Nebula Spectrum ULA Chip Replacement Module
The Nebula is a Spectrum 48 daughterboard, designed to replace the ULA chip found in UK Spectrum motherboards (versions 2 to 6). It was designed by myself, Phil Ruston (PCB design, mostly) and Alessandro Dorigatti (logic core) between 2015-2016. Many thanks to Chris Smith for his excellent book: “The ZX Spectrum ULA – How to design a microcomputer” without which the module could not have been made.
The Nebula is based on a programmable logic chip with some external components to handle the analogue systems. The core has only the features that the original Spectrum ULA has – nothing more (the CPLD is full anyway!). Emulation aims to be cycle perfect – it passes the FUSE tests and runs programs that rely on the floating bus behaviour. (The ’emulation’ is based on a 6Cxxx era ULA so the display position and EAR input are as would be expected when a 6Cxxx ULA is fitted.)
The module’s video output is the same as that provided by the original ULA (IE: YUV to drive the Spectrum’s modulator circuit. Naturally the best picture is obtained when the Spectrum has been composite modded but RF still works). When fitting to issue 2 boards, VR1 and VR2 need to be
adjusted to correct the colour offsets – as would have to be done when replacing a normal ULA.
The Nebula has been tested extensively on standard Spectrum 48 motherboard revisions 2, 3B and 6A and has been found to be stable on all. (Of course, absolute perfection can never be guaranteed – we don’t have access to vast quantities of peripherals and test equipment. At the end of the day, we’re just hobbyists).
Fitting: On rubber key Spectrums, space between the ULA socket and top cover is extremely limited. If some of the nearby logic chips (IC3, IC4, IC24, IC25) are in sockets they may cause tension in the case when re-assembling. It’s is strongly advised that these chips are soldered directly to the spectrum PCB.
If you’re interested in buying a Nebula, I am making up some boards for sale. They will be available from store.retroleum.co.uk (under Spectrum Chips) but please bear in mind they are hand made which is very time consuming and so supplies will be limited. Also, please read these important fitting notes.
Q. What tests have been run?
A. The following programs ran and behaved the same as with a normal ULA chip: Fusetest, Ulatest3, FloatSpy, IR Contention, BTime, STime, Shock Megademo, Aquaplane, Darkstar, Sidewize, Vectron, Arkanoid, (and many other games..)
Q. What does Nebula stand for?
A. It’s not really an acronym – it’s just a nice word that has ULA in it :)
Q. Will it work on [insert freaky foreign Spectrum clone here] ?
A. If the clone in question takes a standard Spectrum PAL 5cxxx or 6cxxx ULA, then “probably?”
Q. What are the contacts for (upper right)?
A. Programming the CPLD. (Yes, it does come pre-programmed:)
Q. Any plans for similar devices for other machines?
Q. Any plans for an NTSC version.