Another new board

[Alan Cox]

The SC126 was almost my ideal retrobrew Z80/Z180 system but with a couple of niggles and lack of a convenient case option. Steve fixed the case option but not in the direction that I wanted - of expandability.

This is the same core Z180 CPU/RAM/ROM design taken the other direction.

The base decode for the RAM/ROM is the same approach as the SC126, but with two RAMs and one ROM. That allows you to switch out the low ROM and go all RAM. The upper RAM can also be switched out for the RC2014 RAM, although I need to tweak that slightly to make it transparent. Ideally it'll then be possible to boot and switch to an external 512/512K banked memory card and run banked memory expecting software too.

On the I2C and SPI side I swapped all the glue for a 82C55 chip. That provides enough lines for the SPI, I2C, joystick, keyboard, mouse and memory control. Instead of gates the pull up/down stuff is done with signal diodes and this section is very much influenced by the N8, so hopefully it'll all work out 8)

The slots are UEXT with some 3.3 and some 5v slots, as well as a proper mux/demux to allow for 8 SPI devices and for SPI devices that rudely don't share the bus.

At the moment I've got the core of my test board running an unmodified RC2014-Z180 ROMWBW. It'll need some changes to the code because the onboard floppy is mapped differently as is the SPI and of course the keyboard/mouse are not handled. With 1MB RAM though the nice thing is it ought to be possible to run some totally unrelated firmware and load ROMWBW into half of the RAM when wanted.

Next step is to build and test the floppy then UEXT and SPI/I2C parts, then the RTC.

The board files are at https://github.com/EtchedPixels/Z180MiniITX and I'll need to make at least one more revision to address a 5v line in a 3v3 UEXT port and to figure out what to do with the graphics plan. I added support for a cheap i2c teletext device as video out. Since I designed the board last summer, said chip has gone from someone having a pile of them cheap on ebay to rocking horse droppings.

I've not lined the RC2014 slot up anywhere precisely with the PC case because I am assuming in most cases you'd use a small backplane as a "riser" with the cards over the CPU board Mini-ITX PC style.

Alan

[Chris Brunner]

Outstanding work!  Exactly what I'm looking for!  I'm a big fan of my SC126 and I still have uses for it, but this scratches the itch I had.  Looking forward to seeing the final revision!

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[Mark T]

Hi Alan, were you looking at the SAA5243? Still seems to be available from utsource. I got a few last year but not tested yet.

[Alan Cox]

On Wed, 5 Jan 2022 at 03:11, Mark T <mark...@gmail.com> wrote:

Hi Alan, were you looking at the SAA5243? Still seems to be available from utsource. I got a few last year but not tested yet.

I was looking at the 5348 because it seemed to be nicely integrated.

Schematic for the curious

[Nige C]

I am building an SC126 kit now. The PCB is excellent for soldering overall, and the kit almost perfect. Steve Cousins kit and support website is brilliant.

My only issue is  getting the angled side expansion connector to fit in the holes!!!!!!! For soldering. 

[Nige C]

For case options. I have two 3D printers, a Prusa i3 Mk3S+  and an Ultimaker2+Just in case.

[Steve Cousins]

Yes, those angled expansion connectors can be difficult. I find getting one end inserted and then working along with a small screwdriver to help align each pin gets the job done. A slight variant that also seems to work is to tilt the connector so you can get one row in (usually quite easy) then use the screwdriver to help align the second row.