| Electronics > Projects, Designs, and Technical Stuff |
| Cloning a Commodore PET-2001 |
| << < (17/33) > >> |
| GK:
--- Quote from: LaserSteve on April 26, 2018, 06:56:00 pm ---Edit, Yep.. Lots of them... https://www.muppetlabs.com/~breadbox/software/trs80-oneliners.html --- End quote --- Now that's pretty cool; added to bookmarks. |
| jthomas:
Looking forward to seeing the rest of the schematic! I'd love to have a go at laying out my own single-board PCB for the thing... a clone of a clone if you will :D I have a possibly silly question about the video section though - what do the two ROMs U117/118 do? The rest of the circuit looks very similar to one I've already built in the past but I can't seem to figure out on my own what their role is in this case. Just about the only thing I can think of is converting X/Y addressing to straight binary or vice-versa (so as not to leave "holes" of unused memory)... I'd appreciate having it explained to me as not knowing will annoy me! |O |
| GK:
Those ROMs just map the 1000 video memory character locations called by the CPUs operating system (40 horizontal characters [columns] x 25 vertical characters [rows] = 1000 character locations) to the ones in the video RAM that actually correspond to those locations on the screen. In the video RAM there are 14 unused character column positions per horizontal row (40 of 64 used) and there are a total of 39 rows, of which only 25 are used = a total of 2496 memory locations. This is why the video SRAM has a 12-bit address, while the actual video board address bus is only 10 bits. Using look-up table ROM for this just simplifies the circuitry as you can then just address the video RAM directly from the pixel and line counters while drawing the display. The original PET hardware instead used a separate address counter for the video RAM (that was continuously stopped and started during the visible field) in combination with a required address location memory/latch, and some convoluted logic to control it all. Back in the day it wouldn't have been economical to waste video memory this way, but now we have 256k-bit SRAMs for pennies. |
| jthomas:
Thank you for the explanation :D |
| ebastler:
--- Quote from: james_s on April 26, 2018, 04:52:00 pm ---That really is pretty cool. The surprising aspect is just how few parts are on that board even using individual components. --- End quote --- Well, there weren't too many components on the original board. Combine the RAMs and ROMs into higher capacity ICs, and there are not that many chips left: |
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