New Hardware Projects

[sirmorris]

Greetings!

The interface is working solidly apart from hires timings. I'm in the process of hacking up the prototype board to get the hires working flawlessly and then I'll be ordering a second prototype board to ensure all is well.

I'm considering some changes in the light of some conversations and reflection in time off from the project for vacation.

Currently there's support for a 32k ROM on board and RAM in the 8-40k regions. I'm going to enable the RAM region to be remapped primarily in order to allow the 32k adventure programs by Gus and Xav to work. The memory map will be reconfigurable in software so that, for instance, you can have RAM at 8-40K but then switch to RAM at 0-32k. This will enable ROM images to be loaded and then swapped into the map.

Currently I've hacked the '81 ROM and overwritten the character data with the SD loader code. It's very simple but it prevents some pseudo-hires programs from looking their best so I am aiming to allow a slightly-patched-but-reliant-on-ROM-at-8k version to live in the EPROM too. The switching, again, would be software controlled.

I'm hoping to start production in Q3.

[Mike3]

Sounds Good... I'm excited about this project.. and much appreciate your efforts..

Regards... mike

[wilf_zx]

Greetings SirMorris,

Just catching up on all the wonderful ZedEx developments including your versatile mmc interface.

The hires problem you mentioned is most likely caused by the 251 decoder which should be enabled with MREQ OR RFSH to guarantee RAM CS set up time for reading hires pixel data during refresh.

You may have already considered using RD with the 251 D0 input to conditionally access 0-2000h RAM so that the modified ROM image in RAM, including the new code between 1E00h and 2000h is enabled during RD while the ZX81 ROM video pattern data (for normal video and pseudo Hires) is enabled during RFSH.

As you mentioned, the ZX ROM overlay code can be boot loaded with the PIC from MMC to RAM (8000h-9FFFh) during powerup/reset. Nice way to update ZX ROM overlay code revisons.

On the hardware side, the pricey I/O expander can probably be replaced with two cheap 74HC595 chips.

best,

wilf

Greetings!

The interface is working solidly apart from hires timings. I'm in the process of hacking up the prototype board to get the hires working flawlessly and then I'll be ordering a second prototype board to ensure all is well.

I'm considering some changes in the light of some conversations and reflection in time off from the project for vacation.

Currently there's support for a 32k ROM on board and RAM in the 8-40k regions. I'm going to enable the RAM region to be remapped primarily in order to allow the 32k adventure programs by Gus and Xav to work. The memory map will be reconfigurable in software so that, for instance, you can have RAM at 8-40K but then switch to RAM at 0-32k. This will enable ROM images to be loaded and then swapped into the map.

Currently I've hacked the '81 ROM and overwritten the character data with the SD loader code. It's very simple but it prevents some pseudo-hires programs from looking their best so I am aiming to allow a slightly-patched-but-reliant-on-ROM-at-8k version to live in the EPROM too. The switching, again, would be software controlled.

I'm hoping to start production in Q3.

[sirmorris]

Nice to see you back, Wilf!

You haven't caught up completely unless I'm mistaken

There are some youtube videos up, one of which features a little program you might recognise..!

[Mike3]

still looking forward to this ...any updates?

Regards... Mike

[Moggy]

still looking forward to this ...any updates?

Regards... Mike

Me too Mike I get a feeling this is going to be good!

Regards Moggy

[sirmorris]

Hiya,

I've hit a bit of a development wall att he moment

The GALs that I'm using to replace all the TTL have required something of a learning curve

I've had the memory running once, but only after some serious reworking of the GAL program. I think my problems are all timing related - the GAL appears to be a bit too fast. I'm almost at the point of considering a hybrid approach using GAL for the IO and ttl for the memory. Though a _third_ prototype may well be out of the question

Oh stop being pessimistic SM. It'll be fine

C

[siggi]

I think my problems are all timing related - the GAL appears to be a bit too fast.

Maybe the same timing problem I described here?

viewtopic.php?t=300#p2620

Siggi

[sirmorris]

Hi Siggi,

I've used that post as inspiration for a couple of changes to the GAL. My problem at the moment is trying to reduce the equations

[RWAP]

A useful little hardware project, which would be fairly simple to produce and would have market appeal, would be a circuit to fix the video display on early ULAs (which cause the black cursor on a very dark screen).

There is such a circuit supposedly for all ZX81s at (translated from Spanish):
http://translate.google.com/translate?j ... ml&act=url

Does anyone fancy making a batch of these?