Expanding memory of Atari 400 (Piggy Backing)

[doctorclu]

I would like to learn more about piggy backing memory. When I got my first Atari computer we had enough money to buy a Atari 400 and my dad used his experience working with the home built Sol Terminal 20 to piggy back the memory and boost the ram card from 16k to 48K. Worked great.

 

How exactly does this process work when you are piggy backing ram? What needs to happen to make it work?

 

And if you don't know, but have photo examples you have found that you want to show off, Im would find that fascinating as well.

 

I found this article for piggy backing on a Z-80:

 

http://www.classic-computers.org.nz/system-80/hardware_mods_memory_upgrade.htm

 

And the attached pictures were a piggy backed board I found in a collection. I plan on adding pictures of my dad's work soon.

 

 

[Rybags]

With Dynamic RAMs, the addressing is by Row/Column and each chip often supplies 1 or 4 bits of data that go to make up the byte being accessed.

 

Doing by Row/Col allows less pins to be used, e.g. 7 pins for row/col + 2 pins for Ras/Cas = 9 pins to give 14 bits of addressing = 16k worth of addresses which would otherwise need 14 pins e.g. on Static RAMs.

 

Since so many pins are shared among most/all the RAMs, piggybacking can be done. Usually two or more pin of the IC on top will be directed elsewhere. Obviously the data pin/s, and usually at least one of the address select pins.

 

Insofar as reliability though, the practice can be asking for trouble and can look sloppy as well. In the modern day there's usually more elegant solutions, in fact by the mid 1980s you could have 64K of RAM sitting on 2 chips although they can't be used as direct plug replacement for the old 1-bit chips that were in common use beforehand.

 

Also, some piggybacking methods simply replace the chips lying underneath - sort of lazy. Usually easy to detect as the chip below might have the power pins snipped off.

Edited December 26, 2013 by Rybags

[doctorclu]

That actually explains a lot. Also liked. So either the piggy backed chips are adding to the existing chips or completely replacing them. interesting.

[doctorclu]

Took apart my Atari 400 to take a look at what went into it, and here is the way my dad worked on it:

 

First what needed to be jumpered under the ram board itself.

 

Here is what was changed on the top of the motherboard next to the ram shot. A Texas Instruments 7047A SN7432N was used.

 

On the ram board itself the upper left was a DM/SM7437N N58032N 127

To the right of that chip the ship was S7639 N823 1298A

 

The ram chips used were 4116 GP MK 4332D Mostek BA10

 

 

Cool to look at. Basically I'm hoping that if I can understand how this works maybe I can help find a way to expand the memory of a non-Atari system that only has 2K (in this case, the ECS for the Intellivision).

 

For fun and understanding I might actually take a Atari 400 and see if I could expand the memory.

 

Here is the first question I would have about piggy backing: Can the same ram chips be used?

 

My dad's example of using Mostek 4116 GP MK 4332D chips instead of the more standard AM9016DPC 8249WP chips that Atari used was probably because those were chips he had lying around from other projects. But could you stack AM9016DPC 8249WP chips on the existing AM9016DPC 8249WP chips and just wire those together?

 

Are there instructions out there for a step by step on how to piggy back the ram for the Atari 400?

 

 

 

 

Edited December 26, 2013 by doctorclu

[Rybags]

It's not uncommon for Atari machines untouched since the factory to have different brands of RAM coexisting.

 

Mixing different brands is generally OK.

 

Intellivision, not sure but it might use SRams. It was often cheaper to make small Ram systems using Static RAM. Although the Ram itself is generally a fair bit costlier, the support logic for SRam is much less.

 

Vic-20, Atari 7800 use Static Ram. Earlier C64 uses SRam for the colour memory.

[doctorclu]

You mention the ram is a bit costlier. Is even like 1K and 2K chips still costly?

 

A lot to take in.

[Rybags]

There was a time when upgrading DRam came to over $1 per K.

 

Modern times, SRam is still more expensive. But for the smaller sizes we'd be interested in the prices are fairly low, as in under $10 for 64K.

[Kyle22]

I would like to learn more about piggy backing memory. When I got my first Atari computer we had enough money to buy a Atari 400 and my dad used his experience working with the home built Sol Terminal 20 to piggy back the memory and boost the ram card from 16k to 48K. Worked great.

 

How exactly does this process work when you are piggy backing ram? What needs to happen to make it work?

 

And if you don't know, but have photo examples you have found that you want to show off, Im would find that fascinating as well.

 

I found this article for piggy backing on a Z-80:

 

http://www.classic-computers.org.nz/system-80/hardware_mods_memory_upgrade.htm

 

And the attached pictures were a piggy backed board I found in a collection. I plan on adding pictures of my dad's work soon.

 

 

That first picture is a good example of Dead Bugging

[ClausB]

My dad's example of using Mostek 4116 GP MK 4332D chips instead of the more standard AM9016DPC 8249WP chips that Atari used was probably because those were chips he had lying around from other projects. But could you stack AM9016DPC 8249WP chips on the existing AM9016DPC 8249WP chips and just wire those together?

 

Are there instructions out there for a step by step on how to piggy back the ram for the Atari 400?

 

Those MK4332 chips are unusual. Each one is two standard 16-pin 4116 DRAMs on an 18-pin carrier with separate RAS and CAS lines. Your dad piggybacked them onto the existing 4116s (your AM9016 chips), tied all the RAS lines together, and ran the three banks' CAS lines to the other piggybacked TTL chips, giving 48K total RAM.

 

 

See also: http://eda360insider.wordpress.com/2011/11/09/3d-thursday-early-steve%E2%80%99s-improbable-history-of-3d-ics-six-decades-of-3d-electronic-packaging/

 

I bet the MK4332s are hard to find but you could do the same thing by stacking three 4116s and keeping their CAS (pin 15) lines separate. Or you could replace the 4116s with 4164s and not have to piggyback them. Those instructions are here:

http://atariage.com/forums/topic/123207-expanding-the-400/?do=findComment&comment=1488304

Edited December 27, 2013 by ClausB