This delay is considered as inactive time and the RS line is at negative logic state V. Handshaking is the process of interchanging information signals between the sender transmitter and receiver. These signals build a communication link between the transmitter and receiver. In RS, there are two types of handshaking. They are hardware handshaking and software handshaking.
The connectors DB9 and Db25 are used for handshaking purpose. To send and receive the information without loss of data, it is necessary to maintain robust communication between the transmitter and receiver.
To do that, buffer is used. Buffer is a temporary storage location which allows the transmitter and receiver to store the data until the information is processed by each other at different speeds. In the above diagram, the transmitter and receiver have their own buffer.
The transmit buffer holds the characters to be sent to the receiver. While the receive buffer holds the characters received from the transmitter. If the transmitter sends data at a higher speed, the receiver may fail to receive. To avoid this, handshaking is used. Handshaking allows the transmitter and receiver device to agree before the communication is going to start.
The flow control of data transmission and reception is done using hardware handshaking. It stops the data being replaced in the receiver buffer. Hence, this is known as software flow control.
This handshaking requires 3 wires. In this state no data is transmitted. Now, the data transfer takes place. This stops the DTE to transmit the data. In this way, handshaking takes place by DTE request, taking control of the communication link and lets the DCE transfer data.
Apart from this, they both supports half duplex and full duplex communication. It sends and receives the data in serial form. The advantages of RS make it as a standard serial interface for system to system communication and also for the following benefits. The RS standard strongly implies to provide a 23 — pin and D — Type connector in order to support the entire functional utility of RS There are three kinds of signals in RS, i.
These function according to their direction in communication, signal types and with a variation of pins. Therefore, a much decreased function of 9-pin connector is in use mostly.
It is known as DE -9 which is a D —Type subminiature connector. Functional Features. RS officially is known to be a complete standard that can define more than electrical and mechanical features. R standard has the abilities of defining functions of various signals used in the interface. Procedural Specifications. In order to send data from router to Modem, the following procedure must be used In RS standard devices one wire transmits a changing voltage and other wire is connected to the ground as the wires has a single end.
Noise induced by differences in ground voltages of driver and receiver circuit affects the Single-ended signals. The information or data in RS standard is serially transmitted only in one direction over a single data line.
At any moment a byte of information can be passes given the fact, the previous byte of data has been transmitted already. RS strictly follows asynchronous communication protocol i. Hence, it needs start and stop bits to inform the receiver when to check for data.
There is a delay of certain time between the transmissions of each bit. First, the transmitter i. If parity bit is used, the maximum bits of 8 can be transmitted and if parity bit is not used then 9 data bits can be transmitted. After successfully sending the data the transmitter sends the stop bits which can be 1 bit, 2 bits or 5 bits long.
Going by the fact that RS is a complete standard, many manufacturers does not follow the standards. Some of them abide by the complete identifications while some only partly follow the specifications.
This is because this variation in implementation of the RS standard is that not all devices and applications require the full specifications and functionality of the RS Protocol.
For instance, a serial model which is using a RS may require more control lines than a serial mouse using the serial port. The process of transmitting and receiving which uses different identifications altogether is backed up by another process called the Handshaking.
Handshaking is a process that actively places the parameters of a communication between the transmitter and receiver before communication beginning. The requirement of handshaking is dependent on the speed of the transmitter at which it sends the data to the receiver and the speed at which the receiver receives it. In case of asynchronous data transmission system, there can also be no requirement of handshaking.
In few transmissions where handshaking is not used, the receiver DCE should read the data that is already received by it before the transmitter DTE sends the next data. The receiver needs to use a special memory location called Buffer, as it is used at the end of the receiver it is known as Receiver Buffer. The received data gets stored in the buffer before it is read by the receiver. The Receiver Buffer can usually store a single bit of data and this data must be read before the next data arrives and if it is not cleared, the present data will be overwritten along with the new data.
RS is only difficult if you try to use all the facilities that the standard provides. You really don't need to consider all the signals when using RS on the bench for simple communication between your hardware and a PC i.
If you want to make RS work over long distances then you must use all the signals including handshake signals as these ensure correct operation. For metres I only ever use the following:. These work for speeds of baud and beyond. This is a cable where pin 1 connects to pin 1 at the other end, pin 2 connects to pin 2 at the other end etc. If you don't know what cable type you have then use a multimeter to test on its buzz setting.
To make sure you don't drop everything on the floor stick a piece of wire in the female connector and wrap the other end of the wire around the multimeter probe - then you can concentrate on probing the pin in the male connector. To keep things simple use a straight through cable. Note: Serial cables are never marked as null modem or modem - they all look the same!
The key to getting an RS system to work is knowing exactly what cable you have and its pin connections. You can not make the crossover at the PC end as you just plug the serial cable into the PC - and that's fine. At this point you have direct connections from the Serial connector at the back of the PC all the way to your development board. So you know exactly where you are - all connections to the development board are straight through from the PC.
The final step is to make the cross over on the board. I have labeled the serial connector with the connections located at the PC - this makes it easier to visualize the whole system.
You can think of the PC and cable as one unit connecting directly to the development board and you do not need to worry about null modem cables, crossover cables, gender changers, cross over boxes etc. You only need to concentrate on the schematic with the PC serial port as an extension of the schematic.
All you do next is to connect the serial connector labeled TX which is the serial input to the development board from the PC to the receiver on your development board labeled R1IN. This makes the cross-over on your board. Next connect the grounds together GND and 0 volts.
In each program there will be a setting that defines the usage of the bits transmitted between the PC and the microcontroller.
To make RS work these settings must be identical. As a starting point use the following settings at each end. If you want to change anything just change one thing at a time but again keep the same value s in both bits of software.
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