ULA revistited.

Any discussions related to the creation of new hardware or software for the ZX80 or ZX81
VinCBR900
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Re: ULA revistited.

Post by VinCBR900 »

Andy, If you used the original modulator then a pair of 75-300 ohm dc blocking TV matching transformers wired back to back could provide galvanic isolation from the hot mains TV chassis.

I dont think this woudl work with composite as the sync would be lost and need re-introducing, so a modification of your zx81 composite video mod http://homepage.ntlworld.com/deborah.cl ... idmod3.jpg by putting a transistor driver and isolation transformer before Q1 and powering the LM1881 using something like a MAX250.

Cheers, Vincent
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Andy Rea
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Re: ULA revistited.

Post by Andy Rea »

that may be worth a try, i've been a bit quiet on the zeddy front just lately.

Thanks for the suggestion

Andy
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Andy Rea
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Re: ULA revistited.

Post by Andy Rea »

Been a while since i've posted anything on here..... but unless my cheif tester says otherwise, i'm looking at a slight re-design on the PCB using SMD where possible and through hole dip-header fro added mechanical strength alonh with buikt in emmiter follower circuit for direct composite video outoput.

Thats all for now.

Regards Andy.
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Paul
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Re: ULA revistited.

Post by Paul »

Andy Rea wrote:slight re-design on the PCB using SMD where possible and through hole dip-header fro added mechanical strength
Did you consider using 90 degree bended pins instead of through holes? I sometimes use these "round pins" from a precision socket (don't know the english name). I take away the plastic having the round pins only. They can be solder quite good onto pads. I hope the photo will make clear what I mean.
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Paul
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Re: ULA revistited.

Post by Paul »

Prime, superflo,
may I ask if you are also going further in your tries?
Are there any news that I missed during the last month?
I'm not trying to push, I'm just curious ;)
In theory, there is no difference between theory and practice. But, in practice, there is.
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Andy Rea
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Re: ULA revistited.

Post by Andy Rea »

Mark 2 boards in the pipeline, single chip, board size 20% smaller than the last one ! Watch this space.

Andy
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Andy Rea
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Re: ULA revistited.

Post by Andy Rea »

This might sound like a silly question but.... i am thinking instead of using an io-port to control extended features i could use peek and poke instead, enabling the features to be controlled from basic as well as machien code, BUT i need to find a rom location that already holds $FF (255) becuase i can only drive data lines low, not high.

So if you would be so kind, when running a ZX81 rom whatever flavour, SG81, old rom, new rom, ZXpand rom...

do thid for me and post your result.

PRINT PEEK 101

hopefully everyone will get the result 255 which means i'll used that rom location, the annotated online rom lists it as a spare location, so hopefully it will be the same in all versions of the ZX81 rom.

Thanks Andy
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sirmorris
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Re: ULA revistited.

Post by sirmorris »

That's one of the only locations I haven't touched yet ;)

I'll make sure all future revisions of the zxpand ROM preserve it. AFAIK all previous versions have it as FF.

Unimproved, improved, SG81 - all FF.

C
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PokeMon
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Re: ULA revistited.

Post by PokeMon »

Why not use the first exisiting ROM Location, 3 (PRINT PEEK 3).
This is in the beginning to initiate RAM Test ...

Code: Select all

;; START
L0000:  OUT     ($FD),A         ; Turn off the NMI generator if this ROM is 
                                ; running in ZX81 hardware. This does nothing 
                                ; if this ROM is running within an upgraded
                                ; ZX80.
        LD      BC,$7FFF        ; Set BC to the top of possible RAM.
                                ; The higher unpopulated addresses are used for
                                ; video generation.
        JP      L03CB           ; Jump forward to RAM-CHECK.
I have one Issue One Board with maybe an early ROM version which has the same beginning code. ;)
And it's not just an empty spare with hopefully filled with $FF - I think all ROMs do the memory check from $7FFF backwards ...
Anyway 101 is $FF in my ROM too.
Last edited by PokeMon on Thu Oct 20, 2011 10:14 pm, edited 1 time in total.
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Paul
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Re: ULA revistited.

Post by Paul »

Because the first five bytes of the ROM are written to by the Basic as the result of an error in the maths.
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