Thursday 25 August 2016

VIC20 PAL to NTSC conversion

This is an old post, preserved for reference.
The products and services mentioned within are no longer available.

Many home computers of the 1980s were produced in various versions with television outputs to suit the region. Most had NTSC versions for North America, and PAL for the UK, and some also had SECAM for France. The differences were sometimes just down to a jumper, or needed various modifications, even different circuit boards.
The original version of the VIC20 (2 pin power connector) had different boards for PAL and NTSC versions, these had different clock crystals (14.31818 MHz for NTSC, 8.867238 MHz for PAL), different VIC chips (6560 for NTSC, 6561 for PAL), and also a different Kernal ROM (901486-06 for NTSC, 901486-07 for PAL). There were also differences in the supporting circuitry, the PAL version had an additional 74S74 chip to divide the clock down by 2 to give 4.43618MHz. That's it on the far right, outside of the outline of the screened cage, entirely failing to disguise the fact it's a bit of an afterthought.
When they cost reduced the board for the later version (7 pin round power connector, as used in the C64), they went for a single board. On the VIC20-CR, the PAL crystal was swapped for a 4.43618MHz crystal, so the additional divide by two stage was not required.
Here in the UK, I have loads of PAL VIC20s, but no NTSC versions, and I was looking for one to use for testing the Penultimate Cartridge, it turned out that Paul Rickards was in the USA with a selection of NTSC VIC20s, and was interested in a PAL one, so we concocted a plan to swap one. Rather than sending a whole VIC20, we just exchanged crystal, VIC chip and kernal ROM. That should be easy enough, just swap those bits over and away you go. Here's is Paul's proof that his NTSC VIC20 is working:
Here is the matching proof from my TV that my VIC20-CR is indeed PAL.
The photo Paul sent of the NTSC version looks very similar to my PAL version, doesn't it?
Well, looking closer at the photo of the NTSC and PAL versions of the VIC20-CR, there are quire a few differences.
The NTSC version has a PCB trace linking pins 2 and 4 of the 7402. The PAL version has the trace cut and a 1nF capacitor fitted
Capacitor C13 is a 220pF ceramic disc capacitor on the NTSC version, which should be horizontal in the outline in the centre of the picture below. On the PAL version, it is not fitted, but a 220pF axial capacitor is connected at 45 degrees between one pad of C13 and the top of R10.
The NTSC version has two ferrite beads, FB4 and FB5 (bottom right). On the PAL version, there is 1 ferrite bead and one yellow wire and they are crossed over.
Finally, on the back of the board, on the PAL version only, a 56pF ceramic disc capacitor is soldered across R5.
I have not been able to locate a decent schematic for the PAL version of the VIC20-CR, and only some ropey scans of the NTSC version, this is stitched together from two of those scans to show the video output section.
I have marked on the differences that have been made to the PAL version. All the PAL VIC20-CR boards I have checked have these same modifications. It's surprising that they must have made hundreds of thousands with these mods, rather than designing a PAL specific board, or even a dual purpose one could easily be made for either standard.
In order to convert my PAL VIC20 to NTSC, I need to get from that modified schematic back to the original. I removed the modified parts and assembled a pack of bits to send over to Paul so that he could do the reverse, and convert an NTSC VIC20-CR to PAL.
There were a few less parts for the reverse conversion, these are the NTSC bits I got back, for my PAL to NTSC conversion. The markings have rubbed off the 14.31818 MHz crystal. It has an unusual central ground pin, but the PCB is designed for a 2 or 3 pin crystal.
I had already removed the PAL specific parts, so all I had to do was fit a new 220pF capacitor as C13, replace the cut track next to the 7402 with a wire link, and install the two ferrite beads, FB4 and FB5 (borrowed from a scrap board). Finally, the NTSC crystal, VIC and ROM.
Once all that was done, it looked fairly close to the original version, and worked first time. It still needs a bit of tweaking, but it's a reasonable NTSC picture.
The TV confirms the signal is now NTSC. Time to install a Penultimate Cartridge and do some testing. Many people are already using it on NTSC VIC20s, but it's nice to be able to test it myself. The yellow comes out better than the PAL version, as does the red below.
More testing is required, but all is working well so far with the converted VIC20. One of the first things you notice is that some cartridge games which used to start offset to the left now start properly centred. Oh dear, I guess I'm just going to have to play some more Donkey Kong.

The PAL bits have now arrived in the states, and Paul's NTSC machine has now been converted. FB4 removed and one end of FB5 swapped over to the end of FB4 end a wire link fitted from the spare end of FB4 to the spare end of FB5, C13 removed and the new 220nF axial capacitor fitted linking to the top of R10, the track between pins 2 and 4 of the 7402 broken and a 1nF capacitor fitted between the two vias, and finally a 56pF capacitor added on the back across R5.
With the PAL crystal, VIC and ROM now installed, the ex-NTSC machine is running as a PAL machine.
Everyone's a winner!

Sunday 14 August 2016


This is an old post, preserved for reference.
The products and services mentioned within are no longer available.

This is something I've been working on for a while, and have finally put it all together.
The top half is a 6502 socket (with pass through), a GAL chip, some DIP switches, 32K RAM and a ROM chip with various ROM images on. The RAM and data bus buffer/switch are hidden under the chips. So far it is basically a squashed up version of my 6502 ROM/RAM boards.
The large chip is a 1K dual port RAM chip. That is 1K of RAM which can be independently read and written from two different address and data busses. One side is attached to the PET bus, where it sits at 0x8000-0x8400, the screen RAM. It is configured as write only, so whenever anything is written to the PET screen RAM, it is written to the board, and to this copy of the RAM.  This is the result of writing 0,1,2,3,4 etc. to the video RAM.
The PET video system is a fairly simple character based 40x25 display, you write characters to the 1K screen RAM buffer and they appear on the screen. No pixel based graphics or custom characters can be shown, it is limited to the 256 characters in the PETSCII character set. Well, sort of, there are two character sets, normal and graphics, which can be selected by an IO line which drives an address line on the ROM. It's actually 128 characters, plus those 128 again but inverted.
There is almost an isolation gap between to top and bottom halves of the board, with only power, and reset joining them, Yes, there are a couple of bodge wires, and an annoying bodge resistor. I had one pin pulled low on the schematic that should have been pulled high, so it was attached to the ground plane. I ended up drilling out the pad and wiring the bodge resistor directly to the pin.
The other side of the RAM chip is attached to an ATmega1284P microcontroller running at 20MHz. This is connected via a 3.3V level shift to a 2.2" 240x320 LCD module.
What does all this do then? Well, if you plug this into the CPU socket of a PET, whenever the PET writes to it's video RAM, it will also be written to the dual port RAM chip. The microcontroller is continually polling this RAM via the other port and converts that to a display on the LCD.
You effectively get a copy of what is on the PET screen on the LCD, updating in real time. To generate this, I have converted the PET character ROM into a data array and draw each character pixel by pixel on the display. The 240x320 layout fits well with the 40 character, 25 row display, each character is 8x8, so 40x8 = 320, 25x8=200, with 20 pixels space top and bottom. This gives a very clear, pixel perfect version of the screen.
 In business graphics mode, the PET introduced two pixels gap between the lines, so this can be fitted in to 225 pixels. I've fixed it in the graphics character set for the moment, as it needs to tap off a one of the pins on the VIA line to change font, I'll be adding that to a later revision, probably by duplicating the port with a single 8 bit register or even just a single flip flop.
This opens up a lot of interesting potential. This is really proof of concept stage. The initial idea was this would be a diagnostics tool. You can plug this into any PET and see if it is running, even if the video output on the PET itself isn't working, as long as it is writing to screen RAM, this will show the state of the system.
It can also be used in conjunction with my 6502 diagnostics board, and will show the results of ROM and RAM testing on it's LCD. So even if the ROM, RAM and video display are all faulty, it should be possible to diagnose this and fix it bit by bit.
Since this is a 40 column display, it's also a potential way to add a 40 column display (albeit a small one) onto an 80 column PET. The board has various ROM sets on there, so the 40 column editor ROM can be selected and the PET will run in 40 column mode. The actual monitor will show the wrong thing, but the LCD will show the correct output.
The update rate is reasonable, fast enough to play space invaders anyway. I tried a few demos, and they all ran OK. This is just the first testing, I imagine I can streamline the code to make updating a bit faster.
Another thing to consider is now that I have a way of collecting the screen data, it could be used to drive something larger than this LCD. It could instead be sent to an HDMI monitor or even generate a video signal for the PET monitor or a composite output, although I think I would need an external shift register to handle the pixel clock rate. Driving the PET monitor add the option of letting you switch between 40 and 80 columns on an 8032, which would be very useful to expand the range of available software. Lots of potential though.
One final thought, this is a 6502 CPU, ROM, RAM and a display. Stick in clock, reset and some IO, and you've got yourself an SBC. Add the appropriate ROMs and IO and you've got yourself a mini PET........

2022 Update: A Mini PET? That's an interesting idea. An idea that later became a kit and now a preessembled replacement board. Both available from The Future Was 8 bit.

Thursday 4 August 2016

VIC20 Penultimate Cartridge User Guide

This is an old post, preserved for reference.
The products and services mentioned within are no longer available.

Congratulations on purchasing your new VIC20 Penultimate Cartridge!

This is the point where people normally stop reading the instructions. You can stop if you like, it should be reasonably intuitive to use so you shouldn't need instructions. But read on, there are a few hints and tips that may be useful.
These instructions relate the firmware revision 5.x on the menu driven carts. Those with version 4.x firmware can get updates, contact me for more information. The cartridge PCB can be used as is, or can be fitted in the shell of an old cartridge.
The section at the top sticks out of the slot at the top of the case to allow you to access the menu button when it is closed.
There will be a fully cased version available soon with a custom injection moulded cartridge case from The Future Was 8 Bit.
Step 1, install the cartridge. Should be obvious, but the VIC20 should be turned off first, and the cartridge inserted with the label facing upwards. With the PCB version, it should be located centrally in the edge connector.
With the cartridge installed, power up the VIC20. You should see the first page of the menu. If not, check the cartridge is properly installed.
From this page you can do many things. You can get back to this page at any time by pressing the menu button on the left hand side of the cartridge. If you have a reset button on the right, this will reset the VIC20 and restart the current selection or exit to BASIC. Use the menu button to get back to this screen.
You can use the joystick to navigate the list of ROM cartridge images. As soon as you move the joystick, one of the titles will be highlighted. Up and down scroll through the list, left and right change page. Once you find the one you want and click fire. The cartridge ROM will be switched in and the game should immediately start.
Some VIC20 cartridges will start offset to the left on PAL machines, this is normal and how the originals behave.
Many of these cartridges let you use the arrow keys or joystick to centre the screen.
When you are finished with a game, press the menu button on the cartridge to get back to the menu.
You can also use the arrow keys or < and > to scroll through the pages of ROM cartridge images. To load a cartridge, press the number highlighted in white.
For example, Omage Race is 23, with the 3 highlighted, so press 3 to start Omega Race.
That ROM will be switched in and the game should immediately start.
The forth page contains some text adventure games. These used to require the user to type 'SYS 32592' to start, this is now automated, so just press the number and the adventure will begin.
The fifth page contains some utility ROMs. The VICKIT 4+5 option is a tape speed loader and assembly language monitor.
BASIC 4 is an update to VIC20 BASIC 2 which adds disk handling commands such as DIRECTORY and DLOAD found on the PET and Plus/4 etc. Note when this is selected, block 5 is used by the BASIC 4 ROM, so is not available. There are two unlisted shortcut keys here, from any screen, press V to start VICKIT 4+5 + 27K RAM, or B to start BASIC 4 + 27K RAM.
Super Expander is provided with the original 3K option, or the maximum 27K available.
As well as the ROM cartridge images, there are a series of direct start options, as follows:

  • f1 - Exit to BASIC with 0K expanded memory
  • f2 - Exit to BASIC with 3K expanded memory (block 0)
  • f3 - Exit to BASIC with 32K expanded memory (blocks 1,2,3 and 5)
  • f4 - Exit to BASIC with 8K expanded memory (block 1)
  • f5 - Exit to BASIC with 35K expanded memory (blocks 0, 1, 2, 3 and 5)
  • f6 - Exit to BASIC with 24K expanded memory (blocks 1, 2 and 3)
  • f7 - Exit to BASIC with 35K expanded  memory and start the SD2IEC file browser
  • f8 - Exit to BASIC with 35K expanded  memory and start the default program from disk

The default program is the first file on the disk, the one which would be loaded with LOAD "*",8. The last two options load from drive 8 by default, but you can press D on the menu screen to cycle through 8-11 if you don't want to start from drive 8.
When selecting RAM in block 5, it will be cleared before the VIC is started. This is to remove any auto-start cartridge images that may have been copied there on previous sessions.
From the main menu, you can also press A which will switch to the advanced menu.
Here you can select from various combinations and RAM using the number keys 1-8, then press f1 to exit to BASIC, f3 to lauch the SD2IEC file browser and f5 to load the default program from disk. If you have a Jiffy DOS ROM installed in your VIC20, that will speed up access to loading from disk.
It is possible to create you own ROM image for the Penultimate Cartridge if you want to change the selection of cartridges or add your own. I will provide full instructions on that later if there is any interest.
If you have got this far and haven't ordered your Penultimate Cartridge yet, go and preorder one now. Or if you prefer, buy an uncased version now.

The first run of cased cartridges is now here, more photos of those and the test program.

2022 Update: The production version of the Penultimate Cartridge is now available from  The Future Was 8 bit