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Data Terminal/Communications Equipment 2.2.4 Protocol Analyzer 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 analyzer 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 analyzer, which are discussed in the following. Y "Cable" A Y "Cable" is not just some cable, but also contains electronics - assuming it is not a low quality 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 analyzer. 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. 9
RS-232 Connections 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. Man-in-the-Middle In this scenario the analyzer sits in the middle between the DTE and DCE. It is basically some device (e.g. a PC) with two serial interfaces. The analyzer mirrors each signal from one site to the other site, and also displays the traffic. +-----+ serial +----------+ serial +-----+ | DTE |----------| Analyzer |----------| DCE | +-----+ +----------+ +-----+ In principle, a simple version of such an analyzer 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 analyzers 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 analyzer can't be used to analyze faults due to signal voltage level problems. Nevertheless, any kind of protocol analyzer is much better than nothing at all. Even the most simple analyzer is very useful. Others See Setting up a Development Environment (for modem development) 1 for some more information. 2.2.5 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. 1 http://en.wikibooks.org/wiki/<strong>Serial</strong>_<strong>Programming</strong>%3AModems%20and%20AT%20Commands% 23Setting%20up%20a%20Development%20Environment 10
Page 1 and 2: Serial Programming Wikibooks.org
Page 3 and 4: Contents 1 Introduction and OSI Mod
Page 6 and 7: 1 Introduction and OSI Model 1.1 In
Page 8 and 9: Software Examples • Serial Networ
Page 10 and 11: 2 RS-232 Connections 2.1 Introducti
Page 14 and 15: Data Terminal/Communications Equipm
Page 16 and 17: Connection Types Figure 1 Female DB
Page 18 and 19: Connection Types Figure 5 mini-ster
Page 20 and 21: Wiring Pins Explained One thing to
Page 22 and 23: Baud Rates Explained 2.4.9 RI (Ring
Page 24 and 25: Baud Rates Explained 2.5.1 Modems E
Page 26 and 27: Signal Bits 2.6.3 Parity Bit To hel
Page 28 and 29: External References Baud Rate Maxim
Page 30 and 31: 3 8250 UART Programming 3.1 Introdu
Page 32 and 33: 8086 I/O ports char test; test = 25
Page 34 and 35: x86 Processor Interrupts 3.3.2 Inte
Page 36 and 37: 8259 PIC (Programmable Interrupt Co
Page 38 and 39: Serial COM Port Memory and I/O Allo
Page 40 and 41: UART Registers UART Registers Base
Page 42 and 43: UART Registers Divisor Latch Byte V
Page 44 and 45: UART Registers The Received Data in
Page 46 and 47: UART Registers When you are writing
Page 48 and 49: UART Registers these to a logical "
Page 50 and 51: UART Registers Each bit the data po
Page 52 and 53: UART Registers Line Status Register
Page 54 and 55: UART Registers Bits 7 and 6 are dir
Page 56 and 57: External References Read the value
Page 58 and 59: 4 Serial DOS 4.1 Introduction It is
Page 60 and 61: Finding the Port I/O Address for th
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Making modifications to UART Regist
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Basic Serial Input 4.5.1 Polling th
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Interrupt Drivers in DOS 4.6 Interr
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Interrupt Drivers in DOS able to us
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Interrupt Drivers in DOS Interrupt
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Terminal Program Revisited Port[Com
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Terminal Program Revisited RBR = 0;
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Terminal Program Revisited build a
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Serial Linux Classic Unix systems o
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Serial Linux 1. Terminals/teletypes
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Serial Linux for more information t
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Serial Linux signal and trap that s
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Serial Linux 5.2.6 uucp Overview Uu
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6 Serial Java 6.1 Using Java for Se
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Using Java for Serial Communication
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JavaComm API 6.2.2 Download & Insta
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JavaComm API wantedPortName); Syste
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JavaComm API 6.2.4 Simple Data Tran
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JavaComm API Before going into deta
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JavaComm API // makes sense to enab
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JavaComm API /** * Clear the buffer
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JavaComm API } } start = i; } retur
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RxTx Like with any other particular
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See also • SerialIO has a free tr
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Forming Data Packets • The footer
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Forming Data Packets bitrate or raw
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8 Error Correction Methods 8.1 Intr
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ACK-NAK A 1-bit sequence number (al
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combination If it sends a message b
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9 Appendex A:Modems and AT Commands
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Introduction forms the base for pro
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Introduction can still also be used
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Modem Programming Basics • By loo
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Modem Programming Basics Whatever i
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Modem Programming Basics 9.2.5 Char
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Changing State 9.4 Changing State 9
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AT Commands with the exception of #
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Result Codes Description: Result
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10 Contributors Edits User 10 Adrig
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List of Figures • GFDL: Gnu Free
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List of Figures 1 GFDL 2 User Mike1
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from that copyright holder, and you
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