Black Box IEEE 1284 Guía de usuario descargar pdf (Pagina 78)

Page 78 of 98 The Black Box Toolkit

Once zoom mode is activated you can click and drag over a region you want to zoom into.

This will be indicated by white dashed vertical lines. When the mouse is released the

region will be enlarged and will fill the whole graph area. You are free to zoom in as many

times as required to achieve exact positioning of the measurement cursors.

In the screen shot below the cursors have been positioned exactly on the leading edge of

the image display (opto-detector on line 3) and the leading edge of the simulated response

(switch closure/left mouse button down on line 7).

On line 3 we can see each refresh of the image display as the bitmap was drawn on the

CRTs screen. The measure between the display leading edge and the simulated

responses leading edge is exactly 300 milliseconds as intended. This tells us that a

response was fed into the paradigm on the remote PC at exactly the right time relative to

the appearance of the bitmap. If we look at the red cursor and trace up the vertical cross

hair and count the number of refreshes to the right of it we can see that the image was

displayed for longer than it should have been. Remember it should have terminated to a

black screen once a response was detected. Here we can see that it was displayed for an

additional 6 refreshes. At a refresh rate of 100Hz (10ms redraw) this means it was

displayed for 60ms longer that it should have been. This is a significant display error, with

the image being displayed 20% longer than it should have been under ideal conditions.

The actual image was displayed for 360 milliseconds (36x10ms). We can arrive at the

same conclusion by simply measuring the first and last leading and trailing edge. However

we must remember to add half the refresh interval (between peaks) to the front and end of

the run as we positioned the opto-detector mid screen, so there is a screen area above

and below the sensor where data is not collected. Here we can see what this would look

like on the graph once accounted for.

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Black Box IEEE 1284 Guía de usuario Pagina 78