6847 Tester

for bbc micro/electron hardware, peripherals & programming issues (NOT emulators!)
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bprosman
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6847 Tester

Post by bprosman » Mon Mar 25, 2019 7:29 pm

Found this "on the net", does anyone can (and wants to) explain briefly how they work ?
mc6847test.png
And the 2nd (IC1 is an Eprom).
55719298_698983820503780_7307952107582128128_n.png
Thanks a lot for the effort.

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IanS
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Re: 6847 Tester

Post by IanS » Mon Mar 25, 2019 8:51 pm

The first one just uses the address outputs to feed data into the data pins, so you'll see a binary count on the display output. In text modes it will just display the character set. e.g. http://homebrewcpu.blogspot.com/2009/02 ... rator.html
The second one appears to be using a ROM (the 2732) to display a number of static test screens. I haven't looked in detail at the circuit so it may be doing something more clever.

Prime
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Re: 6847 Tester

Post by Prime » Mon Mar 25, 2019 11:16 pm

First circuit : what Ian said......

The second one is the cicuit used in the Dragon200e, which adds an external character ROM to the 6847 to define an external character set for the text modes.

The data from the video memory along with a 4 bit character row counter (row of pixels within a character), is used to address the character rom, the output from this is the pattern of pixels that is displayed on the screen. The second circuit along with the RGB circuitry from the AtomColour is the basis of the DragonRGB board some of you may have seen my pictures of on Facebook.

You will note that all but the bottom address line from the 6847 are not connected to anything, because in the Dragon and CoCo the video ram is addressed and controlled by an external chip (the Syncronus Address Multiplexer), which also does the memory refreshing, some decoding, clock generation and arbitration.

I did considder trying to shoehorn this circuit onto the AtomColour, but to do so would be quite involved due to lack of available space within the Atom.....

Cheers.

Phill.

bprosman
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Re: 6847 Tester

Post by bprosman » Tue Mar 26, 2019 10:35 am

Thanks both for your very valuable reply.
Last edited by bprosman on Tue Mar 26, 2019 10:35 am, edited 1 time in total.

bprosman
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Re: 6847 Tester

Post by bprosman » Wed Jun 12, 2019 12:27 pm

Would this also work for the 6845 ?

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