🇬🇧 Sinclair ZX Spectrum: Part 5 (RGB cable & more diagnostics) [TCE

Progress? It has been known to occur occasionally on my channel.
I decided to switch from composite video output to RGB via vRetro's vLA82. Thus I made up an 8-pin DIN RGBS cable.
It has the same pinout as the cable for the Spectrum +2A, but no need for mono-audio in the cable as the vLA82 provides that via a standard 3.5mm socket next to the DIN socket.
So the pinout is:
Pin 2 = ground
Pin 4 = sync
Pin 6 = green
Pin 7 = red
Pin 8 = blue
(other pins remain unconnected)
I have the Retroleum diagnostic ROM installed on the Speccy board in place of the Sinclair ZX Spectrum ROM. And ... the text is readable now! Well, after a cold boot. Once the machine has warmed up a little it starts misbehaving.
blog.retroleum....
So finally I have something concrete to work with - it still does appear to be an addressing issue with the lower 16KByte of RAM (where the screen memory is).
Time to do some research ...
Photos on my Patreon (free or paid members):
www.patreon.co...
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From Part 3:
Ivan very kindly donated one of his 128KByte RAM expansion boards so I am looking forward to trying that out - my first package from 🇷🇸 Serbia! In order for it to work you need to disable the upper 32KBytes of internal RAM, which I've done by adding a switch to inhibit the !CAS signal for that bank of RAM (make one OR gate input on IC23 always HIGH by connecting to +5V thus forcing the output HIGH, thus disabling !CAS).
I've verified the switch works by executing "PRINT PEEK 23732 + 256 * PEEK 23733" : I get 65535 (full 48KBytes), or 32767 (upper 32KBytes disabled).
However, after this it seems things started to go awry (nothing to do with Ivan's board).
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Music by Karl Casey @ White Bat Audio
karlcasey.band...
/ @whitebataudio

[@Brfff]

07:46 ... okay, having the documentation for Retroleum's diagnostics ... this could indicate an issue with the lower RAM (16KByte) multiplexors ... ICs 3 & 4, both 74LS157s, next to the ULA

[@WacKEDmaN]

interesting symptoms... and nice lttle RGB addon.. ..CPC needs one of them VLAs with extra features... 256 color out would be schweet!..i actually read earlier today someone is working on one using an arm micro instead of a FPGA solution...
so when are you gonna design and build your own.. "Yet Another Speccy Clone" ? :P

[@charlesdorval394]

I would have thought you already had all the various possible cables already made.. :P

[@Brfff]

Yes, I should (with my +2A) … but do you think I could find the bloody thing! And I don’t normally have DIN8 plugs as none of the other machines use that size, so I had to order one just to make up a new cable

[@MrWaalkman]

That's a nice little setup. Tidy!