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Re: Need more folks doing PCB layout, not fewer
Thinking_J   6/5/2014 5:57:32 PM
John_Bass. you are right, a lot of design work will never require a individual gate. I too have used programmable logic as you have described for many of the same reasons.

But individual gates are still used - I certainly wouldn't put in a programmable logic part when only one gate was required. A recent design I worked on, a TI Sitara 1GHZ, Cortex A7 processor design required a single gate for interaction between the processor and the power regulator specifically designed for this processor. No need for any programmable logic anywhere else in the design. Please don't ask me to explain why they couldn't have elimiinated this requirement.

The point I am trying to make: the schematic symbols for logic of any type, should NOT try to represent the pcb shape or pcb layout. It SHOULD try to represent the best way of conveying the function of the circuitry.

Granted, programmable logicc is problematic. Blocks representing the power and special purpose pins are easy enough to keep separate from the rest of the "logic" when creating a schematic symbol. But the best way to represent the logic could be very different on products. (same IC on different designs).. few of us want to create different symbols for the same part - based on the usage. Still , it can be done.

I get requests for processor pins that have 17 different functions.. to be identified ON THE SCHEMATIC SYMBOL ... which function will be used on a given schematic (yes , a different schematic symbol for each usage of the part).
The identified Signal using the pins not enough documentation for some.

While you often can improve the layout on a pcb by forcing pin choices on a programmable logic device, often this will be at the expense of performance (internal) of the device. Altera, Xilinix, etc.. most will recommend letting their software select pin assignments to optimize their performance.

There is no one right answer for all situations.

The customers for the "schematic"  (future engineering support, test dept, repair tech) a very different customer from the "pcb layout" (pcb designer,) or purchasing ( layer count , pcb area, drill hole count, etc), or manufacturing (ease of assembly with processes available, component selection based on a pricing and availability), component placement based on thermal/mechanical considerations of the product. The list of issues to consider is large.

To really do the job "right".. requires consideration of all these customers/departments. Your work is judged by all of them.

I assume none of the preferences from each of these "customers" will remain stationary... making our jobs that much harder (sigh).

Of couse, we could just "blow off" the concerns of others (we know best)..


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Re: Need more folks doing PCB layout, not fewer
John.Bass   6/4/2014 9:44:54 PM
Thinking_J: I don't think I've designed with a real nand gate since the early 1980's. TTL octal buffers and latches yes, logic no.  Everything in the 80's was TI or MMI PLD's after I bought my Structured Designs programmer. Quickly migrated to bigger PLD's and FPGA's after that, simply to reduce board area, cost, and improve ability to apply ECO's to the design without extensive rework.

Most client designs reflect the same.

I've also designed around stocked, or easily sourced parts, doing the PCB layout first, by what is easily placed/routed on the board keeping in mind signal integrity. Then keeping the Schematic current so that the designs netlist is synced between the schematic and pcb tools.

With PLD/FPGA designs this helps simplify the pcb design because pin placement is a function of layout, and avoids horrible trace routing from poor pin assignments.

It's frequently better to layout control/data bus traces first between parts, then assign pins/functions to keep the traces net flat with no/minimal vias. I normally avoid strict manhatten layouts, carefully using any direction "flows" for buses, end to end on the same layer, making sure to leave space for control signals.

This flow allows choosing parts to make the layout smaller, faster, easier ... and avoids the trap of trying to layout from schematic, parts that do not easily fit on boards, creating a nightmare of vias to invert/descramble connections that are not flat/via-less

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Re: PCB Layout Guys Not Going Anywhere
BruceH808   5/29/2014 11:39:19 AM
I see few openings for just PCB design nowadays. Maybe I am not looking hard enough, but over the years PCB design has become increasingly complex and the tools more integrated and sophisticated. 

The idea that PCB design can be automated has always been absurd, but this notion persists amongst some management circles. Moreover, autorouters have given way to routing-assist algorythms. Engineers today have more tools to assist them in developing a product, but it still takes a team of professionals to make real money and the cost of doing so are increasing.

I have been many things in my engineering career, and now I am considered a Hardware engineer. I design circuits and perform PCB design. It is my responsibility to get fuctioning hardware into the hands of a software team that will polish it into a real product. 

Who designs the PCB and what their role and credentials are may depend on the industry in question, but it appears there are far more degreed engineers doing this work today than in the past.

Unfrtunately, these individuals often have little knowlege of drafting standards and give little thought to information transfer and reuse.







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Re: Need more folks doing PCB layout, not fewer
Thinking_J   4/25/2014 8:46:14 PM
Physical shape represented with the schematic shape...

I admit... I prefer my schematic connector symbols to closely represent their physical characteristics. Makes everyone's job easier.

You are correct .. We should be a bit pragmatic in use of "rules". Know when to break em.

But a NAND gate... should be NAND gate (unless you want to play with it's signal - active- polarity).


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Re: Need more folks doing PCB layout, not fewer
betajet   4/25/2014 8:44:50 PM
Thinking_J asked: When was the last time you looked at a schematic and thought... "art"?

Well, pretty much whenever I look at a schematic drawn by me :-)

Heathkit schematics are superb.  But then, they were designed to be used by many, many people so was worth the time to make them excellent.

I like to have schematic symbols match the physical pinout.  [update: I'm referring to complex ICs and connectors, not circuit elements or gates.]  When I'm drawing a schematic for a product, I'm going to be using it many, many times when debugging the board and its programmable logic.  The latter is very important, because even though the board is going to be correct the first time or within a couple of iterations, the board may need to be probed every time a new feature is added to the FPGA.  I like to have the FPGA schematic symbol and pins match up with the physical pins if it's a QFP.  It makes debugging ever so much more pleasant and the small amount of time invested in creating a useful schematic symbol pays for itself very quickly at debug time.

With BGAs, I like to make a separate symbol for power and dedicated balls, and then have the schematic pins match up with the I/O pad order inside the FPGA.

Another thing I do is to number components sequentially through multiple sheets.  If there are multiple copies of an I/O port or daughter board connectors, I like the corresponding components to have similar names, e.g., a series resistor R34 for port A is R44 for port B and R54 for port C, etc.  I may skip component numbers so I can make the names similar.

Often I have to work with schematics drawn by someone who used automatic component numbering to assign component names automatically.  Sure, this saves a day drawing the schematic, but makes it really hard to find components in the schematics or on the board later on.  It's a pain to have a node with pull-up resistor R11 and pull-down R317, instead of R11A and R11B.


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Re: Need more folks doing PCB layout, not fewer
zeeglen   4/25/2014 8:35:18 PM
@Thinking_J - You bring up some good points

Does anyone remember the schematics that came with Heathkit products from the 60s and 70s?

Yes, in my younger years I worked at a Heathkit retail store and built dozens of store display models at home.  Very logically and clearly drawn, and the circuit descriptions were great.  Even now my work is influenced by those early Heathkit years; I try to draw new schematics clearly; and write detailed circuit descriptions for colleagues and techs even though they (for some strange reason) are usually not required in the project plans set by manageers.

When someone is trouble shooting a design (years later).. the primary documentation they should look to is the schematic. For me, this person (tech) is the primary customer the schematic is made for.

So true, when CAD schematic capture first started the emphasis was on turning it into a netlist; the concept that someone might later use it for debug seemed to have been overlooked.  Simple things like drawing a diode bridge at 45 degrees, or placing a slight slope to a wire end 'pointing' to the other end of the same wire on the opposite side of the page.  Or drawing a basic timing diagram with text notes.  Early CAD would not allow that - I hated it!

A good example is a company I once worked for inverted the Cadence grid location borders to match company standards.  But they overlooked the off-page references, so trying to follow connections between pages one had to remember that the off-page designations were mirror-imaged and vertically flipped from the border grid.

They included operational notes , expected voltages , expected waveforms - all in the schematic.

The drawing package I currently use allows exactly that - I can place simulations and scope captures directly onto the schematic.  CAD has made much progress.

Do we think of our audience is pcb designer? or the tech supporting the product years later?

Usually the PCB designer does not even look at the schematic.  Hesh follows the stack-up dimensions documents.

When was the last time you looked at a schematic and thought... "art"?

Every time I see a well-drawn schematic.  Actually a while back there was an article in EE Times about art in electronics and schematics, can't find it.

(But don't agree with):
- making schematic symbols that look like pcb shapes.

This depends on the purpose of the schematic.  I do a lot of reverse engineering in that I take unknown boards and turn them back into schematics.  The first pass is very physical, components on the schematic are placed in the same relative position as they occur on the pcb, color coded as to which side they are located.  Integrated circuits are drawn in their pcb shapes with the logic elements shown inside.  Then I take sections and turn them into logical schematics that show the circuit functionality.


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Re: Don't think so
Thinking_J   4/25/2014 7:01:27 PM

Agree with you. And I have had similar experience.

Except for one item: I do believe the EE SHOULD know production materials, processes and manufacturing practices as much as he knows circuit design, components available and math. Even if they don't do pcb layout work.

Reason: They will never know what is possible to build .. until they have a passing knowledge of all the elements involved. They certainly will need to be able to accurately assess risks involved with any "state of the art" projects being considered... it is part of their job.

If the engineer is stuck working older tech.. yea , sure , he can get away with limiting his knowledge base. I don't think this is a good career path.

For the same reason, I recommend PCB designers to continue their education on materials and processes (and their costs!) used in production... so they continue to add knowledgeable insight in their work and in working with others.

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Re: Yes they are unfortunately
Thinking_J   4/25/2014 6:27:28 PM
r3son8trbluz...Your point is understood.

But this reasoning is based on the assumption:

 EEs are higher paid that pcb designers.

This isn't always the case. And for good reason. As noted by at least one commentor, he is a pcb designer that IS also a EE.

The best pcb designers are often EEs that just happen to like working 3 dimensional puzzles (with several other "dimensions" added.. aka space, power, time, etc..)

You are right , pcb layout work isn't time effective for all engineers.

But I wouldn't assume this to be a common limitation on all EEs.

Worst , I wouldn't recommend a EE to avoid this work based on any generalizations put in place by others.

Try it, You may find you can do it faster and better than you can hand it off to others. I find it relaxing.

Hey, we EE aren't really generally known as "great communicators".. (another generalization that really isn't fair)



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Re: Need more folks doing PCB layout, not fewer
Thinking_J   4/25/2014 5:52:29 PM
zeeglen...I understand why it is done (explaining where to put decoupling caps)

Just like I understand (But don't agree with):

- making schematic symbols that look like pcb shapes.

- explaining / or expecting the production supply chain/ to "know" the impedances of DDR2 or DDR3 signals within a schematic.

- or explaining good (basic) layout practices for switching power supplies.

- or explaining how to recognise and layout tank (resonate) circuits

- or explaining good emi/rfi practices...

- etc....

The point:  If we can't make the schematic specify EVERTHING about the pcb design, why make it hard to read for it's intended audience? For the sake of covering just a few of the issues because we can't trust HR to hire good pcb designers? (at least they got the de-coupling caps right.. the rest of the layout was poor).

I acknowlege each of us will pick the battles we think we can win within a given organization.. and sucume to poor work arounds when forced to.

When someone is trouble shooting a design (years later).. the primary documentation they should look to is the schematic. For me, this person (tech) is the primary customer the schematic is made for. They aren't going to be trying to re-layout the design. They want to know how the product was supposed to function. It is likely they won't have the production tools (test fixtures, text instructions, access to the engineer(s), etc...) and may be happy they at least have a schematic.

I could be pointing at windmills .. maybe everyone expects their design to be completely un-supportable after production stops. A result of "throw away" technology becoming so prevalant.

Does anyone remember the schematics that came with Heathkit products from the 60s and 70s?

They included operational notes , expected voltages , expected waveforms - all in the schematic. They were made for novice trying to understand what they were building. (I am not advocating this level of detail in all schematics)
Very little about physical layout. They were "art" (defined by: adding or substracting only reduced their level of perfection).

Perfection because they clearly understood their audience. Do we think of our audience is pcb designer? or the tech supporting the product years later?

When was the last time you looked at a schematic and thought... "art"?

Max The Magnificent
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Re: I agree
Max The Magnificent   4/24/2014 9:32:34 AM
@Jesus Castane: The more accessible is the knowledge the more engineers will go into the PCB Design Industry.

I hadn't realized that IPC 2222 cost 114 dollars -- wow!

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