Data Acquisition

When a data communication link won’t work, the following five very useful devices can beused to assist in analyzing the problem:• A digital multimeter (and paper clips?)• An LED• A breakout box• PC based protocol analyzer (including software)• Dedicated protocol analyzer (e.g. Hewlett Packard)6.9.1 The breakout boxThe breakout box, as shown in Figure 6.17 is an inexpensive tool that provides most of theinformation necessary to identify and fix problems on data communications circuits.Figure 6.17Breakout box showing test pointsA breakout box is connected into the data cable, to bring out all conductors in the cable toaccessible test points. Many versions of this equipment are available on the market, from‘homemade’ versions using a back-to-back pair of male and female DB-25 sockets, to fairlysophisticated test units with built in diodes, switches and test points.Breakout boxes usually have a male and a female socket, and by using 2 standard serialcables, the box can be connected in series with a communication link. The 25 test points canbe monitored by LEDs, a simple digital multimeter, an oscilloscope, or a protocol analyzer.In addition, a switch in each line can be opened or closed while trying to identify where theproblem is.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.6.9.2 Null modemNull modems look like DB-25 ‘straight-through’ connectors and are often used wheninterfacing two devices of the same gender (e.g. DTE–DTE, DCE–DCE) or devices fromdifferent manufacturers with different handshaking requirements. A null modem, as shown in