Data Acquisition

Figure 6.18, has appropriate internal connections between handshaking pins that ‘trick’ theterminal into believing conditions are correct for passing data. A similar result can beachieved by soldering extra 'loops' inside the DB-25 plug. Null modems generally cause moreproblems than they cure and should be used with extreme caution (preferably avoided!).Figure 6.18Null modem connections6.9.3 Loop back plugThis is a hardware plug that loops back the transmit data pin to the receive data pin andsimilarly for the hardware handshaking lines. This is another quick way of verifying theoperation of the serial interface without connecting to another system.6.9.4 Protocol analyzerA protocol analyzer is used to display the actual bits on the data line, as well as the specialcontrol codes, such as X-ON, X-OFF, LF, CR, etc. The protocol analyzer can be used tomonitor the data bits as they are sent down the line and compared with what should be on theline. This helps to confirm that the transmitting terminal is sending the correct data and thatthe receiving device is receiving it. The protocol analyzer is useful in identifying incorrectsetting of baud rate, parity, stop bit, noise or incorrect wiring and connection. It also makes itpossible to analyze the format of the message and look for protocol errors.When the problem has been shown not to be due to the connections, baud rate, bits, orparity, then the content of the message will have to be analyzed for errors or inconsistencies.Protocol analyzers can quickly identify these problems.Purpose-built protocol analyzers are expensive devices and it is often difficult to justify thecost when it is unlikely that the unit will be used very often. Fortunately, software has beendeveloped that enables a normal PC to be used as a protocol analyzer. The use of PCs, as testdevices for many applications, is a growing field.6.9.5 The PC as a protocol analyzerThe PC is proving to be a useful tool for monitoring serial line activity. The general means ofconfiguring such a package is discussed in the following paragraphs.Basic setup parametersThe initial step is to configure the package appropriately. This requires the following basicparameters to be appropriately set up:• Baud rate, parity, stop and data bits