Practical Industrial Data Networks:Design, Installation and ...

EIA-232 overview 45TestPointsDTEDCEFigure 3.6Breakout box showing test pointsA breakout box is connected to the data cable, to bring out all conductors in the cable toaccessible test points. Many versions of this equipment are available on the market, fromthe ‘homemade’ using a back-to-back pair of male and female DB-25 sockets to fairlysophisticated test units with built-in LEDs, switches and test points.Breakout boxes usually have a male and a female socket and by using two standardserial cables, the box can be connected in series with the communication link. The 25 testpoints can be monitored by LEDs, a simple digital multimeter, an oscilloscope or aprotocol analyzer. In addition, a switch in each line can be opened or closed while tryingto identify the problem.The major weakness of the breakout box is that while one can interrupt any of the datalines, it does not help much with the interpretation of the flow of bits on the datacommunication lines. A protocol analyzer is required for this purpose.Null modemNull modems look like DB-25 ‘through’ connectors and are used when interfacing twodevices of the same gender (e.g. DTE to DTE, DCE to DCE) or devices from differentmanufacturers with different handshaking requirements. A null modem has appropriateinternal connections between handshaking pins that ‘trick’ the terminal into believingconditions are correct for passing data. A similar result can be achieved by soldering extraloops inside the DB-25 plug. Null modems generally cause more problems than they cureand should be used with extreme caution and preferably avoided.