Amstrad CPC6128 and dual CRTC

August 9th, 2014

This time in English, as this blurb might benefit the international Google-using audience, too. As you might know, Amstrad CPC6128s came equipped with various versions of the CRTC chip: most of them clones of the Motorola 6845 (read more here). The problem from a demoscener’s point of view is that they weren’t identical in every respect and, thus, very low-level demo stuff might not run. Both of my machines happened to have a CRTC0, which meant that I couldn’t run one of the top prods, From Scratch. Some older demos might be the opposite, namely CRTC0-only.

Various versions of the chip are occasionally available on eBay and I happened to come across a promising CRTC1. Changing the whole chip seems a bit brutal, but according to some accounts on CPC forums, you can piggyback two chips on top of each other and install a switch – unfortunately nobody seemed to give exact instructions. As my second 6128 was a bit busted anyway, I decided to give it a go. The best bet from a layman’s perspective seemed to be installing the switch to Vcc: bend pin 20 up on both chips, solder together the rest, and install a two-position switch that directs 5V to one CRTC or the other.

And it works! From Scratch runs now and DTC reports the version correctly depending on the switch position. See below how the kludge looks like:


According to some rumors some people have installed even three chips on top of each other. After that you start getting close to the keyboard, tho. The contacts might be a bit hard to get working flawlessly, I guess. Then again, from a demo perspective a CRTC2 is a useless piece of junk – not much will work anyway. Funnily enough, the problem child is the genuine Motorola chip 🙂 CRTC3 and 4 are part of the ASICs of the very last last CPC models, so they don’t exist in the wild as separate chips.

Filed under: demoskene,laitteet,retro

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