As part of me trying to resurrect a non-functional VZ300 (rebranded VTech Laser 310) I've been going down the Z80 investigation rabbit hole.. the machine in question just throws garbage on the display at boot. I've swapped out a bunch of stuff including the ROM, all the DRAM, the Z80 - all to no avail. These machines have a few custom gate array chips that have zero official doco but some work on reverse engineering has been done.
Here's the basic machine, from the early 80's. That metal shield over the board was a complete bear to remove!
All of this reignited my interest in Z80-based machines, and I decided to build one up from bits... I started just with a Z80 and 555-based clock circuit and everything else either pulled hi or low as required to get it to basically run NOP's forever (instruction fetch from any address results in the same hardwired data). I've progressed and it can now access an EEPROM to read code. I coded up a simple loop by hand and programmed the EEPROM, and I can see it fetching and executing. Next step might be some SRAM or an IO device and see if I can get it to talk to other devices. Will need some more complicated address decoding for that though.
Breadboard mess! No magic smoke was released.
Scope showing /M1 line in blue and /MREQ in yellow. This is exactly what I was hoping to see!
Here's the basic machine, from the early 80's. That metal shield over the board was a complete bear to remove!
All of this reignited my interest in Z80-based machines, and I decided to build one up from bits... I started just with a Z80 and 555-based clock circuit and everything else either pulled hi or low as required to get it to basically run NOP's forever (instruction fetch from any address results in the same hardwired data). I've progressed and it can now access an EEPROM to read code. I coded up a simple loop by hand and programmed the EEPROM, and I can see it fetching and executing. Next step might be some SRAM or an IO device and see if I can get it to talk to other devices. Will need some more complicated address decoding for that though.
Breadboard mess! No magic smoke was released.
Scope showing /M1 line in blue and /MREQ in yellow. This is exactly what I was hoping to see!