Seeking RAM for upgrade (US)

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bjchapm
Posts: 5
Joined: 23 Apr 2021, 23:05

Seeking RAM for upgrade (US)

Post by bjchapm »

Hello all,

I'm looking for the HM628128LP-7 to upgrade my Gigatron to 64K, but I haven't found it at Mouser or Digi-key. I have found listings on Ebay, but they are shipping from China. Does anyone have any suggestions for US sources? Also, should I be searching for something else instead?

Many thanks,

Ben
lb3361
Posts: 186
Joined: 17 Feb 2021, 23:07

Re: Seeking RAM for upgrade (US)

Post by lb3361 »

You can get a 128KB one and wire the extra address pins together.
https://www.digikey.com/en/products/det ... CN/4234576

Or you can bite the bullet and build a 128KB expansion which also gives you SPI port usable with a SD card. (see viewtopic.php?p=2788#p2788 and viewtopic.php?t=332). I have a couple spare PCBs and GALs. Just send me a private message if you want them. They just take space on my desk.
denjhang
Posts: 50
Joined: 02 May 2021, 01:25
Location: yuenan
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Re: Seeking RAM for upgrade (US)

Post by denjhang »

taobao.com has any chips I need, they are cheap and easy to get (of course there are many fakes). But apparently these chips are shipped from China.
bjchapm
Posts: 5
Joined: 23 Apr 2021, 23:05

Re: Seeking RAM for upgrade (US)

Post by bjchapm »

lb3361 wrote: 23 Mar 2022, 21:29 You can get a 128KB one and wire the extra address pins together.
https://www.digikey.com/en/products/det ... CN/4234576
Thank you for this suggestion! I did not realize this part was compatible. I've ordered two - they are $4.00 each (plus shipping) for anyone else who is looking.
lb3361 wrote: 23 Mar 2022, 21:29 Or you can bite the bullet and build a 128KB expansion which also gives you SPI port usable with a SD card. (see viewtopic.php?p=2788#p2788 and viewtopic.php?t=332). I have a couple spare PCBs and GALs. Just send me a private message if you want them. They just take space on my desk.
Thank you for the offer! I'm putting together a Pluggy Reloaded this weekend, which will get me some of this functionality.
at67
Posts: 479
Joined: 14 May 2018, 08:29

Re: Seeking RAM for upgrade (US)

Post by at67 »

bjchapm
Posts: 5
Joined: 23 Apr 2021, 23:05

Re: Seeking RAM for upgrade (US)

Post by bjchapm »

Thanks everyone! I am now running with 64K of RAM (interestingly ROM 5A says 128K; the RAM is a 128K, but I am sure that only 64K is available). Thanks to lb3361 - the recommended RAM from Digi-key worked perfectly. Also, thanks to at67 for the instructions on how to connect the chip.
bmwtcu
Posts: 111
Joined: 01 Nov 2018, 12:02

Re: Seeking RAM for upgrade (US)

Post by bmwtcu »

That 128k detection in ROMv5a is a known bug that has been addressed in DEVROM. I believe it was also fixed in SDROM (which is based on ROMv5a).
Sugarplum
Posts: 59
Joined: 30 Sep 2020, 22:19

Re: Seeking RAM for upgrade (US)

Post by Sugarplum »

lb3361 wrote: 23 Mar 2022, 21:29 You can get a 128KB one and wire the extra address pins together.
https://www.digikey.com/en/products/det ... CN/4234576

Or you can bite the bullet and build a 128KB expansion which also gives you SPI port usable with a SD card. (see viewtopic.php?p=2788#p2788 and viewtopic.php?t=332). I have a couple spare PCBs and GALs. Just send me a private message if you want them. They just take space on my desk.
Why tie extra address lines together? Why not just tie any extras to ground or put that on a PCB with a jumper on it to ground where the jumper can be removed as a breakout for the extra line if you need it in the future?
at67
Posts: 479
Joined: 14 May 2018, 08:29

Re: Seeking RAM for upgrade (US)

Post by at67 »

Sugarplum wrote: 31 Mar 2022, 00:00 Why tie extra address lines together? Why not just tie any extras to ground or put that on a PCB with a jumper on it to ground where the jumper can be removed as a breakout for the extra line if you need it in the future?
It's an assembly/soldering optimisation, just solder the pins together and you don't have to worry about extra wires, routing, etc. The RAM could care less which bytes your address lines access, as long as the address mapping is always consistent and always matches the 64k address space in a 1:1 fashion, (i.e. not a hash), then it will always work.

Unused address inputs must not float, tying them together is the easiest way to accomplish this.
Sugarplum
Posts: 59
Joined: 30 Sep 2020, 22:19

Re: Seeking RAM for upgrade (US)

Post by Sugarplum »

at67 wrote: 31 Mar 2022, 01:51 It's an assembly/soldering optimisation, just solder the pins together and you don't have to worry about extra wires, routing, etc. The RAM could care less which bytes your address lines access, as long as the address mapping is always consistent and always matches the 64k address space in a 1:1 fashion, (i.e. not a hash), then it will always work.

Unused address inputs must not float, tying them together is the easiest way to accomplish this.
I understood not floating. That is why I suggested a jumper to ground. And it can be removed to be used for later MMU upgrades.
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