Atari 800 PCB rear 28-pin connectors?

[ldelsarte]

Dear Atari 8-bit fellow,

 

A silly question probably: Can anyone explain what this (28 pins?) connector at the bottom of the Atari 400 & 800 pcb/motherboard is for?
(On the 800, it's right behind the daughterboard that houses the CPU, etc)
It's anyway unavailable to the user, as the computer case is closed there.
Is this an earlier version of PBI/ECI (but with missing pins)?
Is this something useful for testing the machine in the factory?
Is this a feature that Atari removed at the last moment?

What else?

Does anybody know?

 

Thank you!

Laurent

 

Image credits: https://www.the-liberator.net/

 

[Beeblebrox]

@ldelsarte  afaik this was only used by Atari BITD to test machine in the factory.

 

I don't think it's been utlised by anyone in any mods or upgrades, especially given it's inaccessible once the case it on, etc.

 

EDIT: 

 

See a previous discussion here from a few years back (always worth searching AA for previous threads as chances are these things have already been asked):

 

 

Edited June 25, 2022 by Beeblebrox

[TGB1718]

On the circuit diagram, this port is labelled "Test Points", so as @Beeblebrox says, it was for factory and maybe

service department use only.

 

Bit of a shame really, if they had made this available to the user, who knows what "goodies" would have appeared.

 

Edited June 25, 2022 by TGB1718

[Gunstar]

27 minutes ago, Beeblebrox said:

@ldelsarte  afaik this was only used by Atari BITD to test machine in the factory.

 

I don't think it's been utlised by anyone in any mods or upgrades, especially given it's inaccessible once the case it on, etc.

 

EDIT: 

 

See a previous discussion here from a few years back (always worth searching AA for previous threads as chances are these things have already been asked?

 

 

I was once-upon-a-time planning on using that 800 edge connector for a PBI, making use of any PBI signal lines that just might happen to be there and for the rest cutting the traces behind the edge connector and wiring up my own PBI grabbing whatever wasn't there from the Incognito PBI output. But in the end, I could not rationalize all the electronic hacking and shielding/case hacking to accomplish this just to make use of that edge connector. So I ended up making a standard PBI port off the back of the 800 outside the shielding making use of the Incognito's PBI abilities.

 

however, I could see making use of some signals from that edge connector for some internal mods and upgrades so no need to hack the shield and case hardware (or at least not much and have it hidden and discreetly wired up.

Edited June 25, 2022 by Gunstar

[Gunstar]

Oh, and by the way, I make it a 56-pin edge connector...28-pins per side of the backplane...or am I mis-remembering and the bottom pins are dummies not connected to anything? Or are connected to the same traces as on top and therefore only 28? I don't recall ever coming across an edge connector like that before though. it's been a while since I had the old gal opened and studying that edge connector so I don't recall anymore.

 

In any case, I do remember enough top and bottom pins to connect the full PBI buss too, and then I was going to create a reduction adapter down to the normal PBI edge connector size.

 

Edited June 25, 2022 by Gunstar

[TGB1718]

4 hours ago, Gunstar said:

Or are connected to the same traces as on top and therefore only 28?

Had another look at the cct diagram, looks like about 38 of the pins are used, some are numbers

some are letters, so both sides are used.

[Rybags]

I'm not sure using the connector would have any advantage vs the current method on 800 of using the OS or a RAM slot + wiring mod to add extra needed signals.

To use the connector would probably involve hacking away part of the shielding and possibly the outer case as well and from what I can see from schematics not a lot of signals are present.

[Lynxpro]

I thought Joe Decuir said over in either the Atari Museum or Atari 8-Bit Computers Facebook group(s) that the edge connector was originally meant as an option for external expansion, much like the 1090 was meant to do with the PBI connector. 

[stepho]

Sounds like how Jobs wanted no expansion capabilities on Apple boards, so the design team put "diagnostic" connections on.

 

Quite possibly the Atari design team planned for an expansion port but Atari chose to not use it.
Pity, the S100 bus and other similar backplanes opened up a lot of potential.

[Rybags]

Opening at the back for that as an expansion option would add extra FCC headaches.  Though of course even for the time the design was overkill.

[Gunstar]

This reminds me of my Bally Arcade I restored and am upgrading into a full computer as per old vaporware that Bally never released, building some of it myself and using other's DIY plans and/or third party released upgrades that upgrades the Bally to a full computer, if not to the extent Bally's plans originally intended. But different revision mobo's have hidden edge connectors too, and though not as robust as the 800's, Bally had to deal with the ridiculous FCC rules at the time too, so they have a full shielding and hidden I/O underneath too on early models, pre-1979/80 when the FCC rules were relaxed.

 

While doing the upgrades and mods, I've put to use a hidden edge connector in my Bally, and depending on the motherboard revision and model (Bally brand or later Astrocade brand) some have all the original hidden edge connectors and some don't. Because of this there was quite a bit of confusion in an upgrade I was planning by connecting to one of these hidden connectors, because nobody else in the group, apparently, had the exact same model/revision mobo that I have. So when I was referring to one of the hidden edge connectors, removed on later models and revisions, they all thought I was referring to the one rear buss connector that remained for expansion throughout it's production by the two companies that owned it. And I, only having experience with the one mobo revision I own, had no idea that most Bally/Astrocades didn't have this other edge connector.

 

But once we all finally figured out the issue through photos and and me pointing it out on the mobo schematics, and discussion, and all were enlightened, even some who had owned Bally's since back in the day, but just never came across the mobo revision I have. It turned out that by mistaken identity, the edge connector being labeled as "keyboard" on the schematics, and being the exact number of pins that the Bally's built-in keypad used, that a keyboard could be plugged in there if made for the Bally.

 

It turned out, after study by me and others, that it's a mis-labeling and it was just a factory test connector as well. But since I was already using the buss expansion for an external memory upgrade, I didn't want to attach a keyboard there like others had done in their personal upgrades as well as third-party upgrades from past had done. So in the end I decided to use the 10-pin edge connector for my external keyboard anyway (DIY custom built Cherry mechanical from scratch) and just cut the traces accessed by the edge connector and re-wired it to the internal keypad so I could still have my separate, unpluggable external keyboard like I wanted, with easy access and not out the back.

 

Anyway, this link is to my unfinished blog, but there are pictures and description of my using the edge connector in it there, just no pictures of the finished mod yet. But I'm posting one of those below the link (picture taken to show of my new dust cover, but it shows the keyboard and keypad). And of course, I always take things a step further than those who came before, and apparently I'm the first where this is concerned regarding the Bally/Astrocade, which is adding not just a real keyboard, but also a real 24-key mechanical keypad that mirrors the calculator style one on the console.

 

 

Edited June 28, 2022 by Gunstar