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 [ 5 posts ] 
 New to PCB's and schematics 
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Joined: Mon Oct 07, 2019 2:41 am
Posts: 255
Later this year, I plan to draw up schematics and PCB's for a mother board and a few cards,
memory,cpu logic and control, and front pannel, and acres of memory for 1974 1.25 us core emulation.
Any know of general guides for TTL (74/LS) hardware, and PCB design? Libriaries for comon stuff like 16 pin IC's and leds and switches and mounting holes and older connectors like S-100 bus or 1.56 inch card edge connectors 44 or 72 pins. Ben.

Thu Jan 02, 2020 7:03 am

Joined: Mon Oct 07, 2019 1:26 pm
Posts: 43
Hi Ben, I think you should use KiCad. It is free and has many users. Alhough I am using another package, more than 10 years old now, called Orcad.

Thu Jan 02, 2020 12:23 pm

Joined: Mon Oct 07, 2019 2:41 am
Posts: 255
I have some sort of version of that. Just what free drill sizes go with what footprints? How thick for
traces as power? Default cap/resistor footprints and holes? 72 pin edge connector? How to layout pre-sized templates? I don't want re-invent the wheel with every pcb.

Thu Jan 02, 2020 5:17 pm

Joined: Tue Dec 11, 2012 8:03 am
Posts: 285
Location: California
The custom PCBs section of the 6502 primer addresses some of this, but I'll repeat part of it here. (I see I could update a couple of little parts anyway.) See the info there about free CADs. Some board manufacturers offer a free CAD that only works at the one board house. Avoid those; because if you use their CAD and then are unhappy with them for whatever reason, you'll have to do your layout again for a different board house. Use CAD that generates industry-standard Gerber 274X photoplotter files and Excellon drill files.

I used one cheap prototyping house perhaps 20 years ago that made you choose from a list of drill sizes. I'm not aware of any that still require that. You can pretty much do whatever sizes you want, for no extra charge. There is a small size tolerance though, so there's probably no point in specifying holes whose sizes are only a couple of thousandths of an inch different from each other. The ones I use most are .008" for vias (.015" until recently, but the PCB-manufacturing technology has now made vias with the higher aspect ratios quite dependable), .025", .030", .035", and .040" for thru-hole 1/8W resistors, capacitors, ICs, and pin headers (although I don't do much thru-hole layout anymore). Even SMT parts will sometimes need holes though, particularly connectors and switches that need more strength so you don't rip the foils off the board with the mechanical stresses of plugging or unplugging, or pushing a switch's lever one direction or another, and sometimes the manufacturers' specifications for hole size on those is quite precise, to prevent these problems. Then there are of course things like mounting holes for standoffs or whatever. Thru-plating was rare and undependable in 1974. Not anymore.

Any CAD should be easy to make your own components in. I don't use the ones that came in the libraries that came with my CAD, because I wanted to re-form the components for the maximum densities our company's products need. It's very quick and easy in my old CAD, but apparently that's not always the situation with other CADs. Regardless, there will always be components that aren't in the libraries. There are just too many to include them all. For example, in a recent board I laid out, I needed a new microUSB connector that didn't exist when the CAD was produced, an SMT 4-conductor-plus-switches 3.5mm phone jack, and a sub-teensy-weensy SPDT SMT slide switch (in addition to the more standarar chip resistor & capacitor, diode, transistor, and IC land patterns).

An .008" trace will carry a half amp with no problem; so the issue is more one of AC performance, not the DC. Your power-supply traces will be bypassed anyway. The better thing is to have at least a ground plane if not also a power plane, and it's not because a narrow trace can't carry the current. It can. (Note that copper pours do not qualify as ground planes! Fast signal edges in digital are RF, not audio, and the rules are different.) A book could be written here on layout guidelines; but fortunately it sounds like you're sticking to rather low rise & fall times, which makes a lot of things more forgiving. Otherwise, you'll want to check the section of the 6502 primer on avoiding AC performance problems in your construction, at, and the links to other forum topics and ap. notes there. Spend a little time in those, lay out your boards, then post pictures here and those of us who have been doing layout as part of our jobs for decades will probably spot things immediately to have you fix so your first experience has a better chance of going smoothly. There are too many things to list here that could cause manufacturing problems, things that the inexperienced might not have thought of.

_________________ lots of 6502 resources

Thu Jan 02, 2020 9:22 pm WWW

Joined: Mon Oct 07, 2019 2:41 am
Posts: 255
I still need a good IMPERIAL set of rules with KiCad, for footprints and tracks and cutouts since this is a non surface mount project. The largest PCB will be the ALU card with 1.5 amps power (LS) and
36 IC's and lots control signals and 36 bus connections.

Fri Feb 28, 2020 6:36 pm
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