A500 trapdoor memory: free design

[PeteAU]

Originally Posted by kinmami

Hello Guys
First of all congratulation for the project development and shared time.
I have written a BOM list draft where the highlighted items need your confirmation regarding the nominal value of some components and the suggested length of socket pins.
In attachment.
Thanks

IMO C3, C6, C23, C11 are not needed - I run happily without them. Pads are there just in-case.

C13/C19 are alternatives. Only one of them is needed. Any value >=4.7uF will be fine. 2.2uF ceramic also good.

C20 is also a bit optional - it might be a good idea on a power hungry expanded machine. Your choice looks fine.

U6 - Should be "F", "HCT" or "AC" only. Your choice is CMOS and probably won't work. Back in the 1980's, AMRAM boards used 74LS and it still worked.

I don't think DIP48 socket will fit. Maybe DIP24 twice will. Most people use 2.54mm pin strips instead.

For the pins to mobo: https://www.aliexpress.com/item/Sing...535232012.html

[Mq]

I confirm C3, C6, C23, C11 are not needed.
I don't use C12 and C19 too. I'm soldering 100nF as C13 for decoupling RTC. I don't solder any big tantalum or ceramic too, but I'm soldering electrolitic capacitor C20 instead of that. I use electrolitic 100uF - I'm sure it's more than enought, but I have a lot of them so I use them - I think it could be 10uF too.

Second matter: Yesterday I built provisional DRAM chips tester.
I attach pictures.
I don't have SOJ42 sockets, but I use SOJ40 sockets from old VGA cards.
I cut one side of sockets so DRAMs can be fitted inside them.
On the last two pins of DRAM there is +5V and GND duplicated with pins on the opposite side. I measured that they are internally connected in chips, so I left them not connected and it works.
In sum I solder 4 sockets, C1, C4, C7, C22, U6, C16 and wirings - that's all.

Tester works stable, and now I can check DRAM chips before I solder them into target extension. I did this device, because I had issues with bad DRAM chips previously, and now I have to build 5 more extensions, so it will be easier.

[kinmami]

Thanks.
Revised list, based on your suggestion and Digikey P/N availability, in attachment.

[Mathesar]

Originally Posted by Mq666

It looks like extension randomly will work or will not work with the HCT. My friend tested 74HCT74, 74F74 and 74ACT74, and Gary adapter don't work with HCT for him. With ACT and F it works fine.
I think 74HCT is risky, so I solder 74F only, and I recomend to use F series, because F series has shorten propagation delay.

I used all HCT logic in my build without any problems. The '139, '00 and '74 are certainly not timing critical.
The two '00 chips that generate the new regen and ramen signals "might" be critical. Otoh, like Pete said; the original amram boards used LS logic...

Where do you source your logic and RAM chips? I source my logic from a dutch webshop and the ram from salvaged simm sticks. I did not use any eBay parts.

[alenppc]

I got logic chips from Digikey and Mouser and ram from AliExpress following PeteAU's recommendation.

[katarakt]

Originally Posted by kinmami

Thanks.
Revised list, based on your suggestion and Digikey P/N availability, in attachment.

Hey, great !! Thanks

Maybe PeteAU can take a look again and give his OK? Some changes in b6 regarding the bom ?

@Mq666
I had the same idea, if i'm going to start the project maybe i'll order the right sockets. Sockets matching perfectly

[Mq]

I bought all my logic in F series from local internet shop, which is good and tested many times, and I haven't any issues with them. HCT series checked by my friend was from old stock which he has.
All DRAM chips I have from old SIMMs. I checked them all in old PC before I desoldered them.

But now it is something interesting.
With my tester I checked some DRAMs today. I have NPN chips which all works perfectly. I have 16pcs of them.
But today I desolder some LGS chips from another SIMM - same as I have in my very first extension which had issues described in the beginning of my "RAM extension adventure".
And I have same issues with them! Three chips works perfectly in each order. But four chips give me various issues in various chips order. In one order I have yellow screen, in other order I have random errors, in yet another order last chip RAM is unvisible. I exchange chips between them, and I found one order in which everything works perfectly.
I think this chips has some critical parameter value which may cause incompatibility.

I did a lot of confusion in this forum thread. I'm sorry if someone is annoyed by this.
Maybe none of you will ever reproduce my problems, because maybe they only concern specific chips, from a particular manufacturer and from a specific series.

[katarakt]

Thanks for info about the LGS chips . Maybe you can post a detailed picture with this LGS chip.

[PeteAU]

Originally Posted by Mq666

I don't use C12 and C19 too. I'm soldering 100nF as C13 for decoupling RTC. I don't solder any big tantalum or ceramic too,.

Just 100nF for the RTC isn't enough. When you flick the power switch off, the voltage on the RTC briefly dips way down (around 2V) and it will sometimes forget time/date. I watched it on a scope. It really needs a few uF on C13 or C12.
Alternatively you could make R5 smaller to let the battery drive harder during these events.

Originally Posted by kinmami

Thanks.
Revised list, based on your suggestion and Digikey P/N availability, in attachment.

-For D1, BAT721C is better due to lower Vf.
-IMO I think you should delete C14,C15,Y1 and recommend only RTC62421/RTC62423/RTC72421/RTC72423.
-And of course R6 & LED are optional.

Let us know if "A412-ND" fits, I wonder if it will.

[kinmami]

Originally Posted by PeteAU

Just 100nF for the RTC isn't enough. When you flick the power switch off, the voltage on the RTC briefly dips way down (around 2V) and it will sometimes forget time/date. I watched it on a scope. It really needs a few uF on C13 or C12.
Alternatively you could make R5 smaller to let the battery drive harder during these events.



-For D1, BAT721C is better due to lower Vf.
-IMO I think you should delete C14,C15,Y1 and recommend only RTC62421/RTC62423/RTC72421/RTC72423.
-And of course R6 & LED are optional.

Let us know if "A412-ND" fits, I wonder if it will.

updated.
pls review and let us know.
thanks

[kinmami]

additional update in attachment.
waiting your kind comment.
thanks

[PeteAU]

Originally Posted by kinmami

additional update in attachment.
waiting your kind comment.
thanks

My only comment is that's too expensive. The sockets, connectors and RTC could be much cheaper elsewhere.

P.S. I have posted rev B.6 (1st page). This should be finished for 2017

[kinmami]

Originally Posted by PeteAU

My only comment is that's too expensive. The sockets, connectors and RTC could be much cheaper elsewhere.

P.S. I have posted rev B.6 (1st page). This should be finished for 2017

unit prices & P/N from digikey are just for reference.

[Mq]

Originally Posted by PeteAU

Just 100nF for the RTC isn't enough. When you flick the power switch off, the voltage on the RTC briefly dips way down (around 2V) and it will sometimes forget time/date. I watched it on a scope. It really needs a few uF on C13 or C12.

PeteAU, You're right. My mistake. I was wrong because I misread the schematic. I have mistaken a group of capacitors C10, C11, C20 with a group C12, C13, C19. I was thinking that electrolytic C20 is doubled with tantalum we talking about. But not. Ok, now I corrected extensions I have, by solder 10uF as C12. Thank You for the tip.

Katarakt: My problematic LGS chips are same as on that picture:
picture
It's actually not my picture, but I have same chips, but my are 9805, 9806, 9807.
I have eight chips like that. I was coupling one set of four chips working properly together and one set of three chips working properly together too. But there is only one working combination. Each other combination had issues. Eighth chip don't works at all, because one day he works great and another day he causes yellow screen. Rest of chips must be in one order that I found working. Exchanging chips to other order causes random errors in last fourth chip as I described in previous posts in this thread.

I don't have more LGS chips, so I leave it as it is.
Any other chips from other manufacturers working great and without any issues for me.

[katarakt]

Thanks Mq666 and PeteAU for rev. b6

[Mathesar]

I have ordered a set of 5 red b6 PCB's today at Elecrow to test.
Do they run faster in red?

[hooverphonique]

Originally Posted by Mathesar

Do they run faster in red?

For sure they do, just like when you paint the brake calipers red on your car - adds 25kW

[demolition]

Originally Posted by hooverphonique

For sure they do, just like when you paint the brake calipers red on your car - adds 25kW

If red brake calipers make the car run faster, does that mean they are no good seeing as they are supposed to stop the car?

[springycz]

Hello, I have a couple of questions regarding this chip ram expansion.
Do I need the Gary adapter in order to expand chip ram to 2MB on A500 plus?
I found several simms containing 42 pin chips that are supposedly 1Mx16, namely:
HY4181164B, KM416C1200AJ-7, TMS418160DZ, AS4C1M165S-60JC, and HM5118160-AJ7
Will anyone of these work with this expansion?
Thank you for your answers!

[demolition]

Gary adapter cannot be used in A500+. To upgrade an A500+ to 2MB chip, you only need the trapdoor board with 1MB installed and no other boards or wires.