Commonly, the baud rate with , , , are used. It determines the speed at which data is to be sent from the transmitter to the receiver. Note: The baud rate has to be same at both the transmitter side and receiver side. DB9 connector has 9 pins and DB25 connector has 25 pins with each pin having its own function.
Apart from the electrical characteristics, RS defined the functions of signals that are used in the serial interface. Some of them are common ground, Data, control and timing signals. Here is a list of signals used in RS pinout. They are Null modem and Straight-cable. Next one, is the Straight-through cable.
As the name implies, it is a one to one connector, i. Apart from connections, the cable length depends upon the wiring capacitance. As per specification, the cable length is nearly 80 feet. The working of RS can be understood by the protocol format. As RS is a point-to-point asynchronous communication protocol, it sends data in a single direction.
Here, no clock is required for synchronizing the transmitter and receiver. The data format is initiated with a start bit followed by 7-bit binary data, parity bit and stop bit which are sent one after another. The parity bit is appended to this data for the receiver validation. The data sent from the transmitter should match at the receiver. Generally, 1 or 2 stop bits can be sent. While sending data, there should be a certain delay between each bit. 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. However bit length is usually set to 8 bits - this is very commonly used.
The RS parity bit is a crude error detection mechanism. You can use either odd parity or even parity or none at all in the diagram above a parity bit is included between the last data bit and the stop bit - Here the parity is indicated as a '1' meaning that there are an odd number of databits.
So the parity in use is odd-parity. The diagram below commonly used in microcontroller work does not use a parity bit. It simply evaluates all the data bits and for odd parity returns a logic one if there is an odd number of data bits that are set. For even parity an even number of data bits that are set, sets the parity bit. At the receiver the parity bit is used to tell if an error occurred during transmission. You can use this in the receiver software by reading a flag in the UART module.
The problem with error detection using the parity bit is that if two bits are in error then the parity check fails. This is because each error cancels the effect of the other in terms of the parity calculation. Any even number of errors causes a failure in error detection. It won't be a problem on a bench top based system that has no critical data transfer. Over a short cable e.
Normally I use no parity and there is no problem at all. For systems running over a long distance or in a noisy environment a better system should be used e.
Adding a cyclic redundancy check to the data stream before and after it is sent over the RS CRCs let you check for and correct quite a few errors without re transmitting the data. The RS stop bit merely gives a period of time before the next start bit can be transmitted.
It is the opposite sense to the start bit and because of this allows the start bit to be seen i. If there was no stop bit then the last bit in the data stream would be the parity bit or data bit if parity is not active.
This would change depending on the data sent so if it had the same sense as the start bit then the start bit could not be seen! At very high baud rates the period from stop bit to start bit assuming data is being sent continuously will be very small e. This can allow the receiver to detect the start bit more easily since if there is capacitance on the line the waveforms will exhibit a CR rise fall time.
Extending the time period allows more capacitive loading. However it also depends on the receiver hardware used. Typical settings for use on the desktop e. See here for more on that. At some point you may want to make a software UART perhaps to save code space in your current design maybe you don't need the receive part - as you are just outputting variables or to use a spare pin.
To create it you need the actual signal diagrams that you see at the microcontroller pin strangely these are hard to find on the web.
The following diagram shows the timed 0V and 5V bit stream at the output pin of the microcontroller. The lower diagram shows the translated signal levels at the RS output drivers which are transmitted over the serial cable. These higher levels are are generated by sending the V logic levels to a transceiver chip e. The lower waveform in the bove diagram above shows the RS signal that you would see using an oscilloscope on the output drive of a translator chip such as the MAX Note that the RS idle voltage is V,.
In the diagram above the lower waveform shows the RS voltage signal that you can expect to see at the output of the RS TX pin also the same levels on the RX pin on an oscilloscope.
Handshake signals are simply a method of stopping data flowing. If some part of the system is busy it may not be able to accept more data and rather than losing it signals are used at each end of the link to tell the other end to stop transmitting data. There are two types used in RS hardware handshake and software handshake. Check back soon for more RealPars blog post!
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What is RS and what is it used for? Today you will learn about RS There are a couple of disadvantages of RS With so much love and excitement,. The RealPars Team. Search for:. By Kevin Cope. Instrument Mechanic. Posted on Sep 17, Checkout your email. Have questions? Sign in. Contact Us.
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