Saturday, August 11, 2018

Understanding design choices

When examining an existing design I try to get "into the head" of the original designers, putting myself in their shoes facing the same design questions. This often helps me understand why they made certain choices.

For example, the top side of the original PCB in the Canon P170-DH calculator uses what I think are carbon ink traces rather than copper. Likely this is because carbon traces can be used as contacts for the conductive pill keypad and left exposed to air without corroding. However, carbon ink traces have significantly higher resistance, which explains why the traces were made so wide.

One of the oddities of this board is that the power supply and printer driver components are mounted on the top of the board, while the calculator electronics are mounted on the bottom. Initially I thought this had to do with fitting into the three-dimensional space available in the shell, and so I did the same with the design of my replacement PCB. In reality there is at least 0.65 inches of space between the PCB and the shell, and the tallest component is barely 0.55 inches tall.

One morning this week, as I sat in traffic, a more likely explanation occurred to me. All of the components on the top of the board are through-hole parts, while all of the components on the bottom of the board are surface-mount parts. All the soldering is done on one side of the board, which makes for easier assembly.

All of the components I'm using are surface-mount, with the exception of a big filter capacitor and the connectors. If I was starting this design from scratch I'd give serious thought to putting all the components on the top, and the conductive-pad switch contacts on the bottom. Although KiCad has the ability to visually "flip" a board, it doesn't reorder the layers (yet!) which makes routing a bit more challenging.

1 comment:

  1. The original link "Original 4004 Schematics (PDF)" in "Useful links" section (and maybe on other post) is now on error-404 from the Intel site.
    However there are captures (14 working ones) by Web-Archive/Way-back-Machine of that page on:

    Linking from the seems to not be prohibited (if I read the FAQ well, especially in "The Wayback Machine->Can I link to old pages on the Wayback Machine?"¹ and "Rights->Can I use this ____ for ____ ?"² sub-sections).

    "The Wayback Machine->Can I link to old pages on the Wayback Machine?"¹:
    Yes! The Wayback Machine is built so that it can be used and referenced. If you find an archived page that you would like to reference on your Web page or in an article, you can copy the URL. You can even use fuzzy URL matching and date specification... but that's a bit more advanced.
    "Rights->Can I use this ____ for ____ ?"²:
    nternet Archive does not itself seek to limit use of its digital materials. However, we cannot give ironclad guarantees as to the copyright status of items in our Collections and cannot guarantee information posted on item details or collection pages regarding copyright or other intellectual property rights. Our terms of use ( require that users make use of Internet Archive's Collections at their own risk and ensure that such use is non-infringing and in accordance with all applicable laws.

    The person who uploads an item often provides information related to use rights, either by way of directly entering it in the description field or by selection of a Creative Commons license. The latter, if included by the uploader, will be viewable via a Creative Commons logo on the details page, which serves as a link to a description of the specific type of license that the uploader has assigned.

    One way to attempt to contact an uploader about information that they have posted is to post a review to the item.


    Have a nice day!
    Nickh ²+.