At 11:31 AM 1/31/2002 -0500, Jeff Adolphs wrote:
>I didn't know what you did was possible. I use net labels on each pin
>such as A0, A1, A2,... I was using comment text to label the buss line
>such as AD<7:0>.
>The net label on each pin really gets the signal to the right place. I
>always use Global nets because I always have multiple sheets and need
>the nets to connect to other pages.

First of all, net labels make actual connections within a sheet. This is 
not changeable. What is optional is how off-sheet connectivity is handled.

Mr. Adolphs uses Net Labels and Ports global. In this case, Ports and Net 
Labels are equivalent. This is not true with the other connectivity scopes. 
With Port and Net Labels global, bus labels have no function. But I would 
recommend using them anyway, instead of text, because someday one might 
want to use the other scopes.

>  I also use a graphic oval as an
>offpage symbol so the net label is what makes the connection.

Like the use of annotation text for net bus labels, this is not only 
unnecessary, it is fraught with risk. Ports are designed for off-sheet 
connections. I highly recommend using -- at least -- Only Ports Global 
connectivity, it forces one to make off-sheet connections visible. I 
actually recommend going beyond that to Sheet Symbol/Port connections, but 
it does require more work. That work returns better intersheet checking and 
clearer documentation.

Here is how they work. Only Ports Global causes Net Labels to only create 
connectivity for the sheet on which they occur. Presumably one allows the 
netlister to add sheet numbers to the nets when using this scope, otherwise 
one will get duplicated net names, which I think doesn't work for PCB, I 
forget. Ports, however, create global nets which connect across sheets to 
Ports on other sheets with the same name. If you set the port electrical 
characteristics correctly, this will also allow you to check a single sheet 
for electrical rules compliance.

Electrical rules compliance is not just a detail. A high percentage of 
schematic errors will show up as a violation of electrical rules, or they 
will at least produce a warning if you have the Rules matrix properly 
configured. I recommend creating a warning for all unconnected pins, and 
then suppressing the warnings with No-ERC directives placed on the pins; 
the markers serve as a visible notation that one is deliberating leaving a 
pin open. If the pin has a net name on it, you might assume that it is 
connected elsewhere, which will not be true if there is some even slight 
variation in Net Label name, such as AD0 and ADO.

Now, Sheet Symbol/Port connectivity is even more clear. It works with 
hierarchical schematics, and any multi-sheet schematic can be set up as a 
hierarchical schematic. One has a project page with sheet symbols on it for 
all the other pages. The same thing is done with the other scopes with flat 
schematics, to tie the sheets together for netlisting and other purposes, 
but with this connectivity, nets are only connected between sheets by 
explicit wires or net labels on a higher-level sheet. So the project page 
might only consist of sheet symbols and connections between them, or it 
might have its own circuitry and then all links between it and subsheets 
are made through the Sheet Symbols. So from one sheet -- in many cases -- 
you can see all the connectivity for the whole project, excepting only 
intrasheet connectivity on subsheets. This can be used to make a highly 
understandable schematic, something which can otherwise become difficult 
where there are multiple sheets.

The advantages of SS/P connectivity become really high with design re-use. 
One does not need to worry about the details of net naming on the 
subsheets. Subsheet Net labels are irrelevant for intersheet connectivity, 
only the explicit connections made at the level above matter. You can have 
a port named /WR on the subsheet, which will connect to a sheet entry of 
the same name on this sheet's sheet symbol. This then could be wired to a 
net named RD on the higher-level sheet.

This connectivity can be a bit tricky to get working, it must be done 
exactly right. But the good news is that if you don't get it right, you 
will get warnings and errors unless you have misconfigured the ERC matrix.

I do not recommend using different names for nets as I mentioned was 
possible, it is only that with design re-use, one can accept such 
differences. A proven subsheet does not need to be edited to make it 
usable, except for adding or changing Ports if necessary. If it is not 
difficult, by all means, make those Ports have the same names as the Net 
Labels. But you don't have to, except with Buses.

Now, to buses with SS/P connectivity. This works:

Subsheet has bus D[0..7] which connects to Net Labels AD0 through AD7. This 
bus is connected to a Port named D[0..7]

Supersheet has a Sheet Symbol referring to the subsheet. That sheet symbol 
contains a Sheet Entry named the same as the Port on the subsheet, in this 
case AD[0..7]. That Entry is connected to a Bus Label (Net Label}, 
presumably with the same name (but this is not necessary -- it appears that 
the nets are assigned based on sequence, not on name. Here is where 
AD[0..7] *might* be different from AD[7..0]). The Bus Label connects to its 
subnet Net Labels on the supersheet.

There is a question about the electical characteristics of Ports and Sheet 
Entries. In my view, these stand in for, i.e., represent, what is 
off-sheet. So an output on one sheet will be connected to an input Port on 
the same sheet. Connecting it to an output port, the way I have my error 
matrix configured, I forget if this is the default, will generate a 
warning. But this input Port is made to look physically like an Output, 
ie., I'd prefer to have it on the right-hand side of the schematic if this 
is easy, and I will definitely make it right-pointing if I can. This port 
then connects to a Sheet Entry on the sheet's Symbol on the next level. 
That Entry will have an Output characteristic, so it will correctly drive 
inputs. It represents what is off-sheet.

Abdulrahman Lomax
Easthampton, Massachusetts USA

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