Z80 Homebrew with PSG

[RetroFocus2015]

Hi,

1. Oh i didnt realize that you were not using the entire 64k address space. With 32k ram and only 2k rom, you have some of that address space available, unless of course you intend to increase the size of the rom, and then you should use i/o anyway. But if you can limit the rom to say 16k, then that gives you one address line that wont be used for addressing anything when the ram isnt selected with A15, so you can use that AND'ed with A15 to access 16k of something else, like your sound chip or similar...

2. An edge connector...
You mean a PC board edge connector? To plug into what, the PC computer or your own mainframe card cage?

1. As I can best describe it, the design is constantly changing in my mind, using parts I now have available. At the moment I have a AT28C256 32K EEPROM on the schematic. With import costs and - I suppose - rarity of older parts it seems a lot easier to obtain the newer parts with the bigger amounts of RAM and ROM on less chips. I think I'll use the 32k EEPROM and cram as much useful stuff on there as possible, like ASCII characters etc.

2. Something like this **broken link removed** You could plug just about anything into these expansion ports like joysticks, RAM upgrades, printers, light pens etc using an interface like this one on a TRS-80 https://www.nightfallcrew.com/09/07...s-80-modello-mc-10-con-espansione-ram-di-16k/

 

[RetroFocus2015]

Is the attached schematic how your IO decoder looks like? Which are the IO lines being used and which are still free?

Allen

Yes, although I've removed the Memory Bank chip select lines and now have only A15 available, which I was going to use a NOT gate inverter on to switch between RAM and ROM.

I am still using exactly the I/O configuration from the book. I've just realised that the segment LEDs used for testing use _DSWR5_, _DSWR6_ and _DSWR7_ and the ports use _DSWR0_, _DSRD0_, and _DSRD4_. If I leave the segment LEDS in at this stage, that leaves me with _DSRD_ 1,2, and 3 and likewise for _DSWR_.

 

[ericgibbs]

Its conventional to use MREQ for ROM/RAM and IORQ for any peripherals.

Is it possible that you could post a copy of the circuit that you are using as a starter.?