7000 series readout system: Difference between revisions

The standard mode of operation for the readout is free-running, i.e. in regular intervals that are asynchronous to the sweep, beam control is switched to the readout generator.  In certain conditions, this can cause visible gaps in the signal display.  When the readout intensity control is turned to minimum, this switching is disabled and the signal trace is not interrupted at all. (In the [[7844]] with its two physically separate beams, readout uses beam 2, and beam 1 is never interrupted.)

In certain mainframes such as the [[7844]], [[7904A]], [[7934]], and [[7104]], turning the readout display intensity control past the maximum intensity setting switches readout into pulsed mode.

The oscilloscope readout system produces a pulse train consisting of 10 successive negative-going pulses representing a possible character in a readout word.  Each time-slot pulse is output at −15 V onto one of ten lines, labeled TS-1 through TS-10 (Time Slots 1 through 10), which are connected to the vertical and horizontal plug-in compartments.

 

In time slot 1, special operations can be requested that affect the display in later time slots. These commands are designed to simplify display changes required by external probes that request beam identification or change input sensitivity.  The "Identify" command switches to an internal program displaying the word "IDENTIFY" instead of any encoded value.  Command R3/C1 requests an additional zero to be added starting in time slot 5, R3/C2 adds two such zeroes, R3/C3 shifts the selected prefix one code to the left (i.e. p → n, n → μ, μ → m, m → space), and R3/C4 shifts the prefix and adds a zero.  (See the [[Media:Tek-plugin-readout.jpg|encoding circuit of the 7A18 plugin]] for an example implementation.)  It is also possible to request a decimal point to be drawn without losing display digits, by encoding row 7 commands in time slot 1.

 

 

Normally the readout will only display the channels of those plugins that are active as per the mode switches (P1018-P2118, channel inhibit), however, individual plugins can override this by pulling the A35 ("override channel inhibit") line low. If the two channels of a plugin are inhibited, i.e. A35 is high at +15 V, two diodes (e.g. CR2163 and CR2162, with current sharing resistors) inject current into the A38/A37 line, effectively deleting the readout data by producing SKIP or space (C-0, R-x) instructions.

It will use TS1 to encode the number of zeros, TS2 to display a down-arrow indicating an inverted input, TS3 to display ">" or "<" if the input is uncalibrated, TS4 for a digit, TS8 for a unit prefix like "μ" and TS9 for a unit like "V".  The other slots are unused (see illustration).

Most 7000 series plugins have [[BNC connector with readout ring|rings around the BNC input sockets]] that allow attached probes to interface with the readout system.  The probe connector includes a contact pin connecting this ring.  A resistor connected to ground encodes the probe attenuation, e.g. 11 kΩ indicates a ×10 probe and 6.8 kΩ a ×100 probe.  If the probe includes an Identify switch at the tip, this connects the readout pin directly to ground.  

 

The [[Media:Tek-plugin-readout.jpg|plug-in circuit]] uses the shift instructions in time slot 1 to implement the change of range, which is done by adding 100 μA (1 step) to the column current in time slot 1 for a ×10 probe, or 200 μA (2 steps) for a ×100 probe.  For example, if the encoded unit was 100 mV, TS4 would encode a "1" digit, TS-8 the "m" prefix, and TS-1 the "add two zeros" command (R3/C2).  Connecting a ×10 probe adds 100 μA in TS1, selecting the "shift prefix left" command (R3/C3) instead of "add two zeros".  This drops the extra zeros and changes the prefix "m" to none, thus the display changes to "1 V".  With a ×100 probe, the command selected would be "shift prefix left and add one zero" (R3/C4) and the resulting display becomes "10 V".

The character generators are custom analog chips that provide X and Y output currents for ten characters of up to seven strokes (eight X/Y coordinate pairs) inscribing each character.  The in-character coordinate points are selected through a triangular analog character scan signal, not a digital code, which in turn activates one of eight groups of three transistors each generating the X/Y output currents.  Each of these groups is fed a constant emitter current (I_E), and the distribution of this current to the three transistors is controlled by their different emitter areas.  The first and second collectors provide X and Y currents respectively, the third ("Z") directs current to the substrate (I_E - I_X - I_Y - I_Z = 0).  Actually, the emitter area itself is not varied in the mask, but out of a fixed number of emitters of equal size, a certain number is connected on each transistor (see illustration). This allowed all character generator ICs to be manufactured with the same silicon layout, differing only in the metallization layers. [[File:7000-readout-chargen-circuit-1.jpg|300px|thumb|ROM circuit for one character (digit "3").  Bases are scanned in turn.  The numbers under the emitters indicate how many of the emitters are connected to the common rail. Ten such character circuits plus the scanning (base drive) circuit are integrated in each ROM chip.  All X and Y collector outputs are connected across characters and chips.]]

The [[7854]] contains a microprocessor-controlled 40x16 character display generator which is also used to display plugin readout data.   

The readout interface therefore contains only the column driver, multiplexer and A/D section but not the character generator - it only feeds readout data to the microprocessor.

Towards the end of the 7000 series lifespan, in the second half of the 1980s, a re-designed readout board (Tek part number 670-8622)

was introduced, in which the character generator is based on digital values stored in a 2732 EPROM instead of Gilbert's analog ROMs,

with the number of display points increased to up to 16 per character.  

 

This board was installed e.g. in the [[7934]], [[R7103]], and [[7904A]] and still used some of the Tek-made readout ICs like the timing generator.  

The design is similar to the readout mechanism in the [[2465]] series.

The presence of this board can be detected from the outside by looking at the readout characters which are straight up but were italicized  

in the analog version (which was done the analog way too, by adding some 10% of the vertical signal to the horizontal channel).   

The character style itself is also different — for example, the "V" in the classic board has just two straight lines, whereas they are angled in the digital version.

| Analog ROM board      || EPROM or μC board

Finally, from 1989/1990 on, a third generation readout board designed around an 80C31 micro-controller replaced the previous versions in the last series of [[7104]], [[R7103]], [[R7844]], [[7904A]], [[7603]] and [[7623B]] models. This version no longer used any Tek-made special ICs, manufacture of which had been discontinued by that time. The board is part SMD, part through-hole.  

It is described in  

[[Media:070-4593-00.pdf|this version of the 7904A manual]]

(pages 288 through 291 describe the EPROM version above, while the 80C31 version is shown on pages 380 through 391).

Microcontroller based readout board block diagram.jpg | Block diagram  

Late 7603 with digital readout.jpg | Late model [[7603]] with μP readout

* Barrie Gilbert, ''[https://ieeexplore.ieee.org/abstract/document/1050157 Monolithic Analog READ-ONLY Memory for Character Generation]'', IEEE J. of Solid-State Circuits, 1971.