Pixelated ground pour on an old computer's PCB: why/what for? - Page 1
Forum section: electronics / general

author: shapirus
author_rank: Super Contributor
timestamp: June 09, 2025, 10:00:24 am
content: So I've been watching the new video by Mend it Mark...where he repairs an old Commodore Amiga computer, and spotted a very curious ground pour pattern:This got me interested. As you can see, it's pixelated, because its edges couldn't be made sloping, for whatever reason, only vertical or horizontal, and the resolution of the fill area is quite low. Lines (horizontal in this image) can be seen all across the ground copper. What did it result from? It's clearly not a hardware limitation, as the traces are not pixelated.So it must have been a software thing. I don't think they designed this board like that on purpose, so it must have been some pecularity that made it possible to create smooth angled traces, but not the copper pour. I'm sure someone here knows more, perhaps someone even designed some PCBs back in the 80's and can share some details.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: June 09, 2025, 10:08:04 am
content: In a lot of PCB layout programs, the copper pours are constructed of lines. If you decrease the line distance, this will increase the resolution of the copper pour, but also decrease speed.At the time when the Amiga was designed, memory and CPU resources were still a main concern, so I guess the line spacing used there was rather coarse.In a modern layout tool, the lines would still end with a circular shape, not with a rectangle. So, either this also related to limited CPU resources or maybe the copper pours case where actually implemented on rectangles instead of lines.

author: shapirus
author_rank: Super Contributor
timestamp: June 09, 2025, 10:15:17 am
content: Quote from: 0xdeadbeef on June 09, 2025, 10:08:04 amThat makes sense, assuming that the traces had to have higher priority in resource allocation, since high resolution actually mattered for them, unlike the ground pour (at those frequencies anyway), so they could save some memory and CPU there.However, I don't think I have ever seen another PCB like this from that era. Maybe I've never paid attention, and only this one caught my eye, I'm not sure.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: June 09, 2025, 10:34:55 am
content: Quote from: shapirus on June 09, 2025, 10:15:17 amIt was certainly not common. Earlier PCBs are still created manually and usually don't have these "general" copper pours.My best guess would be that in this specific case, the developers decreased the copper pour's resolution during development and forgot to increase it for the final product.

author: AndyC_772
author_rank: Super Contributor
timestamp: June 09, 2025, 10:35:52 am
content: Possibly a Gerber file format limitation. Photo masks for PCBs were produced using an X-Y plotter, with a head consisting of a light source and an interchangeable aperture which determines the shape which is projected.The Gerber format describes the board in terms of how that X-Y head should move and which aperture should be used. It can't describe how to fill a solid area, other than to select a reasonably wide aperture and then draw a series of parallel lines to fill the space.The format has been extended over the years, of course, but that board is ~35 yrs old...

author: tom66
author_rank: Super Contributor
timestamp: June 09, 2025, 11:02:02 am
content: If you look at even modern PCB software like Altium, the polygon solver takes noticeable time (multiple seconds) - so much so that you have to repour a polygon every time you do something that alters it. So I can imagine on an 80s/90s computer, polygon fill solving could take upwards of several minutes and doing it with a lower resolution was considered a reasonable trade off.

author: nctnico
author_rank: Super Contributor
timestamp: June 09, 2025, 11:54:28 am
content: Quote from: tom66 on June 09, 2025, 11:02:02 amNo, Altium is just extremely slow. If you look at packages like Orcad Allegro, you need to have dozens of polygons on a 10 layer board before it starts to slow down somewhat. And that is on a computer with a slow (passively cooled) videocard.

author: tom66
author_rank: Super Contributor
timestamp: June 09, 2025, 12:12:15 pm
content: Quote from: nctnico on June 09, 2025, 11:54:28 amOrCAD might be smart enough to multithread the operation. Certainly KiCAD 6's polygon solver is also similarly slow but it seems to run one 'layer' at a time when it is an embarrassingly parallel problem for multilayer boards. They may have fixed that since though. In the 80's though, multicore computers were unheard of outside of high end computer labs.

author: Haenk
author_rank: Super Contributor
timestamp: June 09, 2025, 02:15:20 pm
content: I would think they had to do the fills manually (on screen), or used some wonky selfwritten software.Note the two small "dots" attached to the trace and fill area right of that below the R654 printing.And the "blob" trace left of R654 printing.And the trace below R202/3 printing, with a kink in it.And the offset in some via connecting traces.And the trace changing width south of R203 printing.Either the software did not work, or they were in a real hurry to get the board out.

author: jmelson
author_rank: Super Contributor
timestamp: June 09, 2025, 03:41:01 pm
content: Quote from: AndyC_772 on June 09, 2025, 10:35:52 amThe original Gerber photoplotter had an aperture wheel that was loaded with films with large pictures of various donuts, circles, etc. These could be drawn in X or Y to make lines, or drawn at any angle to make slopes. So, the stepped angle in the picture would not have been required. The various software I used back in the 1980's and '90s would not have done things like that. Even if the master artwork was made on a pen plotter, they could draw at an angle, too. So, I don't know what was the cause of that, but most likely some particular software shortcut. My software (Orcad, then Tango, then Protel) could use a grid of overlapping traces to copper pour an area, but it would always wrap the filled grid with traces to cover the rounded endpoints of the traces.Jon

author: nctnico
author_rank: Super Contributor
timestamp: June 09, 2025, 04:12:27 pm
content: Quote from: tom66 on June 09, 2025, 12:12:15 pmNo. It is not about multi-threading. It is all about a way better (more efficient) algorithm. My 12 year old Celeron laptop can run Orcad Allegro smoothly with a decent sized design. I have used quite a few PCB packages (including ones running on MS-DOS on 386 PCs) over the decades and Altium is the slowest one by far.

author: tom66
author_rank: Super Contributor
timestamp: June 09, 2025, 07:01:35 pm
content: Quote from: nctnico on June 09, 2025, 04:12:27 pmOk, but either way, the algorithm to do polygon hole finding and solving into a simpler polygon to fill is decidably non-trivial and is at least O(n log n) time where n is the number of vertices. So perhaps OrCAD guys have spent a lot of time optimising and caching the results, or they are taking shortcuts. I can believe that a polygon fill algorithm in the 80s would be very slow without all of the advances that have come in computer graphics wrt to triangulation and fill algorithms that have since been researched.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 09, 2025, 10:31:09 pm
content: Yes, it's not surprising to me. Just shows that the shown PCB was probably designed using software, on probably modest machines.Even early versions of KiCad used to have pretty rough polygon approximations for copper fills IIRC.Most software CAD tools do use polygon approximations and obviously, for "round" shapes, the better the approximation and the more vertices you need. Which then becomes computing-intensive as the algorithms are not linear.