vLA81 ULA and 16/32K internal RAM

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Lardo Boffin
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vLA81 ULA and 16/32K internal RAM

Post by Lardo Boffin »

Hello all

Does having a vLA81 have an impact on fitting 16 or 32K of internal RAM? I see from the user guide it allows 32K decoding but am not really sure how to go about using this.

I have done a number of internal 16K mods following Mark’s guide (http://8and16bit.blogspot.com/2015/08/m ... o.html?m=1) but have never done the 32K version.

Does having this ULA mean that the 32K is easier?

Any suggestions welcome. :D

Thanks

Lardo
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
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TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
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Paul
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Re: vLA81 ULA and 16/32K internal RAM

Post by Paul »

Hello Lardo,
It does affect the memory Extension in so far as no address decoding is necessary.
Just use RAMCS and vLA81 does the Magic.
Kind regards
Paul
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1024MAK
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Re: vLA81 ULA and 16/32K internal RAM

Post by 1024MAK »

More help about the vLA81 ZX81 ULA in these threads

The maker describes the vLA81

soft mode changes

I must find time to play with mine some more :oops:

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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Lardo Boffin
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Re: vLA81 ULA and 16/32K internal RAM

Post by Lardo Boffin »

Thank for the links. I was wondering how using one of these to get 32K would work in relation to your mod process?

Presumably the same tracks would be cut. Would the part where you reconnect the two cut tracks be connected directly to a pin on the vLA?
99296B80-6AF1-4C58-9EED-2F0452085BE3.jpeg
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
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Erm
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Re: vLA81 ULA and 16/32K internal RAM

Post by Erm »

I've done a few of these (and have done a lot of fiddling with various configurations and written a lot of emails bothering Mr Ingley with silly questions) and there is no need to cut any tracks, just bend out 4 pins (1,2,23,26) of the 32k RAM chip and connect to address lines A11 to A14.

Note that earlier versions of the manual were incorrect and the memory mode 2 (with lower 32k ram) can only be achieved by connecting A13 to the redundant (non clock) pin of the vLA (pin 35 from memory). Modes 0 and 1 don't need the extra connection.
I'm Chris, a Brit in Germany (Chr$ on the QL forum). https:\\QXL.WIN < QL related collection.
Lardo Boffin
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Re: vLA81 ULA and 16/32K internal RAM

Post by Lardo Boffin »

Erm wrote: Sat Dec 14, 2019 6:41 pm I've done a few of these (and have done a lot of fiddling with various configurations and written a lot of emails bothering Mr Ingley with silly questions) and there is no need to cut any tracks, just bend out 4 pins (1,2,23,26) of the 32k RAM chip and connect to address lines A11 to A14.

Note that earlier versions of the manual were incorrect and the memory mode 2 (with lower 32k ram) can only be achieved by connecting A13 to the redundant (non clock) pin of the vLA (pin 35 from memory). Modes 0 and 1 don't need the extra connection.
Excellent! Any chance of a few photos please? Always makes it easier for an electronics-ally challenged person like me. :D
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
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Erm
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Re: vLA81 ULA and 16/32K internal RAM

Post by Erm »

There are a few places you can get the A11-14 from (also depends on if Iss1 or Iss3 and how convenient/neat you want to be). Here's an example, the wires from the RAM go through the board and connect with pins 1 - 4 of the Z80:
20190917_235642_resized_1.jpg
There are also convenient places on the Iss1 board just above the RAM, like this:
20191206_134005_resized.jpg
I'm Chris, a Brit in Germany (Chr$ on the QL forum). https:\\QXL.WIN < QL related collection.
Lardo Boffin
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Re: vLA81 ULA and 16/32K internal RAM

Post by Lardo Boffin »

Thanks Chris.

Just one or two questions.... :D :D

* I assume L1 needs to be changed to L2?
* With the mod as shown the zeddy has 32K of RAM?
* Does that leave it in a state where most / all commercial software will still work? (Concerned that games that expect things to be in a specific RAM location will be unaffected)
* Does it support the common hires schemes in this config?

I have never truly got my head around how RAM maps on the zedddy! :oops:

Thanks!!!
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
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Erm
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Re: vLA81 ULA and 16/32K internal RAM

Post by Erm »

Lardo Boffin wrote: Sat Dec 14, 2019 9:08 pm * I assume L1 needs to be changed to L2?
* With the mod as shown the zeddy has 32K of RAM?
* Does that leave it in a state where most / all commercial software will still work? (Concerned that games that expect things to be in a specific RAM location will be unaffected)
* Does it support the common hires schemes in this config?
1. Yes.
2. Yes, with the vLA switched to mode 1 (so switch 3 on, switch 4 off). Or with 3 and 4 both off you get your normal 16k.
3. Yes, very much so. 3D Monster Maze, and anything else for 16k machines that I've tried, all run.
4. Yes, and it works with anything HiRes I've thrown at it. WRX HiRes needs the other 32k RAM mode, and that needs the extra cable (the A13 to vLA pin 35)
I'm Chris, a Brit in Germany (Chr$ on the QL forum). https:\\QXL.WIN < QL related collection.
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1024MAK
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Re: vLA81 ULA and 16/32K internal RAM

Post by 1024MAK »

Erm wrote: Sat Dec 14, 2019 6:41 pm I've done a few of these (and have done a lot of fiddling with various configurations and written a lot of emails bothering Mr Ingley with silly questions) and there is no need to cut any tracks, just bend out 4 pins (1,2,23,26) of the 32k RAM chip and connect to address lines A11 to A14.

Note that earlier versions of the manual were incorrect and the memory mode 2 (with lower 32k ram) can only be achieved by connecting A13 to the redundant (non clock) pin of the vLA (pin 35 from memory). Modes 0 and 1 don't need the extra connection.
On my blog, in this entry I show how I modify an issue 1 ZX81 PCBs so that a 32K SRAM chip can be put in a socket without having to bend any pins. It’s intended for machines that came with two 2114 SRAM chips. Before soldering in a 28 pin socket, I show which tracks to cut to isolate the upper four address pins for 62256 32K byte SRAM chips. I also show the extra wiring needed to route the needed extra address lines to these pins (A11, A12 and A13).

This leaves one address line on the 62256 unaccounted for. So if you are only interested in having 16K bytes, I show a wire link positioned to connect pin 2 to +5V (VCC). Pin 2 is one of the address pins on the SRAM chip. It has to be connected to a valid logic level and can’t be left floating. The reason for the wire link is simply to allow people to move up from a 16K byte modification to a 32K byte modification without having to desolder the socket or bend pins.

To use the full 32K bytes of capacity instead of connecting pin 2 to +5V (VCC), if you are using a vLA81 module, connect it to A14 that comes from the Z80 CPU (I’ll post a photo later).

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
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