V9958 - First tests successful!

A project log for rosco_m68k

A full-featured Motorola 68k retro computer, starring a 68010 running at 8MHz

Ross BamfordRoss Bamford 05/16/2020 at 22:210 Comments

It's been another dizzying couple of weeks on the rosco_m68k project, with lots of community activity going on, Tindie orders to ship and more stock to source, but I've still had time for some good old-fashioned hacking. I've been focusing on the video card mainly, and it's now at a point where prototype boards are ordered and have been shipped by JLCPCB.

In it's current state, the board looks like this:

 (Well, it's changed a bit since that photo as the video RAM at bottom right understandably didn't work in this configuration :D) 

The centrepiece here is the V9958 (on it's SDIP->Breadboard adapter centre right), supported by a couple of 16V8 GALs for decoding and glue, all hooked into a prototype bus board (top left). The main board is top-right. I was pretty impressed it worked at all, and it took a little tinkering to get it going, not because of issues with the design so much as issues with breadboard in general. 

To begin with, I didn't have any suitable DRAMs, so while I waited on their arrival I decided to try hooking up an old (battery-damaged) Amiga trapdoor expansion PCB to see if it would work. Results were encouraging:

Obviously it's not quite there, but there *is* some output, so I knew I was heading in the right direction. Again, the ability to prove this out in MAME was a massive help, this is what the screen above looked like in the emulator:

Eventually the DRAMs arrived, which is when I took the shot of the board you saw in the first picture. This improved things somewhat, but also made other things weirder:

(all the H's are deliberate, I changed the character I was clearing the screen with to check I was actually writing the DRAM - MAME couldn't help here as it was all about cycle times and refresh).

This is the point where I took the first picture in this post, where the DRAM was tacked on to yet another breadboard and jumper wired in. Of course I suspected this to be the problem, and sure enough, after moving the DRAMs onto the same board as the V9958 itself and making sure the wiring was sound, I was finally able to get a decent "Hello World" out of the VDP!

With this success, I finalised the schematics for the prototype board with some input from everyone over on Discord, and drew up a quick board design to send off for manufacture. It's on it's way, and I'm really looking forward to building it and proving the design before moving onto the final design of the board.

The PCB is quite compact (it's a prototype, and I'm working hard to keep prototype costs to a minimum after spending far too much on prototype RAM and bus boards):

The final design is going to incorporate some great community suggestions (like the amplified RGB and sync signals going to the header as well as the DIN, and the colour bus being on a header too), but for now I just want to make sure the basic thing works, and this will do just that.