Test equipment that the user requires in the premises data communications environment varies from very simple devices to extremely complicated devices.


At the low end are simple line monitors referred to by a variety of different names. Telebyte calls its data line analyzer the Model 301 which is shown in Figure 21. This typical low end unit is made up of two components. It includes a data line monitor, the Model 43 Micropeeper, that allows the user to determine the actual status of the seven key signals of the RS-232 data path. All of the 25 signal leads are usually passed through with the seven key signals TD, RD, RTS, CTS, DSR, DCD and DTR being monitored and their status indicated on a set of dual colored LEDís. The LEDís show whether or not the EIA-232 pin is high, low or has no signal. The Model 301 also includes the Model 151 Mini-Patch Box to allow altering the connections in the data path.


A step up from such low end units are add-on cards for the PC, which allow the personal computer to perform as a full featured line monitor. These cards plug into a PC and convert it to a test instrument. They typically have the capability of viewing the bi-directional data and the controls signals of any EIA-232 communication link. The PC display allows visual presentation of the data on a serial line. It allows the operator to understand, quickly, the relationships between transmit data, receive data and the various control leads such as Request-to-Send and Clear-to-Send.

An example of such a test unit is Telebyteís Model 903 PC Comscope shown in Figure 22. The Model 903 consists of a plug in card and a T cable, which allows the PC to tap into and capture the data flowing between data equipment devices. The PC Comscope allows the protocol that is used in the data transmission to be verified. It enables maintenance personnel easily to identify open data leads, missing clock signals, parity errors, incomplete handshakes, etc. It also allows you to emulate protocols such as BISYNC, X.25 and HDLC. Bit Error Rate Test (BERT) capability is also provided by the unit. This test device effectively lets you design your own protocol analyzer. The Model 903 operates in PC AT/386/486 machines or compatibles including lap tops.

While test instruments like the Model 903 are convenient they do have a significant limitation. They depend upon a spare slot being available in the PC platform which is their home.

Figure 22: Low Cost Data Line Analyzer

Figure 23: Model 903 PC Comscope

With the proliferation of add-on boards a slot for the Model 903 may not always be available. This is especially a problem if one wants to use a notebook PC. Telebyte addressed this issue by developing the Model 904 PC Notebook Comscope Protocol Analyzer. It is a small portable unit which is external to the PC but works with it via its serial port. The Model 904 is illustrated in Figure 23.

Figure 24: Model 904 PC Notebook Comscope Protocol Analyzer


Beyond this are datascopes which both monitor and emulate data sources. These allow quality assurance of communication circuits by doing a variety of tests. Datascopes are not usually put in as part of the PC but stand-alone with their own keyboards and screens. Depending on what the user is willing to pay he can have increasing speed and more and more features. Telebyteís Model 901 Netscope was an example of such a tester. A photograph of the Model 901 is shown in Figure 24. However, such test units have lost ground to the PC based equipment.


It is always dangerous to predict the future. Nonetheless, at present, we appear to be in the midst of a revolution with respect to datacom test equipment and certain trends are evident. The PC has become ubiquitous. As a result, field service technicians can generally count on a PC being present and available for their use no matter where they are sent. The technician need only put a card like the Model 903 in his attache case when going into the field and then commandeer an available PC. This we see as the "new portability."

Where does this leave the stand alone test units like the Model 901 Netscope? While the Model 901 was retired other stand alone types will not disappear. Rather, they will increase in capabilities especially in the areas of display and archival memory. The increase in capabilities will be matched by a high price. Their use will thus be limited to the laboratory rather than as a portable tool for the traveling technician.

Figure 25: Model 901 Netscope


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