Optical time-domain reflectometer user interface design - in reference to the Hewlett-Packard HP 8145A Optical Time-Domain Reflectometer - technical

Hewlett-Packard Journal, Dec, 1988 by Joachim Vobis

Optical Time-Domain Reflectometer User Interface DEsign

THE HP 8145A OPTICAL Time-Domain Reflectometer described in this issue is a highly sophisticated machine. Its hardware and its theory of operation are difficult to understand. On the other hand, most users want to see an interface that is self-explanatory and helps them do everything from simple to complex tasks easily.

The creation of a friendly user interface for both local and remote control is the task of a large portion of the firmware built into the HP 8145A. The firmware can be divided into five parts, as shown in Fig. 1:

* A central data base holds the machine's states in ten registers (0-9) and the measurement results in thirteen internal registers (0-12) and 114 registers in an external plug-in module. Setting register 0 holds the current parameter settings and measurement register 0 holds the current measurement.

* The keyboard/display process provides input and output between the data base and the front panel. This part of the firmware implements the local human interface.

* The HP-IB process does the same for remote control, conforming to IEEE 488.1 and 488.2. This process also dumps measurement results to a plotter or an HP ThinkJet printer without an external computer.

* The measurement process executes the measurement by reading setting register 0, programming the hardware and processing the data received. It stores its results in measurement register 0. This part of the firmware is described in the article on page 29.

* The check process implements test and verification routines such as the self-test during power-on.

The largest piece of firmware is the keyboard/display process. While it was possible to develop the other parts mostly in the lab, the human interface was grown in an interactive way. Successive prototypes were shown to customers.

The feedback allowed us to optimize the human interface for the requirements of typical applications, and to add features wanted for advanced tasks. Because users can be divided into three groups, three levels of use are offered:

* Standard use. The user wants to select the distance range and run the measurement. Results and markers are obtained directly from the display or by creating a hard copy or a printer or plotter.

* Very easy use. Most of the hard keys and softkeys can be disabled (blanked) by HP-IB commands. Typically only the keys to recall settings and to start and stop the measurement are left. A user can recall the proper settings and execute the measurement, but can't do anything wrong by accidentaly pressing an unwanted buttom.

* Sophisticated use. For documentation, special measurements, and comparison of measurement results, a set of features is available to the skilled user. Some of the features are not offered by other OTDRs, so they are designed to be sufficiently self-explanatory that the user can become acquainted with them without a lot of training or reference to manuals.

Front-Panel Interface

The hard keys on the front panel are divided into four groups that provide the functions most often needed (Fig. 2). The START, STOP, and CONTinue keys control the execution of measurements. The digit keys together with the knob and the vernier allow numerical entry to set or modify parameters. The marker keys activate different markers to calculate distances, attenuation, and losses. STO stores an instrument state in one of the setting registers (1-9) and RCL recalls it. RCL 0 can be used for reset (load factory-defined setting) and RCL ENTER rescales the X and Y axes of the display to show the complete measurement result.

Fig. 2 shows a typical CRT image. An X-Y grid contains the measured curve. The grid's Y scale (offset and dB/div) is on the left side and the X scale (start and stop position) is directly under the grid. Below the X scale, a measurement bar shows the measurement range and the calibration values.

In the upper right corner is a readout of marker values. The left and right markers are used to measure distances, losses (dB), and attenuation (dB/km, least-squares approximation). The splice marker can be used automatically (by default) or manually to calculate the loss of splices or connectors.

Softkeys

The right side of the display contains the softkey labels, which offer functions not provided by the hard keys. Because there are many more tasks than softkeys, the softkey functions are arranged hierarchically, the most important on the highest level and the less important in one of the sublevels.

The main level is split into two halves. The first half controls the X and Y axes (start or center position, distance span, dB/div, and vertical offset) while the second half of the main level manipulates the measurement parameters (continuous averaging or refresh mode, auto or manual pulse width, 1300 or 1540 nm wavelength). In the continuous average mode, all modifications are editor operations, that is, changing a value affects only the display and not the measurement. This can be easily demonstrated with the X scaling: changing the start/center position or the span has a zooming effect, but the measurement as shown in the bar at the bottom runs with the same distance range. Pressing START to initiate a measurement forces the edited values to become the actual parameters for the hardware.