Custom PB Duo RAM?

[MinerAl]

On eBay right now there are many RAM cards for Duos.  Most of them are in the <$20 range for 8M cards.  The 40M cards are in the $150 range and the one 48M card available is listed at $300.  (This is in no way a criticism of the prices; if they can get that for their RAM, then God bless em.)

So naturally I wondered if I could get one of the cheap 8M cards and (have someone) solder higher capacity chips onto it.  It looks like the 8M cards have 16 x  28-pin individual RAM (TMS44800DZ 70-ns, for example) chips on them. Are there larger RAM chips in the same form factor with which these could be replaced?  The pictures of the 40M and 48M cards show the same general configuration and pin count.

Here's an auction with a good pair of images of what I'm talking about: http://www.ebay.com/itm/APPLE-8MB-POWERBOOK-DUO-210-230-270-280-8MB-MEMORY-/221737632350

RAM is voodoo to me, so if this is a stupid idea, please be gentle.  If it'd work, and people will pay hundreds for high capacity cards... seems like it might be a business opportunity for a blister-fingered solder-jockey.

 

[techknight]

You need to figure out the pinout of the RAM connection so you can pull out the additional addressing lines needed to expand a small cards capacity.

They may be in the developer note or a newer version of the guide to macintosh family hardware.

I need to do this for a pb540 someday.

 

[MinerAl]

So, the traces on the module itself are the limitation?

 

[techknight]

If your missing the required address lines to expand the RAM, then yes. 

 

[techknight]

Page 89:

http://powerbook.micahgartman.com/dev/dn_duo_system.pdf

Since those JAE connectors are long since gone except for maybe a few chinese parts brokers, You wont be making any new cards without pulling the connectors from old cards. 

Or, like you said, modding old cards and running the extra row address strobes. 

 

[MinerAl]

What would be involved in modding old cards?

 

[Elfen]

Some of the 16MB boards I have, have a blank other side, so soldering another 16MB to the second side would bring it to 32MB.

And the 190/5300 RAM cards I are only partially filled with RAM, 1 row filled with 3 empty ones. These would be easy to upgrade if one can find the right chips to put in.

 

[Bunsen]

Since those JAE connectors are long since gone except for maybe a few chinese parts brokers, You wont be making any new cards without pulling the connectors from old cards.

What's wrong with Chinese/ebay parts brokers?  It's not like anyone is going to be building 10,000 of these.  A dozen or so would be enough for a test run.

 

[Trash80toHP_Mini]

Populating unimplemented pads to double(normally) any given DRAM Card's capacity would work fine AFAIK. Apple's 4/8MB DRAM PCBs are likely not easily converted to higher capacities. CAS and RAS line availability is the limiting factor for converting low capacity cards and designing higher capacity cards.

Duo System DevNote: C H A P T E R 6 - DRAM Expansion Cards:

When designing a DRAM expansion card, you should include logic for address and
control circuitry, since all signals required are available at the expansion connector. There
are ten DRAM address bits (FRA0:9), and 32 data lines. Data bits 24 through 31 are
buffered (DDATA[31:24]). Other data (Data 23:0) is not buffered.

Five Banks available on the Memory Card:

The 4MB expansion card houses its DRAMs in banks two (2) and three (3). Banks four (4)
and five (5) are empty. The eight-megabyte expansion card uses all four memory banks.
Banks zero through three (0-3) will support only two megabytes of DRAM each. Banks
four (4) and five (5) will support eight megabytes per bank, providing 16-Mbit DRAMs
are used. If banks two through five (2-5) are fully populated, it is possible to design an
expansion card that provides a total capacity of 24 MB. (See Figure 6-2.)
Using a different packaging technique, it is theoretically possible to expand memory
capacity up to 32 MB. Although the Apple DRAM expansion card is physically
constrained to four banks, appropriate column address and row address strobe signals
are provided to support a fifth bank. Using a special IC packaging technique, such as
tape automated bonding (TAB), an expansion card using the fifth bank could provide
an additional 8 MB of DRAM, for a total memory capacity of 3 2MB (28 MB on the
expansion card, and 4 MB on the main logic board). This configuration has not been
tested or approved by Apple Computer.

Possible Complicating issues:

Buffering of Data bits 24-31

ROM limitation nastiness

Upside:

JAE Connector required is a thru hole component, readily available and easily harvested for reuse.

Modem/Power Card bay and PCB plane is contiguous with DRAM Card bay so a combo Memory/Power Card PCB is doable.

Additional surface area for custom PCB consisting of available Power Card surface area could be very handy. [}] ]'>

JAE Connector and Power Buttons on utterly useless 14.4 Modems are eminently available for harvest.

JAE Board interconnect needed for additional support/alignment integrity.

Modem Connector has its own power budget and connections, ADB hack optional! [] ]'>

WAG:

While Memory is ROM limited as in 1400 and the Compact Mac Series, any available Addressing CAS/RAS signals from Banks 1 and 2 a/o the Memory Controller itself on the Logic Board might be hijacked by patching to trace cut contacts on the Modem/Power Card's JAE board interconnect.

The equivalent of Compact Virtual on the 4MB limited SE could allow for additional DRAM to be used as Silicon Disk. mapped to unimplemented.

Question:

Might an Init/Driver be developed to allow the memory space unused in Banks 1 & 2 on the MoBo to be implemented in DRAM ICs in the Modem Bay cubic?