A500 CHIP RAM ampliation.

[DyLucke]

Hi there, i've just finished soldering some sockets on the spare places for chipRam on my A500.

Everything went fine, till the moment i turned the computer on.
I've fit some dip chips from one spare A500 ram expansion but it seems it doesn't work.

I don't know if i need to do anything else aside fitting some chips there, or maybe the ones i've fitted are not compatible with the actual ones.

Maybe somebody could help?

The ones that came with the A500 are SIEMENS HYB514256B-70
The ones i've fut SAMSUNG KM44C256AP-8

Any ideas?

Thanks!

I've noticed something, it seems both type of chips are compatible, but extra RAM is displayed as fastram and NOT chipram.

What's wrong here?

[8bitbubsy]

You also need the decoupling capacitors below the chip rows. The brown chermaic ones. You can see that the on board-soldered ones do have them, but not on the pre-empty place. Not sure if this will fix the issue, but it sure is important.

[Shadowfire]

As 8bitbubsy said, solder in those bypass capacitors, or else you are going to subject yourself to a whole lot of unnecessary frustration.

The bypass caps will help system stability, but the chip/fast RAM dilemma is a function of your Fat Agnus IC, as well as jumper settings on the motherboard (earlier version motherboards actually needed a modification, but I'm assuming that if the board has blank sockets for the chips, then you only need to change a jumper or two).

[DyLucke]

well, this A500 is one of these "rare" ones that performed one 1mb Agnus, but over an A500+ board. You know, the last A500 to be produced, C= ran out of A500 boards so they assembled an A500 on a PLUS board.

The thing is, i don't have any capacitor to solder. Everything is set there, all the caps are in it's place.

Memory seems to work, but i guess these SIEMENS HYB514256B-70 i've socketed,
as the nomenclature says, it seems they're 256kb each, so it could happen that
the 1mb Agnus gets overrunned, and takes memory as non chip memory.

The fact is, I've tried to run the miggy with just the "supposed" ChipRam and no expansions, and wb came out with 500kb chip and 1mb other mem.

Could i be right here?
It seems i only need some 128k chips to socket and i'll have a full meg ChipRam?

[Zetr0]

@DyLucke

Holla my fellow hacking Amigo!

from your description you have an A500 Rev6a motherboard.

to get 1MB chip you will need to cut 2 tracks and solder two pads together -

I did a howto on www.amibay.com

at the moment the 2nd 512KB of ram is asserted as OTHER memory so you need to drop the location -

heres a plan


heres the motherboard locations



cutting JP2



soldering JP2




cutting JP7A



And that all there is to it =D


With a litte more work (and an appropriate Agnus), utilizing a simple IC and an A501 + 4 more 4x256 ram chips you can have 2MB chip ram

[DyLucke]

WOWOWOWOWOWOW This tuto is SUPERB!!!

Sadly, my mobo revision is 8A (PLUS), but it's not a PLUS miggy.
So i guess your tuto doesn't work for me. But thank you!!!!

As i said before, my A500, is one of the last ones to be produced, and it has all the chips from a
Rev6 A500, BUT on a PLUS motherboard.

Concretelly this one.
http://www.amiga-hardware.com/downlo...lusrev8amb.jpg

Ram is not Texas Instruments, in mine is SIEMENS HYB514256B-70
That's the only difference i can see.

One thing i don't understand is the fact that all the ram chips in my board, and also the board in the picture
have a 256xx nomenclature, that would mean 256k? Then 256k x4 it should be an entire Meg?
Then why is not showing an entire mega but just a half? ?????????????????
Does it come with an entire mega but LIMITED????????????

[8bitbubsy]

Originally Posted by DyLucke

As i said before, my A500, is one of the last ones to be produced, and it has all the chips from a
Rev6 A500, BUT on a PLUS motherboard.

Concretelly this one.
http://www.amiga-hardware.com/downlo...lusrev8amb.jpg

That's one rare baby, afaik!

[DyLucke]

Yes it is, i always heard about this model, but i thought it was just a rumour, even on the Wikipedia it's said as an speculation.

But the fact is, i've found one of these. It's quite peculiar, because as i said, it's a Rev6a A500 on a 8A PLUS motherboard. The main difference between this model and the PLUS, externally is the fact the LOGO has exactly the same aspect than in the PLUS model, but it just says Commodore Amiga 500. Leds are Green and Yellow, otherwise the
PLUS model where they're Red and Green, and caps lock light is red instead the usual green one.

[Zetr0]

Ahhhhhhhh I seee

well this is indeed some what obscure motherboard, I am not sure if you would want to modify it.... however if you do there is only ONE missing components

here is your revision of the A500+ motherboard (rev 8a)
Here is the full 1MB version of the rev 8a

I revised both boards and the only difference is a missing 74F139 (U32) from the 512KB board.

the 74F139 ic is a Decoder/Demultiplexer, exactly what you need to assert memory in and from the right locations =)


512KB A500+ Rev8a (your board)

U34 - 74F373N
U35 - 74F244PC
U32 - empty


1MB A500+ Rev8a (the full version)

U34 - 74F373 N
U35 - 74F244 PC
U32 - 74F139 PC <---- mr multiplexer


I would populate it and see what happened =D I am pretty sure that this asserts the memory address of the second bank of onboard ram into the chip pool.

I will have to hit some specs to confirm but I have just checked this with an existing A500+ (1MB version)

[DyLucke]

Well, it seems you're right Zetr0, i've located the components you said, and efectivelly, U32 is EMPTY.

So, the only thing i need is to fit one of these multiplexers in that place?

I wonder where could i get one. I've been a couple of times to the local
electronic parts store... And they told me they don't have 20pin chips anymore.

[Zetr0]

@DyLuke

you might get lucky salvaging from older components, however I think I have a few 74F139's hit me up your deetails on PM and I will post you one or too if I have enough spare =)

[DyLucke]

And, thus... Zetr0 saves the day... AGAIN...



One last question, master... I do trust completelly in your amigan skills.
However, what could happen to the board if we are wrong about this
multiplexer?

You know, it's somehow a rare model.
The only modifications i've done are just to add some sockets on the
empty spaces, so i will on the U32. No real modifications as you can figure.
But i'm sorta worried about if i could short the board if we fit a chip in
the wrong place?

Thanks again Zetr0

[DyLucke]

PM sent.

[Zetr0]

@DyLucke

looking at the A500+ schematics (which is what your board is) there wont be any problem with adding the 74F139.

I have just found a spare one, and you are welcome to try.

I would imagine in a worse case scenario it just wont work, I dont tihnk it would damage anything, after all, all the signals are there at the point on the board anyway.

imho, I would say plumb it in, in the unlikely event it doesn't work then simply remove the chip.


The multiplexer will assert the first 2MB into chip space (i really have to revise my $address locations I should know these off by heart now... bad zetty) just make sure you have a 2MB capable agnus.

8375 - 3190544-xx would be the component on the A500+

=)


WOW!!! yours is very interesting - it has a 2MB agnus (8375 - but the pin shifted 318069-xx... this is pinout comaptibe with the 8372a/b and 8371) also the 381069 - 8375 vairent is used in the A600 rev1.1 and higher as well the 318069 - 8372B of the A3000 too

very VERY interesting...

[Zetr0]

thinking about this more..

by adding the multiplexer you will turn this unit into a full A500+ this means that A500 specific adapters (like the mini-mega 2mbchip adapter will most likely NOT work, also the 4mb baseboard may has some issues)

however the mini-mega chip upgrade should matter as you can just plum in a 1MB A501+ or vairent. there are some at vesalia for about 10 euros including postage (thanks spock!).

I have to admit I am really excited about this hackery, my friend Thecorfiot and I induging into some A500 mass hacking too =D

[Zetr0]

Oooo before I forget, TC reminded me of the two solder pad bridges on U32

these will need to be removed, so pending on the socket you put in, you might have to change that.

*see attched image

[DyLucke]

Let's see, i feel my brain like ¿overflowed? now. XD LOL. (i don't know how to conjugate that fffxx verb)

The Agnus fitted is indeed an 8375 exactly an 8375 390544-01
I do think it's a 1mb Agnus, SYSInfo currently states that.

So as a resume, if i do cut these two lines on U32, and THEN i fit the multiplexer...

What will happen? Will i have 1mb ChipRam?
Or by some chance will i have ChipRam enabled for 2meg, and then i would only need to fit
chips on all the sockets there?

Sorry about all this asking, but i'm more
familiarized to the A1200 than the A500.

[Zetr0]

@DyLyucke

the picture you linked to here

the board in the picure an 8375 318069-xx Agnus onboard - and can address 2MB

the 8375 3190544-xx Agnus is on the A500+ and can address 2M8

the 8375 Agnii comes in two flavours, 318xxx series and 319xxx series, both these chips address upto 2MB chip however the 319xxx series is pin incompatible with 318 all other agnus series chips.

the 8375 318xx is pin compatible with the 8372 A/B and 8371, although its not all that likey the addressing lines are availbe to have upto 2MB chip on an A500 rev 3. (where the 8371 and 8370 are found)

I did an investigation into the Amiga Agnus a little while back -here -

this goes into more detail about the Agnus =)


Now, please dont cut the lines on u32, If you cut the tracks you will remove the posibilites of these signals being used by the multiplexer. Instead remove the solder bridge with the soldering iron, some flux and some solder wick or other appropriate method.

And theres no need to be sorry for asking questions, after all how else do we learn?

=)

[DyLucke]

Erm yes yes, what i meant by "cutting the lines" was exactly remove the solder bridge.

So then and again Zetr0, remove solder and then fit the multiplexer. That would enable
the 2mb chipram, so i would only need to fit the corresponding ram chips to have a fully
2mb Chip A500+. Right?

No work needed on JP3? I've seen some differences between revisions, and the fully
populated ones.

http://www.amiga-hardware.com/downlo...lusrev8amb.jpg

http://amiga.resource.cx/photos/phot...res=hi&lang=en


Then i assume as you said before, my trapdoor Ram expansion will be after this useless.
For what i know, i can't address more than 2mb between chipram and trapdoor expansion. To address more memory i do need FastRam on the side connector. Am i right?


Thanks for all the inconvenience friend.

[Zetr0]

@Dylucke

okay, I just grabbed rkauers defective A500+ motherboard that I am stripping for him.

(its a surprise so shhh dont tell anyone lol), infact I have been meaning to do this a good couple of weeks ago.


right so... pics --


Heres the mobo, although defective they are beautifull



After removing the F139 heres what U32 should look like without the bridges

(click to enlarge)


And heres U32 from another angle

(click to enlarge)


Here is how JP3 should look

(click to enlarge)



And lastly the Agus


-------------


After performing this modification, you will turn your board back into an A500+ my bet is that this will allow for 2MB Chip

ie - another 1MB via an A501+ or simlar 1MB card attached to the trapdoor slot

Essentially what you have is an A500+ motherboard made to work as an A500, with the planned wor we are just changing the modifications done to it at the factory.


I am a little curious - this pic is this of your board or a stock photo?