Lately several new Vectrex cpu pcbs have started appearing in order to see the Vectrex vector output on a wider variety of vector hardware. The Vectrex cpu pcb is very well suited to this – esp. the digital side is quite simple and the parts are even now still available – not always the exact same but versions in different packaging at least. And there’s plenty of vector fodder in cart. form to try out..
Several small changes do result in noticeable differences to a normal Vectrex, though, since the CPU is slightly different, and the 6522 might be much more so. The sound chip seems less of an issue. I had reports both from Scopetrex owners and from the Fred K. repro. that a specific cartridge line does not work as expected – the 6522 gpio line. And on the current Scopetrex at least the 6522 irq line is not hooked up at all to the cpu (so games like Bedlam do not work).
The 6522 line was initially surprising but after checking the Vectrex schematics I found no dedicated pull-up (default state is ‘1’). And the 6522 IRQ line was incorrect in the official schematics, too, weirdly hooked up to a RAM pin 🙂 . Not unusual – not to always entirely believe the schematics is a good idea anyways, there might be errors and/or different revisions, and this is now another nice, obvious example I know of. However at the back of my mind I ‘knew’ that the 6522 had an internal pull-up – which must be the reason of the Vectrex behavior – but also that there are a lot of 3rd party 6522 manufacturers around with all kinds of differences. Esp. the ‘serial’ issues I knew about and now I know of another one.
Starting with the v1.31 firmware it (hopefully) takes care of these different 6522 on repro. pcbs. But.. sadly.. this takes care about basic functionality, there is still one major issue to be aware of: visual issues.
If you use a repro pcb with a Vectrex monitor everything is fine. Using a scope or arcade vector monitor however likely shows problems. The weirdness is due to several issues – one being the speed with which the monitor reacts to differences in the position. So curves in e.g. Vector Patrol or in 2048 do not work. But much more importantly: the intensity changes are out of sync with the positional changes compared to a Vectrex. These are the most important ones – instability of the drawing is another one for example but for now I want to get back to the primary ones.
The intensity problem – the timing of when you tell the Vectrex to turn on the beam, draw a line for while, and turn it off, is significantly different from a Vectrex monitor to a different output like a scope is not a problem per se – it is a problem because the repro. hw did not take it into account and has some kind of way to calibrate for it. And the additional delay -so far always a delay for the brightness- is quite large, nothing easily handled for by fast drawing functionality. It is ok-ish for really slow drawing functionality – granted that is most of the Vectrex bios – but not nearly enough to get arcade capability via the Vectrex pcb. Which is absolutely possible. What does that mean ? It means that anything trying to get even a bit more performance out of the Vectrex by using hand-coded graphics functionality will look weird at best, unusable at worst.
The current v1.x firmware uses a very fast text output which is fine-tuned for the Vectrex and not well suited to these different monitors. The next one -the ‘arcade’ v2 firmware- will support other vector monitors much better.