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Old 14th September 2006, 02:46 AM   #1
MGH is offline MGH  United States
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Default Benifits of point to point wiring for digital circuit?

Are there advantages or disadvantages of point to point wiring of a digital circuit over a PCB? Much more tedious but worth it?
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Old 14th September 2006, 03:06 AM   #2
clem_o is offline clem_o  Philippines
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Density. You can have many wires on top and crossing each other, which wouldn't be possible with true traces on PCB.

Of course, if there's a wiring mistake it's a dog to debug.

Cheers
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Old 14th September 2006, 03:12 AM   #3
poobah is offline poobah  United States
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In general... no.

There are cases where long parallel traces can cause unwanted coupling between signals. Consider for example a set of data or address lines in a memory array. These signals usually change state as a group... so it doesn't really matter if the edges get fuzzy. They are clocked some time after the transition... when all the fuzz and and ringing have died out.

Now clocks on the other hand are usually given more carefull consideration. Lowering the node impedance, guard traces and other tricks are often used. Jittery clocks are bad... oomkay?

Much depends on the type of logic family employed... not really the speed... but rather the rise and fall times. The steeper the edges, the more coupling you get. Old Schottly, "S" devices could be a PITA that way.

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Old 14th September 2006, 04:04 AM   #4
clem_o is offline clem_o  Philippines
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Hi poobah,

IBM mainframes even up to the era of the 3090 (if I'm not mistaken) were built (orpartially at least) using wire-wrap technology. Done properly and with care, even fairly high speeds were achievable. The physical distance between traces on a pcb versus that of wire-wrapping for parallel lines would indeed be (possibly) smaller on wire-wrap, but not by much.

I agree of course there are limits - depends on the length, distance, rise/fall time of the signals as well as the voltage levels of the logic devices used. But wire-wrap serves pretty well up to several MHz. Just don't run things like hundreds of MHz or GHz on it...

:-)

Cheers!
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Old 14th September 2006, 04:26 AM   #5
poobah is offline poobah  United States
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Yes Sir!

Even into the mid 80's... I know Amdahl computers was "stitching" boards with small welded wires. Their approach to cross talk was to put every wire in straight line, shortest distance between to points. Their studies showed it had the least amount of overall noise coupling.

The boards looked like HELL though!

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Old 14th September 2006, 04:32 AM   #6
clem_o is offline clem_o  Philippines
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Quote:
Originally posted by poobah

The boards looked like HELL though!
Certainly agree with that! Wire-wrapped boards look great - face down, so one can only see the component side!!!

:-)

Cheers
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Old 14th September 2006, 04:44 AM   #7
poobah is offline poobah  United States
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I still do some wire wrapping now and then... Prototyping is getting harder and harder. Can't get everything (the new chips) in DIPs these days.

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Old 14th September 2006, 04:52 AM   #8
clem_o is offline clem_o  Philippines
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Generally I photoprint then use UV-sensitized PCB. But if the circuit is small it may actually be faster to wire-wrap. Sadly though wire-wrap sockets are expensive, and yeah, no such thing for SMT stuff!! :-)

Cheers
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Old 14th September 2006, 06:09 AM   #9
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Quote:
Originally posted by poobah
Prototyping is getting harder and harder.


Oh , I'm not so sure. There is a picture floating around the net of a dead-bugged BGA chip and the magazine RF Design once featured a dead-bugged Spectrum Analyzer.
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Old 14th September 2006, 06:21 AM   #10
clem_o is offline clem_o  Philippines
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Quote:
Originally posted by rfbrw



Oh , I'm not so sure. There is a picture floating around the net of a dead-bugged BGA chip and the magazine RF Design once featured a dead-bugged Spectrum Analyzer.

Geez, wish I had hands steady enough to solder onto a BGA!!
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