[Coco] Glenside IDE controller

Dave Philipsen dave at davebiz.com
Sun Jun 26 05:58:08 EDT 2016

This brings up a point which was discussed here about three months ago.  There are some who believe that gold pins on the card edge of the disk controller are overkill for our application.  At least one person believes that the 'preference for Gold contacts may simply be a matter of prioritizing one thing over another or even a "nobody ever got fired for buying IBM" kind of thing.'

A couple of points were mentioned, supposedly in support of using non-gold-plated fingers on circuit boards for program paks.  One excerpt said,  "Tin-to-gold interfaces are more susceptible to fretting corrosion related failures, and lubricants are not nearly as effective..." 

The other excerpt said, "One caveat: Don’t mate gold with tin as it is subject to fretting
corrosion. As the connection is made, the tin tends to transfer to the gold pins, eventually forming a tin oxide layer on the gold..."

I believe that these are all valid points.  And perhaps gold or gold-plated contacts *are* overkill in our application.  But if you believe these articles that were quoted (and I do) then you do two things; 1) find out how the contacts are plated on the original connectors found in the various CoCos, 2) make sure that connectors you select (whether the edge connector the card plugs into or the fingers on the edge of the board itself) use the *same* type of metal on *both* sides of the connection.

It is known that a certain type of corrosion is caused by mating dissimilar metals.  It is also know (as stated above) that a gold-to-tin connection can introduce a second type of 'fretting' corrosion. So if you're selecting the connector as Kip describes you will find that it is described as "Gold on the mating area, tin on the contact tails, nickel underplate".  That means this connector would be well suited to mate with a circuit board that has gold or gold-plated fingers since its mating area is gold-plated. And while I do not know the composition of all edge connectors used in the various runs and models of the CoCo, I do strongly suspect that the edge connectors on the three CoCo 3s that I own have gold-plated mating surfaces.  Therefore, if I am making a choice of what type of program paks would be best suited for use in *my* CoCos I will choose those with gold-plated contacts to avoid corrosion problems.

Digi-Key is a well-known and reliable source for connectors as these and they have a huge inventory of parts but they do tend to be overpriced on some things and I believe $9.43 is too much to pay for a card edge connector even if it has gold plated mating surfaces.  I do not know anything about the contact composition of Jim's connector but the price is much closer to what I'd be comfortable with paying.  If I were designing a board I wouldn't want to pay more than about 4 bucks  for a 40-position card edge connector as used on the CoCo. And personally I would prefer it to have gold-plated contacts to mate with a cartridge with gold-plated fingers for durability and trouble-free performance.

Dave Philipsen

>> On Jun 26, 2016, at 12:49 AM, RETRO Innovations <go4retro at go4retro.com> wrote:
>> On 6/25/2016 10:37 AM, Kip Koon wrote:
>> Yep, that would be the one.
>> Kip Koon
>> computerdoc at sc.rr.com
>> http://www.cocopedia.com/wiki/index.php/Kip_Koon
>>> -----Original Message-----
>>> From: Coco [mailto:coco-bounces at maltedmedia.com] On Behalf Of Al Hartman
>>> Sent: Saturday, June 25, 2016 11:31 AM
>>> To: CoCoList for Color Computer Enthusiasts
>>> Subject: Re: [Coco] Glenside IDE controller
>>> From a message I got on the list a few years ago from Brian Goers...
>>> "The card edge connector that a floppy controller could plug into is available from Digi-key.
>>> The manufacturer EDAC Part Number is 395-040-520-202."
>>> http://www.digikey.com/product-search/en?keywords=395-040-520-202
>>> $9.43 each
>>> -[ Al ]-
> Not to toot my own horn, but I think the ones I sell for $1.50 will work:
> http://store.go4retro.com/pcb-edge-connector/
> -- 
> Coco mailing list
> Coco at maltedmedia.com
> https://pairlist5.pair.net/mailman/listinfo/coco

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