Teletext Experimenter’s Board – Connecting to RGB displays

Alternative SCART Wiring

Not everyone will need to connect the Experimenter’s Board to a display (if you only need it for decoding pages to your computer, for example) but I suspect most people will.

There are plenty of variations of display that people might want to use, and I can’t cover them all … but hopefully, I can provide enough examples to cover most people. So this blog post is about the RGB output from the Teletext Experimenter’s Board, and how you might connect it to a display you have.

The display output

The Board outputs R, G, B and composite-sync signals, at standard 15kHz refresh rate. This is the same refresh rate as 8-bit and 16-bit era computers, so if you have an RGB monitor for your BBC Micro, Amstrad, Sinclair, Amiga, Atari, Sega Megadrive, Sam Coupe, etc, then it’ll probably work.

Collectors of Acorn computers may note that the pinout I’ve chosen for the 9-way connector happens to match the RGB-output for the A3xx/A4xx/A3000 machines. I just picked the standard with which I was most familiar.

Using the pin-headers

Although I provide a 9-way D-SUB connector with the kit, you don’t have to use it. I recommend it if you’re going to do a lot of plugging/unplugging, or if the board is going to be used in its own right as a Teletext-experimenter’s gadget … but if you’re planning on fitting this inside one of your other projects, then it might save you some space and weight to not fit the connector, and just use the pin-header instead. Electrically, there’s no difference.

CM8833 / 1084 monitors

The CM8833 (which, as far as I can tell, is pretty-much identical spec to a 1084) is a very common 14-inch CRT monitor beloved by retro enthusiasts. It is ideal for this board.

If yours has a SCART connector then continue to the next section. But if it has a 9-pin D-SUB connector, then you’ll want to make a cable with a 9-pin D-SUB male connector on both ends, following this wiring:

Note that as this cable will have identical connectors on either end, be sure to label which is for the monitor and which is for the teletext card.

RGB SCART monitors

If you’re in the UK or France, then your monitor may well have a SCART connector. (I hate SCART, but that’s a rant for another day.) SCART connectors have provision for RGB signals, but it’s up to the monitor manufacturer about whether your model supports it.

But assuming it does … here is the cable you’ll want to make:

(Taken from Acorn User magazine, December 1991)

Alternative SCART Wiring

(Update, 30th May 2020) I’ve discovered an alternative wiring of a SCART cable amongst my retro-computer collection. This version came with my yellowing Archimedes A3000. I have quickly modified the previous image, so it’s easier to see the differences:

In this version, pin 5 on the DIN isn’t used at all, there is no resistor, and instead the composite-sync signal is wired to SCART pins 20 and 16.

Also note that the earth connection is going to pin 18 (“RGB blanking ground”) instead of 13, 9 and 5 (independent ground connections for R, G and B) … though to be honest, I suspect that in the vast majority of SCART monitors, all those ground pins are connected together anyway.

I include this alternative wiring here because it makes a big improvement to picture quality on one of my monitors … but no difference at all to the others. I strongly suspect this is a difference between TTL RGB and Analogue RGB, and some of my monitors are more forgiving than others. I invite people to try both, and see which one they prefer.

HDMI Upscalers

These are very useful gadgets to have in your arsenal of misc-geek-tools. They allow you to connect older RGB hardware (like retro consoles, or this project) to HDMI displays. I own one of these, which can be had for around 25 pounds on Amazon:

This particular model has a RGB-capable SCART input, so just refer to the previous section for cable-wiring details.

Monitor compatibility

I’ve successfully tested it with a 1084S/CM8833-II, a Samsung 32-inch LCD TV, a cheap Polaroid integrated LCD TV+DVD player, a cute clip-on screen for a Sony PSOne, and the above SCART-to-HDMI upscaler box connected to a HDMI monitor. Those all worked fine.

I also tested it with an elderly Samsung LCD TV, and the picture appeared but suffered quite a lot of “ghost” lines. As this is the only monitor I’ve seen this issue with, I’m inclined to blame that TVs temperamental analog-to-digital hardware … but if you happen to be very experienced with RGB electronics and can see that I could make my circuit a little more forgiving, then let me know!

Click here to visit the Teletext Experimenter’s Board main project page.

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