Advice on laying out a PCB...

[floobydust]

Hope movie night with the family was good...

Looking at your board picture.... in addition to other comments already made:

- Your schematic shows a TO-220 cased 5V regulator, albeit your board shows a TO-92 cased regulator. With expansion ports, you should be using a TO-220 with a heatsink. Also, it's a good practice (and generally in the datasheet) to add a 0.1uF cap on the input to the 5V regulator as well (as the output).

- As you're using two logic chips (74HC00 and 74HC139) for memory, I/O decoding and Clock2 qualified RAM timing, I would be more inclined to replace both of these with a single 24-pin PLD... as you can easily get fully qualified /MR and /MW signals, RAM and ROM selects and 5- I/O selects (assuming I/O selects are 32-bytes wide). This would give you a lot of remapping flexibility going forward.

- As you have ample board space, perhaps add a second DS-1813 connected to the NMI line as a panic button. Or use something like a TL7705 to generate both /RESET and RESET lines along with the DS-1813 for the NMI line.

Again, these are basic recommendations to give you a more useful/flexible board. Finally, as this seems to be more of a personal hobby project, I wouldn't try to declare a date to send off to get PCBs made until you're 110% certain that you've got everything you need in the design.... and that it's correct. Just my $0.02 of course.... good luck with the project!

[cbmeeks]

floobydust wrote:

Hope movie night with the family was good...

Actually, it was. We saw The Clock in the Wall with the kids and extended family. Not a bad kids movie and I generally don't like kids movies.

floobydust wrote:

- Your schematic shows a TO-220 cased 5V regulator, albeit your board shows a TO-92 cased regulator. With expansion ports, you should be using a TO-220 with a heatsink. Also, it's a good practice (and generally in the datasheet) to add a 0.1uF cap on the input to the 5V regulator as well (as the output).

Not sure why the rendered 3D view of that shows the TO-92. It's a 7805 (TO-220) and I will be using a heatsink with it. I debated putting a 0.1uF cap on the input. In fact, I've built several power supplies in the past where I've done that. Then I asked somewhere (can't remember where) and someone mentioned he never did that. Not sure why. So I guess I just forgot. I will see about adding it. I like that idea anyway.

floobydust wrote:

- As you're using two logic chips (74HC00 and 74HC139) for memory, I/O decoding and Clock2 qualified RAM timing, I would be more inclined to replace both of these with a single 24-pin PLD... as you can easily get fully qualified /MR and /MW signals, RAM and ROM selects and 5- I/O selects (assuming I/O selects are 32-bytes wide). This would give you a lot of remapping flexibility going forward.

PLD's will certainly be in my next design. In fact, I'm already done much of the schematics on my next design and it uses an ATF1508. However, the name "Potpourri6502" is a play on words for the example computer on the "Circuit Potpourri" detailed on the official primer site. The computer is an "homage" to that computer with my own little flair.

floobydust wrote:

Again, these are basic recommendations to give you a more useful/flexible board. Finally, as this seems to be more of a personal hobby project, I wouldn't try to declare a date to send off to get PCBs made until you're 110% certain that you've got everything you need in the design.... and that it's correct. Just my $0.02 of course.... good luck with the project!

Thanks for the advice. I take it all very seriously.

I totally expect some bodge wires on this first run. Hopefully, it won't be too many but I'm sure I will forget something.

I always do. lol

[drogon]

floobydust wrote:

- Your schematic shows a TO-220 cased 5V regulator, albeit your board shows a TO-92 cased regulator. With expansion ports, you should be using a TO-220 with a heatsink. Also, it's a good practice (and generally in the datasheet) to add a 0.1uF cap on the input to the 5V regulator as well (as the output).

Just a quick note to this, there are now SMPS versions of the age-old 7805 which are direct drop-in replacements. I've been using them for some time now on new designs - they're more efficient, run cool and handle an amp without a heatsink.

Also, I think if making a board with slots like that, I might be tempted to route the input 9v to the slots then put one of these on each card. The old S100 bus had the right idea (8v), just not the SMPS technology at the time.

-Gordon

[Alarm Siren]

Further to Gordon's point about the power supplies, I recommend the Recom R-785.0-0.5 or R-785.0-1.0 (0.5A or 1A respectively). They're pricey compared to a 7805, but as Gordon said they're much more efficient and don't require a heatsink, and they're drop-in replacements - so no fiddling about!

Oh, and thanks for following me on GitHub

[Dr Jefyll]

It's worth remembering that voltages other than Vcc (+5V or +3.3V) are dangerous in terms of unplanned connections which can occur in real life -- for example a wire or metallic tool or voltmeter probe which gets dropped or by other accident ends up in the wrong place on top of the board (or underneath). A board using 5V only (or 3.3V only) can usually survive such a mishap unscathed. But if other voltages are present (eg, +9, or perhaps + and - 12) the result can be blown IC's and even smoke.

The safest thing is to keep all other voltages off the board -- which in this case would mean mounting the regulator on a separate assembly, perhaps only a few inches away, and with appropriate insulation surrounding it. Alternatively the regulator could be on board, but again with precautions regarding insulation of the non-5V components and traces.

Running 9V (eg) to all the slots means a worrisome increase in the area of concern... which is not to say "don't do it," but do be aware of the compromise involved. Are there more pros than cons? Some expansion cards may need an additional voltage because of analog circuitry or relay loads (eg) that are present. In that case you have no choice, and you'll have to make do with whatever precautions you can manage, insulation-wise. (or just try to be super careful... and hope for the best!)

As for using a separate 5V regulator on each card, in my opinion that's an approach whose relevance is not what it once was. Certainly it used to make sense with the old S100 systems, but low-density memory and power-hungry LSI and discrete logic are no longer the norm. With the parts we have nowadays it won't be surprising if the entire system has a current drain comparable to a single card from the good (? ) old days!

-- Jeff

[GARTHWILSON]

You could put voltage converters where needed, but it takes more room. I like to have ±12V (very non-critical, and usually anything from ±9V to ±13V is fine) for things like op amps and D/A converters (especially if their outputs need to go below ground), a speaker amplifier, the negative bias for supertwist LCDs, and RS-232 line drivers like the 16-pin MC145406 triple line driver and receiver which is smaller than the common MAX23x since it does not need all the pins for a charge pump. This kind of thing is why we put ± power supply voltages on the 65SIB interface.

[ElEctric_EyE]

cbmeeks wrote:

Actually, it was. We saw The Clock in the Wall with the kids and extended family. Not a bad kids movie and I generally don't like kids movies.

OT: Do you mean 'A House With A Clock in its Walls'? I saw a preview on IMDB and it does look good!

cbmeeks wrote:

...I totally expect some bodge wires on this first run...

I always do. lol

Yup, no matter how careful you are, it IS going to happen. I'm all for throwing the switch sooner than later. In the end, if you've had 3 iterations of a board design with the final being the 3rd, then I'd say you've done very well.

[cbmeeks]

Alarm Siren wrote:

Further to Gordon's point about the power supplies, I recommend the Recom R-785.0-0.5 or R-785.0-1.0 (0.5A or 1A respectively). They're pricey compared to a 7805, but as Gordon said they're much more efficient and don't require a heatsink, and they're drop-in replacements - so no fiddling about!

Oh, and thanks for following me on GitHub

Thanks for the recommendation on using these. In fact, I really think I want to start using these anyway. I restore vintage computers and I've replaced 7805's before. After seeing Jan Beta (YouTube) use something similar, I started wondering if I should be using those as well.

Since it's just a hobby project, a few extra bucks isn't going to matter. I like what Bil Herd says..."In quantities of a million, pennies count!". So I'm definitely OK with ignoring a few hundred pennies. lol

As for the different voltages....that was something I was probably going to ask in a future question.

One add-on card I know I want to make is a sound card using an AY-3-8910. I've made them before and never used an op-amp. But that's because I ran them through external speakers. So, I'm not sure I would need other voltages.

However, another add-on I've had in my mind for a long time is a cassette interface. I know very little on how they work so I will have to tackle that later. But I imagine it would need other voltages. I suppose I will have to cross that bridge when I get to it.

Oh, speaking of the S100 bus....I take inspiration from the strangest places. Just last night I was reading one of my many BYTE magazines (March 1978) and I saw an add for an S100 expansion bus with 21 (yes, TWENTY ONE) slots. It came in a card-cage inside a large metal box. With two floppy drives on the right.

Big, ugly, metal, utilitarian. And I want one so bad I can taste it. LOL!

[1024MAK]

Conventional 7805 vs. DC/DC switching regulator

For low power circuits where you don’t need a heatsink on the 7805, it’s not really worth fitting a DC/DC switching regulator like a RECOM or Traco Power type.

However, where a 7805 is being used, or specified, and a large heatsink is needed, yes, it’s normally worthwhile fitting a DC/DC switching regulator.

In this link, I put some photos of how I mounted a RECOM in place of an existing 7805 in one of my ZX81 computers.

And here, there are some photos showing a recreated Jupiter ACE kit that I built, but using a RECOM instead of a 7805.

Note that most also come in cases with right angle legs so they can be mounted flat on a board like a 7805. But they cost a bit more money.

Mark

[floobydust]

Murata also make a couple nice ones... one for vertical mount and one for horizontal mount. These are about the size of a 7805 plus the heatsink... prices are also fairly attractive at $4.87 qty. 1:

https://www.mouser.com/ProductDetail/Mu ... GdkUxM8%3d

https://www.mouser.com/ProductDetail/Mu ... QFgJuaU%3d

Depending on the current being drawn, a 7805 may still require a heatsink based on the input voltage... the greater the input voltage, the more heat the 7805 dissipates. Example, with a current draw of 250ma, the voltage drop across the regulator is multiplied by the current being drawn from the load:

With a 9-volt supply: 9-5 = 4 * 0.25 = 1.00 watts
With a 12-volt supply: 12-5 = 7 * 0.25 = 1.75 watts
With a 24-volt supply: 24-5 = 19 * 0.25 = 4.75 watts

[cbmeeks]

More progress today. I think I have all traces and pads routed and accounted for. Next I will inspect each and every one to make sure they go where they are supposed to.

Hopefully, this thing will be in my hands before my vacation at the end of the month.

[cbmeeks]

Small update. I submitted the design just to see if it would pass mustard (yes, mustard...not muster....lol). Turns out, I've got some traces that are too close together. On 2 layer boards, they have to have at least 5 mils of clearance. So I hope to correct that in the next day or so.