Thread: Mk14 vdu
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Old 15th Oct 2020, 7:30 pm   #410
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
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Default Re: Mk14 vdu

Just started the latest round of testing with V1.02 firmware (with the NENIN mods) back in. First the bad news, no combination of 470pF capacitors from NRDS to GND, NENIN to GND or both has any affect on the symptoms. It was worth a try though.

Now then, this question...

Quote:
Originally Posted by Karen O View Post
SH, did you notice any pattern to the display corruption? The first line of text in each half, i.e. lines 1 and 16, are fetched separately from the rest. Are they stable or addled?
....unsurprisingly, proved quite astute. I didn't notice it before but yes, those two lines are stable and furthermore I can edit the contents using the MK14 monitor and they display as edited. The same applies to the first TWO characters on lines 2 and 17, but not the rest of the characters on those lines or the remaining 14 lines per half screen.

I've also noticed that all of the other characters are not completely chaotic, each character is in fact alternating between just two characters although they are doing it in a kind of 'Mexican wave' ripple pattern sweeping continually down across the screen.

Changing to graphics mode, it looks as though I can edit the very first part of each half of the screen which is static, with the rest of the pixels constantly flipping states just as in character mode, but there is one extra anomaly in graphics mode - every other screen seems to be getting rendered half a graphics pixel width offset to the right so the overall image is constantly jumping slightly left/right by half a graphics pixel width.

Still in graphics mode, Looking at the bright vertical line over on the left hand side that too seems to have a transparent extra half pixel width being added onto the right hand side of it, but only in the vertical interval occupied by the 'active portion' of the screen.

The attached photo will hopefully illustrate it better, look carefully at the bright line on the left hand side and you can see it looks thicker inside the vertical range occupied by the 'active' area of the screen, although the extra bit on the right hand edge is transparent as though it is only there every other frame. You probably won't be able to see that the left and right edges of the main graphics block are also transparent to a depth of half a graphics pixel width because the whole screen is zipping back and forth half a graphics pixel width.

If you can work out what is causing this half pixel offset on every other graphics frame, fixing that might well fix the problem in character mode as well.

In character mode, the LHS bright line is 'normal' thickness all the way down, that is, about one graphics pixel width wide.

Hope this gives you some ideas.

You will have noticed from all of the foregoing that the MK14 itself is working virtually normally, except that the two normally empty characters between the address and data fields on the display have a significant about of 'noise' on them so the VDU is upsetting / clashing with the MK14 a little bit, but not enough to make the MK14 fall over.
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