, RS422 And RS485

RS232 

1. Architecturally RS-232 is a bi-directional point to point link.

2. In this type there is 1 driver (master) and 1 receiver.

3. It is a example of single ended point to point bus topology.

4. Communication as defined in the RS232 standard is an asynchronous serial communication method.

5. The word serial means, that the information is sent one bit at a time.

6. Asynchronous tells us that the information is not sent in predefined time slots.

7. In this type [at driver side logic 0 signal means (+5V to +15V) and 1ogic 1 means (-5V to -15V)] and at receiving side logic 0 signal means (+3V to +15V) and logic 1 means (-3V to -15V) .

Fig.1shows point to point communication of RS232 communication

Fig.2 shows connection of driver and receiver

Pin diagram

Fig. 3 shows pin diagram of RS232

Advantage and disadvantage

Advantage
1. Low system cost
2. Simple to implement

Disadvantage
1. Noise and crosstalk
2. Ground shifts
3. Low data rates
4. Low line length

RS422

1. RS422 is a connection follows multi-drop topology.
2. In this connection 1 Driver (master) and multiple receiver.
3. It is an example of differential multi-drop interface.
4. It allows 10 receiver to be interface with 1 driver.

Fig.5 shows multi-drop RS422 communication

Fig. 6 shows differential connection of RS422

 Pin diagram

Fig. 7 shows pin diagram for RS422 and RS485

Advantage and disadvantage
Advantage
1. Low sensitivity to crosstalk
2. Ground noise rejection
3. Common Mode noise rejection
4. High signaling rates(>10 Mbps)
5. Long line length up to 1200 m

Disadvantage
1. Twisted pair cable required
2. Higher cost

RS485

1. It is an example of differential multi-point interface.
2. It allows high data rates communications over long distances.
3. RS-485 was designed for greater distance and higher baud rates than RS-232.
4. According to the standard, 100 kbit/s is the maximum speed and distance up to 4000 feet (1200 meters) can be achieved.

Fig.9 shows multi-point RS485 communication

Fig. 10 shows differential connection of RS485

Summery of all RS232, RS422, RS485

Related articles

• RS232 (for more information)
• RS422 (for more information)
• RS485 (for more information)

For making these notes some data are copied from Texas instruments videos and from internet, only for educational purpose.