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The RS-232 standard is a collection of connection standards between different pieces of equipment. This is a rather old standard, and has been revised many times over the years to accommodate changes to communications technology. A bare-bones connection will have only one wire connected between two pieces of equipment, but usually there is more. Three wires (transmit, receive, and ground) are usually the minimum recommended. A fully implemented RS-232 connection can have as many as 25 wires between each end. Some of the early RS-232 connections were also used to connect terminal equipment to modems, so information about modems is sometimes found with general serial data communication.
[edit] Data Terminal/Communications Equipment
In the world of serial communications, there are two different kinds of equipment:
DTE - Data Terminal Equipment
DCE - Data Communications Equipment
[edit] Straight Serial Connections
In practice the distinction between the two pieces of equipment is really a matter of function rather than any real difference. As mentioned earlier, modems and serial communication equipment have been mixed together, this is another case of that. In this situation, the modem can be thought of as the Data Communications Equipment (DCE) and the terminal that somebody is sitting down and using is the Data Terminal Equipment. In the older days when it was common to use a timeshare computer system (pre 1980s), you would dial up a telephone, stick the handset that you would normally talk with into an acoustical modem, and that modem would be connected to a simple dumb terminal with an RS-232 cable. When we get to baud rates this will make more sense, but the typical connection speed was usually either 50 baud or 110 baud, and really fast connections going at 300 baud.
As a side note, when the very first IMP's (Interconnection Message Processors) that formed the first nodes/routers of ARPAnet (the ancient predecessor of the internet), this was exactly the connection system they were using. This later gave way to other communication systems, but this was the beginning of the internet.
In a more modern setting, imagine a piece of equipment in a very dangerous place, like in a steel processing mill that measures the temperature of the rollers or other steel processing equipment. This would also be a form of what we now refer to as a piece of "Data Communication Equipment" that we would also want to be able to control remotely. The PC that is used in a control room of the mill would be the Data Terminal Equipment. There are many other similar kinds of devices, and RS-232 connections can be found on all kinds of equipment.
The reason this is called a "straight" connection is because when the cabling is put together, each wire on each end of the connection is put to the same pin. This wiring system will be explained further on.
[edit] Null Modems
Often you don't always want to connect a piece of equipment to a computer, but you would also like to connect two computers together. Unfortunately, when connecting two computers with a "straight" serial connection, the two computers are fighting each other on the same wires.
One way to make this work is to connect the two computers to each other with a pair of modems. As explained earlier, this is a very common task, and in the 1980's and early 1990's it was common to have "Bulletin Board Systems" (BBS) where computers would call each other up with modems and exchange all sorts of information.
Now imagine if these two computers are in the very same room. Instead of going through the physical modems, they go through a "null modem", or a modem that really doesn't exist. In order to make this work you have to "cross" some of the wires so when you transmit some information on one end, the other computer is able to detect and receive that same information.
In addition to simply allowing a computer to communicate and transmit data to another computer, a null modem connection can be used to "simulate" the behavior of DCE equipment. This will be particularly important later on with some of the discussion in this series of articles, where you can experiment with writing some of your own serial communication software. In my own experience, I've had to write these "emulators" in many instances, either because the equipment that I was trying to communicate with wasn't finished, or it was difficult to obtain a sample of that equipment and all that I had available to me was the communication protocol specification.
[edit] Loopback Connectors
Sometimes instead of trying to communicate with another computer, you would like to be able to test the transmission equipment itself. One practical way of doing this is to add a "loopback" connector to the terminal device, like a PC with a serial data connection. This connector has no cable attached, but loops the transmit lines to the receive lines. By doing this, you can simulate both the transmission and receiving of data. Generally speaking, this is only done for actually testing the equipment, but can be used for testing software components as well. When this sort of connector is used, you will receive every byte that you transmit. If you separate out the transmission subroutines from the data capture subroutines, it can provide a controlled system for testing your application. at the
[edit] Protocol Analyser
[edit] General
When it starts to get very difficult to examine the serial data being transmitted by the equipment, sometimes it is nice to be able to take a "snapshot" of the information being transmitted. This is done with a protocol analyser of one kind or another.
What is done is a modification of the cabling that allows for a third computer to be able to simply read the data as it is being transmitted. Sometimes the communication protocol can get so complicated that you need to see the whole exchange, and it needs to be examined in "real-time" rather than going through some sort of software debugger. Another purpose of this is to examine the data exchange for purposes of doing some reverse engineering if you are trying to discover how a piece of equipment works. Often, despite written specifications, the actual implementation of what is occurring when transmitting data can be quite a bit different than what was originally planned. Basically, this is a powerful tool for development of serial communications protocols and software, and should not be ignored.
There are common ways to connect a protocol analyser, which are discussed in the following.
[edit] Y "Cable"
A Y "Cable" is not just some cable, but also contains electronics - at least if it is not a cheap junk cable. It is supposed to be placed in between a serial line and it mirrors all signals on a third connector. This third connector can then be connected to a protocol analyzer (e.g. a PC with some display software):
+-----+ serial +---------+ serial +-----+
| DTE |----------| Y Cable |----------| DCE |
+-----+ +---------+ +-----+
|
|
+----------+
| Analyzer |
+----------+
It is recommended not to use a passive Y cable. Such a cable overloads the transmitters at the DTE and DCE, which might result in the destruction of the transmitters. The RS-233 standard requires that transmitters are short-circuit safe. However, modern, highly integrated equipment might no longer be compliant to that particular aspect of the standard.
Often, the line going to the analyzer is also just a serial line, and the analyzer is a PC with a serial interface and some display software. The disadvantage of such a simple Y cable solutions is that it only supports half-duplex communication. That is, only one site (DTE or DCE) can talk at any time. The reason for this is that the two TX lines from the DTE and DCE are combined into one TX line going to the analyser. If the DTE and the DCE both send at the same time, their signals get mixed up on the third line going to the analyzer, and the analyzer probably doesn't see any decodable signal at all.
See http://www.mmvisual.de/fbintermdspy.htm for an example of some simple circuitry for a Y cable.
More advanced Y cable solutions provide the TX data from the DTE and DCE separately to the analyzer. Such analyzers are capable of displaying full-duplex communication. Advanced professional systems not only display the decoded digital information, but also monitor the analog signal levels and timing.
[edit] Man-in-the-Middle
In this scenario the analyser sits in the middle between the DTE and DCE. It is basically some device (e.g. a PC) with two serial interfaces. The analyser mirrors each signal from one site to the other site, and also displays the traffic.
+-----+ serial +----------+ serial +-----+
| DTE |----------| Analyser |----------| DCE |
+-----+ +----------+ +-----+
In principle, a simple version of such an analyser can be built with any PC with two serial interfaces. All that is needed is some software, which is not too difficult to write. Such a device will, however, lack a convenient feature. Professional analysers are able to auto-sense the speed of the serial communication. A home made solution needs to be configured to match the speed of the serial communication. Professional devices are also optimized to ensure minimal delay in the circuitry. Also, a simple homegrown, PC-based analyser can't be used to analyse faults due to signal voltage level problems. Nevertheless, any kind of protocol analyser is much better than nothing at all. Even the most simple analyser is very useful.
[edit] Others
See Setting up a Development Environment (for modem development) for some more information.
[edit] Breakout Box
An RS232 breakout box (a BOB) is a rather nifty piece of hardware which usually combines a number of functions into one. It basically consist of two RS232 connectors, and a patch field (or switches) which allows to change the wiring between the connectors. A patch field and small pieces of wires are preferable over (DIP) switches alone, since the patch field allows access to the signals for other purposes, too.
A breakout box is very useful if the pinout (DTE/DCE) of a particular device is not known. The patch field allows to quickly change the wiring from a straight connection to a null modem connection, or to set up a loopback connection.
Since the patch field provides access to all signals it also allows to use the breakout box to connect a protocol analyser. Better breakout boxes also provide some signal level information on their own, by having LEDs who inform about the signal voltage. This information is useful when trying to identify an unknown pinout. High-end BOBs contain circuitry to measure ground potential difference and pulse traps circuitry to find signal glitches.
Commercial breakout boxes are available in many varieties. It is also possible to build a useful BOB from a handful of simple parts on a circuit board. The patch field can be made from DIL IC sockets, and the wiring of the LEDs is simple if 2-pin dual-color LEDs are used (3-pin LEDs will not work). Each signal line should be connected via such an LED and a 680 Ohm resistor in serial to GND (Signal Ground). The home-made breakout-box is completed with a couple of RS232 connectors, possibly also one to attach a protocol analyser and some simple metal or plastic case.
[edit] Character Sequence Generator
Another nifty piece of hardware and/or software which is useful for developing and testing serial applications and equipment is a character sequence generator. Such a generator produces a repeated sequence of serial line data. For example such a generator might repeat the famous "The quick brown fox ..." sentence in an an endless loop. Another common test sequence is the generation of all 8-bit codes from 0x00 to 0xFF in a loop. Such a loop contains all 7-bit ASCII and 8-bit ISO Latin 1 characters, plus the first 32 non-printable control characters and can e.g. reveal decoding errors or transmission errors. Also very common is a modem test sequence, using generic modem commands (Serial Programming:Modems and AT Commands) to build up a modem connection, send some data and tear the modem connection down in a loop.
Commercial hardware character generators provide a heap of additional features, often combined with a protocol analyser. As such they are rather expensive. However, just like with a BOB, it is possible to build a useful DIY character sequence generator for small cash. This can either happen with software on a normal computer (some simple endless software loop sending the same data again and again to a serial interface), or with a few pieces of cheap electronic components. Some small stand-alone hardware is often more convenient in the field and in development for quick tests than e.g. a PC or laptop with some software.
A simple classic hardware character generator basically consists of a baud-rate generator, a UART (Serial Programming:8250 UART Programming), an (E)EPROM, a binary counter and a line driver (Serial Programming:MAX232 Driver Receiver). Typically, each of these components is a simple single IC. The (E)EPROM is supposed to contain the character sequence(s). The baud-rate generator drives the UART and the binary counter. The binary counter drives the address lines of the (E)EPROM. The result is that the character sequence is produced at the data lines of the (E)EPROM. These data lines are feed into the UART's input. The UART's output is connected to the serial line driver. All this can be easily fitted on a small prototype board in a simple case.
A more modern hardware character generator can be build around one of these small micro controllers (e.g. Atmel AVR). This is particularly easy, since these micro controllers already contain serial interfaces, and just require a little bit of serial programming - which is the topic of this book.
[edit] Connection Types
If you wanted to do a general RS-232 connection, you could take a bunch of long wires and solder them directly to the electronic circuits of the equipment you are using, but this tends to make a big mess and often those solder connections tend to break and other problems can develop. To deal with these issues, and to make it easier to setup or take down equipment, some standard connectors have been developed that is commonly found on most equipment using the RS-232 standards.
These connectors come in two forms: A male and a female connector. The female connector has holes that allow the pins on the male end to be inserted into the connector.
Posted by waqasahmad
1.)In telecommunications, RS-232 (Recommended Standard 232) is a standard for serial binary data signals connecting between a DTE (Data terminal equipment) and a DCE (Data Circuit-terminating Equipment).
2.)Or we can say that RS232 is an electrical signaling specification published by the Electronic Industries Association (EIA).
Working:
It is commonly used in computer serial ports.It supplies the roadmap for the way devices speak to each other using serial ports.RS-232 sets acceptable voltage and signal levels, along with common pin designations, or configurations, for wiring serial connector ports. It also specifies protocols for the control information passed between devices, which includes such events such as indicating the beginning or end of a data stream.
Without standards like RS-232, manufacturers would have no roadmap to build compatible product lines for technology. RS-232, or serial ports, are now used almost exclusively for dial-up modems. Other devices, like mice, that used to use serial ports, now make use of newer USB and Firewire ports. As the marketplace gradually switches away from dial-up modems to DSL, cable and satellite, RS-232 will eventually become an obsolete standard.
Posted by sagitraz
the Electronic Industries Association published three modifications, the most recent being the EIA232F standard introduced in 1997. Besides changing the name from RS232 to EIA232, some signal lines were renamed and various new ones were defined, including a shield conductor.
Posted by sunilpachavazte
RS-232 (Recommended Standard 232) is a standard for serial binary data signals connecting between a DTE (Data terminal equipment) and a DCE (Data Circuit-terminating Equipment). It is commonly used in computer
serial ports. A similar ITU-T standard is V.24.
RS 232 defines:
Electrical signal characteristics such as voltage levels, signaling rate, timing and slew-rate of signals, voltage withstand level, short-circuit behavior, and maximum load capacitance.
Interface mechanical characteristics, pluggable connectors and pin identification.
Functions of each circuit in the interface connector.
Standard subsets of interface circuits for selected telecom applications.
WORKING
In RS-232, data is sent as a time-series of bits. Both synchronous and asynchronous transmissions are supported by the standard. In addition to the data circuits, the standard defines a number of control circuits used to manage the connection between the DTE and DCE. Each data or control circuit only operates in one direction, that is, signaling from a DTE to the attached DCE or the reverse. Since transmit data and receive data are separate circuits, the interface can operate in a full duplex manner, supporting concurrent data flow in both directions. The standard does not define character framing within the data stream, or character encoding.
Posted by mayank2121
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