X381: Difference between revisions

The '''Tektronix X381''' is an experimental 560-series timebase plug-in, probably made in the early 1970s.

It appears to have used a [[3B1]] plug-in as a starting point. It was probably developed by [[Hiro Moriyasu]].

No documentation is available for the X381, so everything in this page is just a guess based on examining the one existing plug-in.

The rotational position of the vernier sweep speed knob is linked by shafts and gears to the rotational position of the main encoder shaft.  

Attached to directly to the main encoder shaft are:

Each angular position of the main encoder shaft produces a distinct pattern of illuminated or not-illuminated in the 26  

optical fibers (or flexible light pipes) that exit the rotary optical encoder.  

These are optical signals are converted to electrical signals and the electrical signals are fed to the nixie controller ICs.

The appropriate encoding is selected based on the position of the front panel SEC/Hz two-position switch.

This potentiometer functions in essentially the same way as the vernier sweep speed on a conventional timebase plug-in such as a [[3B1]].

When the rotary optical encoder is at the end of a revolution, an optical signal (one track on the encoder disk)  

calls for a step of the 1-2-5 rotary sweep speed switch that is concentric with the vernier sweep speed knob.

The position of the main encoder shaft (and corresponding pattern of fiber illumination) is insufficient to disambiguate which direction to rotate the 1-2-5 sweep speed switch.

One possible solution would be to use memory, i.e., to continue in the same direction as recent motion of the vernier knob.

A pair of microswitches is attached to the vernier knob. One closes on clockwise torque; the other microswitch closes on counterclockwise torque.

The direction of the torque of the vernier knob determines the direction of the step of the 1-2-5 sweep speed switch.

This forms an electromechanical counter, similar to an odometer, with the 1-2-5 switch accumulating a count of the rotations of the main encoder shaft.

Within the X381, there is an octagonal aluminum enclosure containing a backlit disk of film that is rotated by a front panel knob.

The disk functions similarly to a quadrature optical encoder, but instead of generating just two signals, it generates several signals.

Since the two numbers have a reciprocal relationship, one number probably represents the sweep speed in time per division

while the other number represents the sweep speed in frequency, e.g., of a waveform that repeats in one horizontal division.

The bulb used for illuminating the encoder wheel is marked "6.3V" and has an integral lens, see photo below.

Each photodetector has two strands of optical fiber entering it, from separate tracks on the encoder disk.

It is possible that the position of the SEC/Hz knob determines which of the two bulbs will be energized, thus selecting one or another subset of the tracks on the encoder disk.

Viewed from a modern perspective, the encoder disk is two ROMs driven by the same address lines, with the two light bulbs serving as chip select lines.

However, the wiring on the SEC/Hz switch suggests that there's more complexity than just selecting between two light bulbs.

The outer knob is the coarse TIME/DIV control, and goes in 1-2-5 steps from 2 sec/div to 1 μs/div.

When the coarse TIME/DIV control is rotated using the front panel knob, the motor's gear is mechanically disengaged from the rotary switch shaft.

The rotary coarse TIME/DIV switch controls the rightmost nixie tube, which shows the TIME/DIV units.

The inner knob, like the outer knob, can be rotated by turning the front-panel knob, but is also motor-controlled, possibly to enable automated measurement.