(转)monitor serial RS232 data

bilbo0214

Introduction on monitoring serial RS232 data
The RS232 standard defines an asynchronous way of communication between DTE,
data terminal equipment (computers, printers, etc.) and DCE,
data communication equipment (modems). This type of communication has become the minority
and nowadays serial communications is mainly between two DTE devices
using a
null modem cable
. Although this is
1:1 communication, it is possible with special cables to monitor the data streams.
RS232 provides 2 data lines for each data channel. One is for transmitting data and the other
for receiving. Because of these two separate lines, data can be send full duplex. This means
that both ends can send and receive data simultaneously without mutual interference. In most situations
however the high level communication protocol only allows half duplex communications because most simple
protocols with external devices work with a master-slave, or question-answer
configuration. One of the parties is the master which is in charge of communications. This master
sends commands and requests to the slave which responds to them. The slave will never by itself
start a communication sequence so in practise the communication is half duplex: There is no single
moment when both sides send data simultaneously.
That most RS232 communication is performed in a half duplex way is important if the data stream has
to be monitored. A half duplex communication protocol can be spied with a computer with just one
serial port attached. This port listens to both RS232 communication lines simultaneously but no
data will be garbled because only one party sends at a time. This type of communication can be spied
with simple software like the terminal emulation program HyperTerminal which is shipped with the
Windows operating system.
In the situation of full duplex communication on a RS232 channel we cannot simply tie both lines
together and listen to it. For this situation you need two separate serial ports on the spionage
computer. Also special sniffer software is handy that listens to both ports simultaneously and outputs
the data of both lines to the screen or to disk.
Half duplex RS232 spy / monitor / sniffer cable
It is not difficult to monitor half duplex RS232 serial communication between two devices
with a PC. To do this you need the RS232 monitor cable which is displayed in the
next picture. Two DB9 connectors are wired straight through. The spy computer is connected
to the third connector. This monitor cable taps communication from two sources
on only one RS232 receiver port. This means that if
the two devices happen to talk simultaneously, the monitored information
will be garbage. In most circumstances communication protocols work half
duplex, in which case this RS232 cable will work without problems. Otherwise
you need the full duplex RS232 monitor cable which is discussed here also.
Half duplex RS232 spy / monitor / sniffer cable

  Connector 1Connector 2SpyFunction
  11-Carrier detect
  222 via R1Rx

Rxspy
  332 via D1Tx

Rxspy
  44-Data terminal ready
  555Signal ground
  66-Data set ready
  77-Request to send
  88-Clear to send
  99-Ring indicator
  --1 + 4 + 6DTR

CD + DSR
  --7 + 8RTS

CTS
The electronic diagram
looks simple and strange at the same time with one diode and one resistor.
The functionality is however straight forward. The spy computer is attached
to the connector in the right bottom. The female connector at the left
is attached to the spied computer and the male connector at the right to the
attached device.
When an RS232 port is in an
idle state
,
it will be in the so-called marking
state with a negative voltage at the transmit output. Assume the computer
connected to the left port is sending data and the peripheral device at the right
side is idle. At that moment the RS232 signal level
on line 3 will change. When the voltage of this line changes to a higher value,
current will flow through the diode to the spy computer. We assume the attached device is
in an idle state. Therefore, the voltage at line 2 is something like -12 Volt,
while at the other end of the resistor +12 Volt is applied. Simple
mathematics learns that a current of approximately 11 mA (=24 Volt/2200 Ohm) flows
through the resistor. This is no problem because most RS232 driver IC's
are capable to deliver at least 45 mA. Because the voltage drop over
the diode is only 0.7 Volt—independent of the current through the
diode—the spy computer will see on its RS232 port (almost) the same voltage levels as
present on the transmit port of the sending computer and data from
the sending computer to the peripheral device is successfully captured.
In the second situation the computer has finished sending data and waits for an answer from the device
at the male connector. The RS232 signal level at line 2 will go to positive values.
The diode will block current to line 3 so the spy computer effectively
only sees the data coming from the peripheral device. Now the spy computer will be able
to pick-up the data send from the device back to the computer.
In the diagram for the half duplex monitor cable some shorts have been made between pins
of the connector of the spying computer. These shorts loopback the handshaking signals of the
computer. In most cases these shorts won't be necessary, but if the spy monitoring software
uses handshaking, this will prevent the monitor software from blocking.
You don't need expensive software to use this RS232 spy cable. A simple
serial terminal emulator like the HyperTerminal program present on all Windows
based computers is enough to spy your communications. The only thing you need
to do is changing the baudrate and start and stop bits settings from the
terminal emulation program to the settings used on the line to monitor.
Full duplex RS232 spy / monitor / sniffer cable
As already discussed, it is not possible to monitor a full duplex RS232 communication
with only one spy port. For this purpose the full duplex monitor cable can be used. This
cable connects to two serial ports on the spy computer where each ports taps one direction
of the communication. You could open two sessions of a terminal emulation program on your
computer, but often better is to use one of the specialized RS232 monitor software products.
In that way the two communication streams are merged in one screen which makes it easier
to analyze the sequence of the communications.
Full duplex RS232 spy / monitor / sniffer cable

  Connector 1Connector 2Spy port 1Spy port 2Description
  11--Carrier detect
  222-Rx

Rx1
  33-2Tx

Rx2
  44--Data terminal ready
  5555Signal ground
  66--Data set ready
  77--Request to send
  88--Clear to send
  99--Ring indicator
  --1 + 4 + 6-DTR

CD + DSR
  --7 + 8-RTS

CTS
  ---1 + 4 + 6DTR

CD + DSR
  ---7 + 8RTS

CTS
The diagram of the full duplex RS232 monitor cable is actually simpler than the diagram of
the half duplex monitor cable. This is because no special circuitry is necessary to combine
two communication lines on one input. Just to be sure, all handshake signals on both spy
connectors have been looped back. This prevents the software from blocking input in case
it checks the CTS, DSR or CD inputs.
Other RS232 monitor solutions
  
Besides the cables mentioned above, there are ready made adapters available on the market which monitor serial communications
on RS232 channels. An interesting product is the
EZ-Tap™ RS-232 Passive Tap Module
from
Stratus Engineering. It allows you to monitor RS232 communications via the USB port.
Software for RS232 logging
A spy cable is not the only requirement to monitor incomming and outgoing RS232
communications. You will also need software which can log the incomming data and store it.
This software should—in case of full duplex communication—be able to log
at least two channels concurrently.
My advice is to use
Comm-logger from Willies Computer Software Co
.
This software can log up-to 32 channels concurrently and will be sufficient for most monitor and log
purposes. It works under all 32 bits Windows versions. The settings are controlled
with an intuitive graphical interface. For silent logging in the background there
is also a command line option available.
               
               
               

本文来自ChinaUnix博客，如果查看原文请点：http://blog.chinaunix.net/u/29291/showart_1841831.html