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Can anyone tell me what the blue dotted outlines of components represent in this schematic?

There are 4 diodes on the LOAD, AND, OR and XOR lines and a not gate on the STORE line. Are these components that were present in an earlier revision but have been removed?

Gigatron Control Unit Schematic

Complete schematic: https://cdn.hackaday.io/files/20781889094304/Schematics%202020-03-20.pdf

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  • \$\begingroup\$ I don't think that's "standard" by any means of the word, but seeing the project has a forum, maybe ask there? Chances are the creators of the schematic answer relatively quickly there. forum.gigatron.io/viewforum.php?f=4&start=125 \$\endgroup\$ Mar 6, 2021 at 15:54
  • \$\begingroup\$ Thanks. I'll try that. Good to know I'm not just missed something basic and commonly understood. \$\endgroup\$
    – digby280
    Mar 6, 2021 at 15:57
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    \$\begingroup\$ @MarcusMüller - did you see the 404 page on that forum site? best 404 page ever! \$\endgroup\$
    – davidbak
    Mar 7, 2021 at 2:47
  • \$\begingroup\$ @davidbak :D lulz! \$\endgroup\$ Mar 7, 2021 at 13:19

3 Answers 3

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Starting off, because the diodes are made on the blue "artwork" layer of the schematic, there are no real diodes there. The blue dashed diodes are merely "comments" or "suggested intent".

So it looks like this is some part of a processor, where the inputs IR5, IR6, and IR7 are being used to decode into a "one hot" signal using the 74HCT138. Following that, there is a "diode ROM" that takes the current decoded instruction and probably converts it to 5 control lines in the processor.

When the decoder receives an input, it pulls one of the output lines low. Pulling one of the output rows low will pull the columns low that have a diode attached. For example, if the inputs A0-A2 are "111", then O0-06 will be high, and O7 will be low. O7 will pull the first, second, and fourth columns low through D34, D35, and D36.

That brings us full circle to D21 - it's not connected to any of the outputs but to the input A2. When A2 is low, O0-O3 will be selected. D21 is functionally equivalent to placing four diodes between column 1 and O0-O3. However, since this looks like a TTL-style computer, it's probably done to save 3 diodes over being easier to read.

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They are trying to convey that D21 is equivalent to the four diodes shown (thus the vertical '= ' sign).

If the A2 line is low, then one of the first 4 outputs is low, so it's logically the same as if you had the four diodes.

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The designers are using the dots for a logical connection without a physical connection.

The dotted inverter does not exist in their final design.

They have a write signal \$ \overline W\$, which they rename INV_B_IN, which is put through an inverting octal driver to create INV_B_OUT, which is the same impact as an inverter. Not all of the inverters on the octal inverter are required for their design, so they appear to be used as required.

I'd guess that in their initial versions, there was a physical inverter, but they realized it was not required, so they could save an inverter, but they left it in to make functionality easier to understand.

enter image description here


There are three inputs IR5-IR7 fed to a 3 to 8 mux. The 8 outputs are ORed via diodes to create 5 commands which are inverted via an octal buffer to create AL and AR0-3 (active high), which is sent to the ALU.

This leaves 3 NOT gates, which they use on the right of the schematic. INV_C, INV_A and INV_B.

The key is:

XXX breakdown symbol into 8 inverter gates

Their intentions were to breakdown the octal buffer into 8 NOT gates and place them as required, but never got around to it. The dotted lines were probably a place holder for this future operation. Instead they used net names to implement the physical connections.

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  • \$\begingroup\$ @W5VO On the left side of OP digby280's image, there are four blue dashed-line diode symbols pointing leftward. On the right side of OP digby280's image (and on the top of StainlessSteelRat's image in this answer), there is a blue dashed-line inverter symbol pointing rightward. \$\endgroup\$
    – Bavi_H
    Mar 7, 2021 at 1:58
  • \$\begingroup\$ @Bavi_H Ahh, that makes much more sense. Thanks! \$\endgroup\$
    – W5VO
    Mar 7, 2021 at 3:36
  • \$\begingroup\$ Typo in the last sentence: s/dealth/dealt/ (insufficient rep to edit for you) \$\endgroup\$ Mar 7, 2021 at 14:34

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