On 2018-06-06 2:08 PM, Paul Koning via cctalk wrote:
>
>
>> On Jun 6, 2018, at 9:48 AM, Noel Chiappa via cctalk <cctalk@classiccmp.org>
>> wrote:
>>
>> Hi, I'm hoping someone here knows the low-level nitty-gritty on how the
>> characters on the CDC 6600 console CRTs were generated.
>>
>> Thornton, "Design of a Computer", says "Control of the beam .. is provided by
>> electrostatic deflection ... electronically converting from the symbol .. to
>> deflection voltages", but alas, doesn't say how that conversion is done. And
>> I
>> looked in some CDC 6600 documentation online, alas, even less detail.
>>
>> But looking at the characters (reproduced on the dust jacket), the curves
>> sure
>> make it look like it wasn't anything simple (e.g. using display vectors, as
>> one source indicated). Does anyone know?
>
> Yes.
>
> It is indeed a digital stroke generator, not a Fourier generator as someone
> suggested. The reason for the odd shapes on the Thornton book cover is the
> AC characteristics of the display electronics.
>
The tweet did, but I found the suggestion surprising.
> There are a couple of parts to the puzzle.
>
> One is the display controller ("synchronizer" in CDC terminology, the module
> that connects to the 6000 I/O channel). The 60125000 manual that was
> mentioned is the "block diagrams" manual for that (and several other)
> controllers. The block diagrams show the overall data flow and the general
> structure of the circuits, but they are not complete schematics.
>
> However, the full schematics also exist:
> http://www.bitsavers.org/pdf/cdc/cyber/cyber_70/fieldEngr/63016700A_6600_Chassis_Tabs_12_Apr65.pdf
>
> The block diagram manual shows the waveforms generated by the controller. As
> you can see, they are pretty angular and straight lined. Each segment
> (between the small marks on the stroke) corresponds to a 100 ms clock cycle,
> with a one or two element step in X and/or Y.
That must be closer to 100 ns? Typo?
>
> Incidentally, the 170 series display controller produces the same waveforms,
> though using a completely different (ROM based) design.
>
> The other part of the puzzle is the DD60 console display. That is fed from
> the 6602/6612 display controller by a bundle of coax cables. The waveforms
> are generated by A/D circuits (quite primitive ones) in the 6602, and travel
> in analog differential form to the DD60. There they go through a string of
> amplifiers and a scaling circuit, for the small/medium/large character size
> selection. Eventually they end up on the deflection plates of large
> electrostatic deflection CRTs. Much of the signal chain is early 1960s
> transistors; the final couple of stages are tubes. You can find the
> schematics at
> http://www.bitsavers.org/pdf/cdc/cyber/cyber_70/fieldEngr/82100010_dd60a_Mar65.pdf
>
> What appears to be going on is that the signal chain in the DD60 has enough
> bandwidth to draw the characters, but only barely. So there is distortion in
> the path, resulting in character shapes on the screen that are not the same
> as the nominal stroke patterns generated by the controller.
>
> I have converted the "chassis tabs 12" wire lists to a VHDL model, which you
> can find on my Subversion server. Run on GHDL, it demonstrates the behavior
> of the circuit and reproduces the documented waveforms.
>
> I have also attempted to create a SPICE model of the DD60 deflection signal
> path. So far that hasn't been all that successful. I probably have bad
> assumptions for the transistor models, and the CRT deflection plate
> capacitance figures are also a complete guess. My hope was to reproduce the
> actual screen patterns, but that hasn't worked yet.
>
> Finally, I did a much more primitive approximation of the DD60 signal path,
> with a couple of IIR filters that very roughly imitate the RC elements in
> that path. That was done as part of my console display emulator program for
> Tom Hunter's DtCyber program. It was somewhat successful in that the
> characters show some of the rounding and distortion that the real display
> has, but unfortunately I can't claim that this is because it's an accurate
> model.
Nice work!
--Toby
>
> By the way, the displays shown on the 170 series console (CDC 565) look
> somewhat different. ...
>
> paul
>
>
