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NEED OF RS232
Simple analog communication over the telephone wires to the typical USB cables for data exchange, we surely have come a long way in the field of communication. RS232 was the first milestone reached in this journey. It was a standard for electromechanical typewriters and modems for digital data exchange introduced in 1962 by the Radio Sector of EIA. It made the data exchange more reliable over analog channel. The standard defined voltage levels that made it immune to noise disturbances and reduced the error in data exchange.
As the technology was growing many electronic devices were being developed during this time like computers, printers, test instrument etc. There came a time where manufacturers felt the need to exchange information between these electronic devices. For example data exchange between a computer and a printer or two computers. But there was no standard or method to accomplish this task. RS232 was the only available standard at the time which was used for data exchange. So, they thought of adopting this standard in electronic devices for digital data exchange. But the standard was unable to fulfill the requirements as it was developed specifically for modem and teletypewriter. To overcome this problem, designers started implementing an RS232 interface compatible to their equipments. Like a computer of HP will only be able to use HP peripheral devices. Because of this the market was flooded with different manufactures with their own standards for their devices. This led to the common problems like non-standard pin assignment of circuits on connectors, and incorrect or missing control signals. The lack of adherence to the standard produced a thriving industry of breakout boxes, patch boxes, test equipments, books and other aids for the connection of dissimilar equipments. So, to put an end to all theses disparities in equipments, syndicate of manufacturers built a transmitter that supplied +5V and -5v and labeled them as “RS-232” compatible and they are same till date. The standard has been revised many times after the initial one and updated by Electronic industries association. The name of standard was also changed from RS232 to EIA232. The Electronic Industries Association published three modifications, the most recent being EIA232F introduced in 1997.
What is RS232 - "RECOMMENDED STANDARD 232"
RS-232 is a standard communication protocol for linking computer and its peripheral devices to allow serial data exchange. In simple terms RS232 defines the voltage for the path used for data exchange between the devices. It specifies common voltage and signal level, common pin wire configuration and minimum, amount of control signals. As mentioned above this standard was designed with specification for electromechanically teletypewriter and modem system and did not define elements such as character encoding, framing of characters, error detection protocols etc that are essential features when data transfer takes place between a computer and a printer. Without which it could not be adopted to transfer data between a computer and a printer. To overcome this problem a single integrated circuit called as UART known as universal asynchronous receiver/transmitter is used in conjunction with RS232.
It is clear from this figure that UART, line drivers and RS232 are three separate parts in the system each having its own characteristic features. UART and line drivers are the parts in RS232 to enhance quality of system during serial data exchange.
A standard definition was given by EIA to define RS232 as “an interface between Data terminal equipment and Data communication equipment”. A typical RS232 system is shown below.
DTE-A DTE stands for data terminal equipment is an end instrument that convert user information into signals or reconverts the receive signal. It is a functional unit of station that serves as data source or data sink and provides for communication control function according to the link protocol. A male connector is used in DTE and has pin out configuration.
DCE-A DCE stands for data communication equipments. It sits between the DTE and data transmission circuit for example modem. A DCE device uses a female connector which has holes on the surface to hold male connector.
A minimum of three signals are required for communication between a DTE and a DCE devices. These signals are a transmission line, a reception line and ground. These two devices communicate with each other by handshaking. It allows a DTE and a DCE device system to acknowledge each other before sending the data.
Handshaking is a process in which a DTE device sends a signal to a DCE device to establish a connection between the devices before the actual transfer of data. It sets the parameters of communication channel established between two equipments before normal communication over the channel begins. It follows physical establishment of the channel and precedes normal information transfer. Handshaking makes it possible to connect relatively heterogeneous systems or equipment over a communication channel without the need for human intervention to set parameters. This same concept is used in RS232 to allow two devices communicate with each other before the actual exchange of information.
All these terms put together gives a complete picture of a RS232 system starting from DTE to DCE with UART, line drivers and RS232 as conjunction between them.