Todd's ELF-ish gets it's STG1861 back #ELF #VIP #ELF2K


When I had started expanding the ELF-ish beyond the basic ELF/VIP capabilities, given all of the limitations with using the CDP1861 (Interrupts, DMA, tight sync with CPU clock, clock rate limited to 1.7897725Mhz) I had disabled the STG1861 (Spare Time Gizmo CDP1861 replacement) with the intention of eventually recovering that board space for possible re-use.

However, there are a number of interesting programs using this low-res graphics system, including VIP games and articles published in Ipso Facto, etc.  So I had a change of heart, and decided to get that up and running again, as an option.

I hadn't dismantled very much, so it was a simple matter of re-making some connections.  I also decided to use a 74HC74 instead of the originally specified 7474, which let me get rid of the 10k pull-up resistors.

Here it is, back in action.

My output is green here as I didn't have anything which can handle NTSC composite, so I'm driving just the Green and SYNC inputs on my scan converter.

But then I spotted this little "video converter" which claimed to be able to handle NTSC composite, and was listed on Amazon (UK) for only £13.99!  So I decided to give it a go.

Of course, to use it, I had to install an RCA jack and add the required resistors to the header to combine the SYNC and Video signals (I used the resistor values that RCA used in the VIP rather than the ones from the original PE PIXIE graphics article).

And much to my surprise, it worked.

As you can see, the picture is a bit noisy compared with the previous green on the good scan converter.  I don't know if that's interference from the computer, the fact that the scan converter only cost £13.99, or that I'm using the 3.579545Mhz / 2 = 1.7897725Mhz clock instead of the CDP1861 specified 1.76064Mhz clock, so the NTSC timings are quite right.

But it's still cool.  I'll need to update the RCA VIP ROM and possibly the CHIP-8 interpreter to support accessing the HEX keypad using the 2-level I/O structure which is now in place in the ELF-ish before I can play Lunar Lander.

Does anyone have dissembled versions of these anywhere?

This has also made me decide that I need to de-couple the CPU clock from things like the CDP1854 UART baud-clock or the AM9511 APU clock so I can vary what the CPU is doing without impacting those other devices.  These minor updates are next on my plate.

Cheers for now,


Hi Again -

Lee Hart's disassembled version of the VIP ROM is located in the files area as VIPROM.PRN

And Josh Bensadon disassembled version of CHIP-8 is located at

Thanks guys!



Hi -

With the helpfully provided disassembled versions of the VIP ROM and CHIP-8, it was easy enough to them to the ELF-ish 2-level I/O design.

First test CHIP-8 program, Kaleidoscope, worked perfectly - just as I remember from way back when.

However, I do have to take back my enthusiasm for that "Video Converter".  Although it works, as shown, it doesn't automatically re-sync if it loses the sync.  For example, using the VIP monitor to enter the Kaleidoscope, then flick Run off (to RESET), shuts off the 1861 and thus the sync.  So the converter goes Blue Screen.

But then flipping Run back up, it stays on blue screen until I cycle through the inputs back to the composite in, and then the screen springs back to life.

I guess, what can one really expect for £13.99?!?

I'm going back to separate Video  and Sync to the proper scan converter.  Green is nice.

Can anyone recommend a NTSC Composite to VGA converter that can resync, but doesn't cost the bank?

In the meantime, I'm going to enter Lunar Lander....