Search found 43 matches

by Mustermann
Thu Mar 16, 2023 10:32 pm
Forum: Development
Topic: Using an external EPROM
Replies: 5
Views: 1358

Re: Using an external EPROM

Might be, this is doing what you want:
https://ws-nbg.de/zx81/main.php
by Mustermann
Thu Mar 16, 2023 9:02 pm
Forum: Development
Topic: Using an external EPROM
Replies: 5
Views: 1358

Re: Using an external EPROM

It takes a lot of effort to display characters from external ROM.

If characters in internal and external ROM are the same and at the same locations you can enable internal ROM in refresh cycle and external ROM in all other states.
by Mustermann
Fri Feb 17, 2023 10:51 pm
Forum: Hardware
Topic: The reason native hi-res is only possible on the 1k zx81??
Replies: 8
Views: 1589

Re: The reason native hi-res is only possible on the 1k zx81??

Freya wrote: Fri Feb 17, 2023 11:56 am So a 32k internal zx81 ram upgrade can use all the different hi-res modes?
There is UDG and CHR$128 HRG. Most of the internal and external memory expansions do not support that.

External a UDG4ZXPAND module is needed in addition to memory expansion.
Internal there is quicksilva UDG board.
by Mustermann
Fri Feb 17, 2023 6:56 pm
Forum: Hardware
Topic: The reason native hi-res is only possible on the 1k zx81??
Replies: 8
Views: 1589

Re: The reason native hi-res is only possible on the 1k zx81??

With some additional logic internal 1k can be mapped at 8k-16k.

I also heard that one of the printer interfaces uses internal 1k to patch ROM.
by Mustermann
Fri Feb 17, 2023 7:00 am
Forum: Hardware
Topic: The reason native hi-res is only possible on the 1k zx81??
Replies: 8
Views: 1589

Re: The reason native hi-res is only possible on the 1k zx81??

The other point, is that some modern SRAM chips are too fast at responding to the control signals, and hence this messes up the timing. Mark This can be fixed by delaying /RAMCS using a capacitor, https://www.sinclairzxworld.com/viewtopic.php?f=7&t=2120 adding some non inverting gates between /RAMC...
by Mustermann
Thu Feb 16, 2023 10:06 pm
Forum: Hardware
Topic: ZX81 Hi-Res
Replies: 40
Views: 14370

Re: ZX81 Hi-Res

If I understood right, this is a classical race condition. ULA load shift register at the falling edge of T4 cycle. /MREQ go to high at the same time. So this is working with the original slow ROM and very slow RAM. With fast RAM the data is invalid prior ULA is reading it. So you need to delay /RAM...
by Mustermann
Sat Mar 06, 2021 5:16 pm
Forum: WANTED
Topic: CHR$X16
Replies: 3
Views: 3497

Re: CHR$X16

I was able to build an internal CHR$16 / CHR$128 mod. There is no PCB, just soldered a RAM on the back of the ROM and build a small dot pcb. https://sinclairzxworld.com/viewtopic.php?p=37302#p37302 This need at least 8-16k range not used by external RAM. I can tell in more detail if needed. I also h...
by Mustermann
Wed Nov 11, 2020 5:57 pm
Forum: Development
Topic: ZX81 Clone Tynemouth Minstrel 3
Replies: 48
Views: 36039

Re: ZX81 Clone Tynemouth Minstrel 3

Be aware that 74HC logic need 3,5V as minimum high level. TTL logic as used in the original ZX81 need only 2V as minimum high level. I struggled with this issue as well when I build my Minstrel. I found four working solutions: - Replace all HC chips by HCT (this will cause some other issues with RC ...
by Mustermann
Sun Oct 04, 2020 1:47 pm
Forum: Hardware
Topic: Top Ten Hardware Mods....
Replies: 9
Views: 10639

Re: Top Ten Hardware Mods....

The RAMKEY of Wilf sounds quite interesting to emulate key presses controlled by a joystick. The hardware is quite clear to me.(Even if I am planning a different implementation) Assembler code is also understandable. But I am confused about the JSBOOT DECIMAL CODE IN 1 REM LINE Line 16516 is ok. Lin...
by Mustermann
Sat Apr 04, 2020 9:07 pm
Forum: Hardware
Topic: ZX81: From UDG DIY to CHR$128 DIY
Replies: 1
Views: 2412

ZX81: From UDG DIY to CHR$128 DIY

There are a couple of ZX81 Hi-Res games existing that are using UDG with 128 user defined characters. This is also named CHR$128. There is not that much information available about CHR$128. One piece of information I found is Kelly Murtas post: https://sinclairzxworld.com/viewtopic.php?t=1495#p15616...