A500 trapdoor memory: free design

[PeteAU]

Originally Posted by springycz

Hello, I have an additional question regarding this trapdoor memory.
I have an A500 with rev. 8A motherboard inside. It´s basically the same motherboard like in A500+s though it has just 0.5MB of chip RAM and a KS1.3 installed, and it´s also missing the RTC circuitry. The rest of chips are the same like on the A500+ motherboard including ECS Denise and Agnus.
I just found Kippers2k thread where he managed to convert this board into A500+ (without RTC which he had on trapdoor expansion).
What I would like to ask is whether this particular trapdoor memory is able to provide 1.5MB of chip RAM so I do not need to install the additional 0.5MB of chip RAM on the motherboard. It would be nice if it works as an all in one the onboard 0.5MB and the additional 1MB trapdoor ram.

That wouldn't work very well thru the trapdoor. You need to add the 0.5MB onboard DIP chips and turn it into a realistic A500+. Then you can use this board to add another 1.0 MB (2.0 MB total CHIP RAM).

Originally Posted by katarakt

About the RTC 62421 ...does it matter if 62421A or 62421B ? What is the difference ?

And about 74F74 ...i found some but what is the difference between 74F74 and 74F74SJ ?? Both ok?

A is more accurate than B or some small difference. I think there are no B around.

Sounds fine, any suffixes on the end are ok, as long as it's the SMD/SOIC-14 type and not DIP.

[springycz]

Ok, thank you for your reply. I will order the MC74F139N and DRAMs and probably will give up on the onboard RTC. It´s probably easier to have it on the expansion, right?

[PeteAU]

Originally Posted by springycz

Ok, thank you for your reply. I will order the MC74F139N and DRAMs and probably will give up on the onboard RTC. It´s probably easier to have it on the expansion, right?

If you convert your 8A to a 1MB A500+ then 74F139 and Gary Adapter are not used!
Edit: I miss-understood, you're putting a 139 on the motherboard for conversion.

Yes I reckon it's easier to put the RTC on expansion.

[Mq]

N suffix is DIL package.

Edit: I miss-understood too. N suffix is DIL, but it's ok if You use it on 8A motherboard.

[katarakt]

Thanks PeteAU, i'm ordering some ICs at the moment. For the new gary adapter...the joints in the middle of the DIP sockets should be cut i think?

[katarakt]

I took a deeper look into the b6 top layer. Now, i have a few questions ...if i don't want to cut traces for different settings between jumper-vias with a knife on the boards (because we have jumper for that!), what traces should be deleted in the layout? I've marked the groups from 1-5 in the picture i attached.

1: clock and exram is connected - both red traces can be deleted?
2: red trace delete?
3: red trace delete?
4: 512-more ...red trace delete?
5: sure, all four must be deleted

[E-Penguin]

Is it possible to switch trapdoor memory between chip and slow/ranger on an A500+? I've seen comments about it but no details on how.

[katarakt]

Maybe the guys here are on vacation?

Regarding the bom b5 ...if C3, C6, C11, C14, C15, C23 are "not connected" ...so not needed...why are these still in the layout b6 ?

[Mathesar]

Originally Posted by katarakt

Maybe the guys here are on vacation?

Regarding the bom b5 ...if C3, C6, C11, C14, C15, C23 are "not connected" ...so not needed...why are these still in the layout b6 ?

Hi, did you make any progress on the memory expansion board?

About the caps; C14 and C15 are only needed if you have an RTC which needs an external crystal. C11 is an extra decoupling cap that is not really needed as there is already plenty of decoupling present.
C3,C11 and C23 are decoupling caps for the middle pins of RAM chips.
Not strictly needed but it doesn't hurt to mount them.

[Mathesar]

Originally Posted by katarakt

I took a deeper look into the b6 top layer. Now, i have a few questions ...if i don't want to cut traces for different settings between jumper-vias with a knife on the boards (because we have jumper for that!), what traces should be deleted in the layout? I've marked the groups from 1-5 in the picture i attached.

1: clock and exram is connected - both red traces can be deleted?
2: red trace delete?
3: red trace delete?
4: 512-more ...red trace delete?
5: sure, all four must be deleted

1: no, leave them where they are. You need them to enable the $C00000 RAM and the RTC (if mounted)
2: not if you are building an 1.0MB chip + 1.5MB slow system.
3: see 2
4: see 2
5: Yes! And you need to mount the 74F139 (or HCT139) to use the Gary adapter.

I hope that clears things up for you. Sorry for the late replies

[Mathesar]

BTW, I build up a complete rev B6 board and Gary adapter.
I am using Texas Instruments 70ns DRAM chips and only HCT logic.
So only 74HCT00, 74HCT74 and 74HCT139.

I am happy to report that b6 works like a charm. It passes all the memory tests and keeps the time perfectly.

QC passed

Some pictures of my nice restomodded rev 5 A500 from 1987 :

[Mathesar]

Originally Posted by E-Penguin

Is it possible to switch trapdoor memory between chip and slow/ranger on an A500+? I've seen comments about it but no details on how.

I *think* you can only use the board as chipram on the A500+.
But maybe other can comment on this one.

[katarakt]

Thank you very much mathesaer!! Sourcing the parts and find out what parts are needed and what not costs so much time because everything is not that easy.

Now i know that everything can be done with HCT parts, thanks for info but i've ordered 74F00, 74F74 and 74F139 a week ago together with some other parts. Sourcing 74F00, 74F74 and 74F139 took very much time, very rare and expensve parts. I'll replace these for future projects in my bom with HCT ones ...easy to find, and cheap

Also, i'm sourcing all wire-cable and connectors ...these are not in the bom, also cost time for me. Must measure how long they have to be.

Ok...i'll add C3,C11 and C23 to my bom...better it is and they are very cheap

About exram jumper ...can you take a look on the pic? These two green circled ones looks like they are the same if i follow the traces ???

Oh, i'm going to build it with 512k chip (because standard A500, 1mb chip not possible) and maximum on slow.

Thanks a lot for new b6 pictures,well done mathesar

[Mq]

Yes, that two green circled ones jumpers are the same. It's your choice which of them you want to use, but probably you will not use them at all:-)
If you build only slow ram extension, then you don't need to use wire from gary adapter to JP2 on motherboard, you don't need to cut JP2, and you must solder only three ram chips on the top of extension board.

Hi, my rev b5 boards finally arived from elecrow (it takes up to 90 days for customs to process a package here) and I got all the parts to build it.
I have a problem so I will describe what i did and maybe someone see something I don't. My amiga is a REV 5 board with a 8372a agnus chip instaled.

First board I built with only one ram chip and the capacitor, i played some games and it worked fine. Then I soldered the remaining memory chips and capacitors as well as the ls139, later on I soldered all the jumper pins for test purposes. I built the gary adapter with all the chips, I could not get the 74ls74 so I used 74hct74 instead, the 00 and 139 are ls chips. This board now give me green screen every time.

Then I built another board with only one memory chip, both capacitors, the jumpers for testing and the 74ls139. I've put the jumpers so I could test with and without the gary adapter and this one always works fine, I can put the CAS jumpers and the JP2 on the mother board and get the 1 meg chip ram or I can remove the CAS jumpers, put the gary adapter in, plug the JP2 to the gary and the SEL wires to get the 1 meg chip ram as well.

- Can the mixed ls and hct chips be a problem?
- I am getting crazy testing connections on the board (can't stand the fluke beep anymore), I don't have the most experienced smd soldering skills but I think I have all the memory soldered fine, can one bad pin give green screen?
- Best course of action would be slowly populate the second board with more ram chips or to built another gary adapter for testing?

Sory for the long text, sory if my english is a little rusty, thank you guys for the help!

edit: forgot the picture

[Mq]

The problem could be with bad soldering. If You haven't good experience with smd chips soldering, then try to look carefully to every soldered pin under the magnifying glass. From my experience I know what mistakes I generally did sometimes. First: if You use a lot of flux then sometimes pin and pad are not soldered at all, but without magnifying glass it looks like it's good soldering. Second: if You use to less flux, then sometimes there can be short circuit between pins. Sometimes it can be very thin short circuit, so You can see it under magnifying glass.
Afterall I don't know if LS ICs could be used. We discused here about various series of TTL's. Mathesar and PeteAU said HCT can be used and they has no problems with them, but my friend had issues with HCT and he use F to fix it. I'm using F only and have no issues with them. So I don't know what to think about it...

[katarakt]

Originally Posted by Mq666

Yes, that two green circled ones jumpers are the same. It's your choice which of them you want to use, but probably you will not use them at all:-)
If you build only slow ram extension, then you don't need to use wire from gary adapter to JP2 on motherboard, you don't need to cut JP2, and you must solder only three ram chips on the top of extension board.

Thanks for info. Ok, if the green circled ones are the same ihave to cut/delete the exram-jumper-trace if i want to add a switch, correct? Sure, probably i won't use them but, in the past, it is allways good if you can switch all these things off.

I want different combinations, some for amiga with 1meg agnus, some without and maybe for some A500+ so all must be possible and hopefully with jumpers only without cutting traces...so....basically, can i do it like this with deleted traces between jumper and use jumper for all ?

Originally Posted by Mq666

The problem could be with bad soldering. If You haven't good experience with smd chips soldering, then try to look carefully to every soldered pin under the magnifying glass. From my experience I know what mistakes I generally did sometimes. First: if You use a lot of flux then sometimes pin and pad are not soldered at all, but without magnifying glass it looks like it's good soldering. Second: if You use to less flux, then sometimes there can be short circuit between pins. Sometimes it can be very thin short circuit, so You can see it under magnifying glass.
Afterall I don't know if LS ICs could be used. We discused here about various series of TTL's. Mathesar and PeteAU said HCT can be used and they has no problems with them, but my friend had issues with HCT and he use F to fix it. I'm using F only and have no issues with them. So I don't know what to think about it...

I should get one of those usb microscopes, my small magnifying glass doesn't help much, but if this can be bad soldering I will definitely check it all again.

SMD 74Fs are just about impossible to find here, i can order ls and hct only. Maybe I can try get all hct chips and build another gary adapter.

thanks for the help.

[Mq]

I have very good eyesight. However, I use 6x magnifying glass after smd soldering, so I think it's worth to use it because I saw something more with that, and sometimes I catch small mistakes.

I'm using F ICs because in my local internet store they have a lot of them and price is the same as LS and HCT in this store.

[PeteAU]

Originally Posted by instorm

Attachment 55278

Hi, my rev b5 boards finally arived from elecrow (it takes up to 90 days for customs to process a package here) and I got all the parts to build it.
I have a problem so I will describe what i did and maybe someone see something I don't. My amiga is a REV 5 board with a 8372a agnus chip instaled.

First board I built with only one ram chip and the capacitor, i played some games and it worked fine. Then I soldered the remaining memory chips and capacitors as well as the ls139, later on I soldered all the jumper pins for test purposes. I built the gary adapter with all the chips, I could not get the 74ls74 so I used 74hct74 instead, the 00 and 139 are ls chips. This board now give me green screen every time.

Then I built another board with only one memory chip, both capacitors, the jumpers for testing and the 74ls139. I've put the jumpers so I could test with and without the gary adapter and this one always works fine, I can put the CAS jumpers and the JP2 on the mother board and get the 1 meg chip ram or I can remove the CAS jumpers, put the gary adapter in, plug the JP2 to the gary and the SEL wires to get the 1 meg chip ram as well.

- Can the mixed ls and hct chips be a problem?
- I am getting crazy testing connections on the board (can't stand the fluke beep anymore), I don't have the most experienced smd soldering skills but I think I have all the memory soldered fine, can one bad pin give green screen?
- Best course of action would be slowly populate the second board with more ram chips or to built another gary adapter for testing?

Sory for the long text, sory if my english is a little rusty, thank you guys for the help!

edit: forgot the picture

Dumb question: Did you close RAM enable jumper? It's needed for any slow RAM. Have you tried 0.5 CHIP + 1.0MB SLOW settings on gary adapter? You could also try swapping the two SEL wires.