Pyramid 4MB RAM for A500 -- all loose

[Eamoe]

Hi folks!

Got myself this Pyramid Eureka 4MB RAM expansion board for the A500. It can be seen on this URL:

Pyramid Eureka 4MB 1994

although mine is the 1992 version (Amiga Hardware Database pics show the 1994 version). Here, you can D/L a picture of mine:

Pyramid Eureka 4MB 1992

It adds 4MB RAM by using an adapter that sits on the Gary slot. The package arrived with these loose wires.

My question: do you guys know what are the correct connexions? What wire should go where, etc? Also, the Gary adapter has a brown wire with no plug ending. Is it meant to be soldered some place on the mobo?

I'm a little afraid of making a wrong connexion and frying up a component...

Any help appreciated!

[Zetr0]

@Eamoe

Hello there, lets have a look,

its very similar to the BaseBoard


Gary Adapter Board

JP1 Header -

3 Pins = use three pin header cable to Main Board JP1
(note that postion 1 is marked on both boards)


JP2 Header -

2 Pins = On Of "rocker" switch


JP4 - Header (wire)

1 wire = Assumption time

Obviously this goes to the A500 motherboard, exactly where wil be dependent on version of the motherboard you are using.

My belief is that this *wire* (when using an A500 Rev 6a) will allow the motherboard to address the first 512KB of RAM as extra CHIP ram

As to where this needs to be attached, I will have to have a look.

my instincs lead me to believe that this will likely solder to JP3, and the need to cut and bridge JP2, but I Would suggest double checking this first.


Either way this device will work without the wire soldered to anything.



best of luck and take lots of pics, I will hit up some more specs to see what other A500 mobo versions might need different configs.

[Eamoe]

Wow, great description you put down here! Thanks. You definitely KNOW your hardware.

As I already made the Chip RAM mod on my REV 6A, does this mean that I could possibly get 1.5MB Chip RAM in total by soldering this brown wire to the right place?

Anyway, I will install the expansion when I get the chance and feedback with pics. Also thought about submitting pictures of this adapter to http://amiga.resource.cx, because there are none at present.

[Jope]

Unless you have a 2MB Agnus, then no.

[Zetr0]

@Eamoe

Unfortunately my friend, the 8372A Agnus thats installed in your A500 Rev6a itwill only address (see) upto 1MB of chip ram.

The 1MB chip mod on the Rev 6a asserts the trap door memory within the CHIP range and allows the Angus to see it (using JP2).

I also believe that to use this board properly you will need to re-connect JP7A on the Rev6a motherboard.

Anyway I have done some quick research and the following is what I would do -

Caution: This is just my 40 minute research, I am human and I do error so please check some schematics if you are not sure.


Gary Adapter -

• JP4 (Wire) - Connects to JP2 [pad 2] center pin

  
  This is my assumption based on the BaseBoard manual and A500 schematics.
  The signal picked up here (JP2) is used for Muliplexing by the trapdoor card to give you the extra 512KB of memory for chip (giving you 1MB total)
  Make sure that you bridge Pads 2 and 3 on JP2
  

*Multiplexing allows the card to address more than just 1MB of ram on the trapdoor port, when the Agnus asserts address 20 (JP2) this allows the adapter card change banks while still aserting or ignoring the EXRAM signal and hence fooling the GARY (Gate ARraY) chip

Using the A500 Rev6a mod, we disconnect the EXRAM signal to GARY (JP7A) allowing the trapdoor ram (or extra 512KB ram populted on the motherboard) to be used as CHIP, alas this limits this to an extra 512KB as you need the EXRAM signal to address more memory.

A500 Motherboard Rev6a

• JP3 (Between U34 and U35 *bottom right of motherboard*)

  
  *if 1MB is installed on the motherboard
  Bridge / jumper pins 1 & 2 (top two pads)
  Cut track between Pins 3 & 4 (bottom two pads)
  

• JP7A (EXRAM Signal)

  
  Jumper pin 2 and 3 (factory default)
  

This reassertment of EXRAM (JP7A) wont change the Agnus from seeing 1MB of chip just route that to trapdoor memory. Ideally the extra 512KB of ram on the motherboard should be removed and placed on the trapdoor card.

I am unsure if you will experience problems with the memory still populated on the motherboard, I would suspect it would shadow whats stored on the first 512KB of extra ram (not chip) of the trapdoor card.

However it is likely it will contain garbled data, so removal is recommended, you could give it a go, but if you do experience problems, then I would suggest removing the extra 512KB of RAM from the motherboard.

Good luck, remember to do your own research to see if this all makes a little sense, above all things though.... TAKE PICTURES =D

[Eamoe]

My my, Zetr0, looks like you could build an Amiga from bottom up! Thanks for all the details, that's very cool, I love it! Haven't understood everything about Multiplexing, although I think I see the implications of it.

OK looks like I've messed something up. Of course I wil not get 1.5MB, because if I put this adapter in my A500, I'll have to PULL OUT and REPLACE the A501 installed in the trap door. I haven't populated the empty RAM slot on the motherboard, but I would very much like to do so. Which leads me to the next question.

I do have the 8375 Agnus and Super Denise installed. Knowing this, is there a method to effectively install and use 2MB Chip RAM on the A500?

[Zetr0]

@Eamoe

LOL not sure about building one from the ground up... maybe I could start a group therapy session for those that read too many schematics found on Aminet ?

I will call it the Aminet Adiction Schematic Society or AaSS for short =)

OR perhaps Amiga Schematic Addiction Society - ASAS ? hmmm sounds cool


anyway - back to your question.

can you have 2MB chip using an 8375 318064-xx series chip on an A500 Rev6 a motherboard?...

Answer = YES

but if the Angus is an 8375 3190544-xx then NO this agnus is PIN incompatible with the A500 rev6a motherboard and will likely go bang and damage the agnus and or the machine, so MAKE SURE the one you have is pin-compatible (thats 8375-318064-xx)

difficulty is a little high....

1. you will need to confirm that the 2MB (Super obese) Agnus works with the A500 rev 6a motherboard.

2. you will need to build a Multiplexing circit for both Address 20 and 21 of the Agnus, this will need a couple of fast 74x logic IC's,
some fooling of the GARY chip
and a 1.5 MB plumbed into the trapdoor*

*by adding the 512KB on the motherboard its seen as trapdoor ram*
unfortunately we have less controll with it populated on the motherboard as we have to decouple the EXRAM signal

Now, you could avoid all that lot of chip piggy backing and jump-wire all over the place by using a MiniMega and doing the 1MB CHIP hack to the motherboard - this would give you 2MB chip, although your trap-door port will be useless as you would have to create an extra circuit to multiplex EXRAM with the Multimega and with whats on the trapdoor port.

Multiplexing, simply put is routing a signal location depending on 2 or more signal inputs

Its a lot of work and somtehing I have planned over the summer, I m confident I can get 2MB chip by building my own adapter, I am less confident this could work with an existing adapter like the 4mb pyrimid or 4mb baseboard.

It would be easier to add FAST ram to the CPU port than via the trapdoor connector after modifying it for 2MB chip

[Eamoe]

OK,

1. what about 318069-16 ?

2. my soldering abilities haven't been tested further than modding A501's and A601's, replacing ready-to-leak, fading accumulators with CR2035 cells. As you'd think, I believe my skills to BUILD A MULTIPLEXING CIRCUIT are way less than sufficient a the moment. I don't even have a clue what's a "fast 74x logic integrated circuit"! Is this a chip you have to burn? Like EEPROM or so?

I don't think I'm the kind of guy to fool Gary... I think rather Gary would easily fool me!

Unfortunately I think I'll leave this 2MB Chip fantasy aside for the moment . Way too advanced for an amateur like me. But I'd be really interested to know how this turns out for you.

Hadn't heard of the MiniMega yet. Interesting. Except I'd like to add Fast RAM too, so I'll also leave this.

Will install the Pyramid Eureka 4MB card, though (thanks for directions, will double check), and will keep you updated with pics.


EDIT: oh yeah, and LOL about the ASAS... schematics are like chinese to me, but I believe it can be consuming when you start to understand them

[Eamoe]

Hi guys,

so... I've been working on that A500 this afternoon. And I'm having no joy yet . Here's a few pics, like I promised.

1. full_mobo.jpg
My REV6A board

2. rev6a.jpg
REV6A: proof...

3. 512k_onboard.jpg
Onboard RAM: only the default 512KB installed

4. jp7a_bridge.jpg
I've bridged the JP7A

5. jp2_bridge_wire.jpg
I've bridged the JP2 (boy did I butcher this one, lifted and nearly destroyed pad 1)

6. jp3_default.jpg
Left JP3 untouched as I haven't any extra mobo RAM installed

7. 1mb_chip.jpg
The Pyramid Eureka adapter seems to give the Miggy the extra 512KB chip RAM, but only if I turn it off (using a rocker switch plugged on JP2 of the RAM-populated board). If I turn it on, the Amiga won't boot, i.e. nothing happens on power up, black screen, no floppy acivity -- nada.

Note: I use a 3.1 ROM. Could an older version work better? I don't see why, though.

Any input appreciated!

[Zetr0]

@Eamoe

hmmm interesting indeed.

I would suggest at this point to disconnect the wire jumper from JP2 and see what happens. (leave the bridge in place though)

[Eamoe]

@Zetr0

Right, I've made more testing (and finally destroyed pad 1 of JP2 -- happily this was an average condition mobo, now back to regular 1.3/no-HD use, modding is over with this one ).

Not getting much more success for now, I'll post detailed results when I get the chance.

[Eamoe]

I'm having no luck with this device.

I can't seem to squeeze more than 2MB out of this expansion card. Here is a schematics of the tests I've made, along with RAM results (all made on a rev. 1.5, KS 3.1):

http://www.1978virgule5.ch/eab/pyramidtests.jpg

There is one "ON" column and one "OFF" column. That corresponds to the state of the switch on the expansion board. Actually it's not really worth mentioning as I never get a boot when it's OFF (only black screen).

Could it be that there actually only are 2MB on the board? It's not likely, I've populated all the slots myself.

Could some DIPs be damaged? If yes, could only part of the memory work?

I've replaced the battery on the adapter and I don't get better results (not like I thought I would anyway...).

Ideas anyone?

[Zetr0]

@Eamoe

the one thing left to try is

Changing JP2 on the motherboard...

1. disconnet the bridge between pins 2 and 3 and connect (make) a bridge between pins 1 and 2.

This should allow Agnus to address (see) 1MB CHIP RAM

Attach the wire from the garry adapter to the bridged made on JP2 (pins 1 and 2) and see if that makes a difference with the internal jumpers on the board.

[Hewitson]

Originally Posted by Zetr0

LOL not sure about building one from the ground up... maybe I could start a group therapy session for those that read too many schematics found on Aminet ?

I will call it the Aminet Adiction Schematic Society or AaSS for short =)

OR perhaps Amiga Schematic Addiction Society - ASAS ? hmmm sounds cool

You're a funny bugger mate.. Laughed at quite a few of your posts :P

[rkauer]

AaSS? OK, I'm in.

I'm an addicted of Aminet schematics.