>>On schematic pdf.pg4, what looks to be the video pixel stream SOVSR (from IC436 on pdf.pg3) feeds an OR gate of IC433, thence inverter in >>IC450, XOR in IC434, combined with some other stuff in OR gate IC464, resistive mixing with the composite sync CMSNC, to produce the >>composite video CMPVD.
>>The XOR of IC434 is the point of interest, the state of the other input of that XOR gate will invert-or-not the pixel stream. >>It is labeled as being fed from RVF (ReVerse Field or Frame?). >>I haven't looked at the schematic long enough to find the source of RVF. Hardware configuration? Bit of an IO port from software configuration? >>But I don't know, going from your pic isn't their a lot more screwed up than just reversed video? Hi Brent, It's nice to know my musings last night were on the right track with IC434. I'd traced the signal back from the composite out and found SOVSR (embarrassingly I didn't work out that CMPVD could mean Composite Video) and decided that RVF must be a reverse video signal. RVF comes from IC426 (LS86) which uses a signal called RF as a source. RF comes from IC427 (LS174 flip-flop) pin 15, the input for that is pin 14 (A0). However, the clock for IC427 is missing and THAT comes from pin 12 of IC430 (LS138) whic is marked TVTL. I didn't get any further than that. Cheers! On 25 September 2017 at 09:48, Brent Hilpert via cctalk < cctalk@classiccmp.org> wrote: > On 2017-Sep-25, at 12:42 AM, Adrian Graham via cctalk wrote: > > I’ve been troubleshooting another one of “uncle” Clive Sinclair’s > offspring (though most folk don’t know his involvement), the Grundy > Newbrain. It’s a typical early 80s micro with Z80 and 32K RAM made up of > 16x 4116 DRAM. > > > > What’s uncommon with this particular one is that it works….mostly. The > annoying thing with the fault with this one is that I watched it happen - > the whole screen is inverted so instead of white-on-black I now get > black-on-white with garbled characters. > > > > Initially the display was fine but with the rightmost line of pixels > missing from each character so I suspected the character ROM (2332 mask > PROM) or 74LS166 bit shifter that supports it, but then as I watched the > whole screen inverted and stayed that way, see pics. > > > > I’ve attached the schematic of the board, but for now my general > question is what could cause the whole video stream to be inverted? ISTR > that with the ZX80 you could invert the screen with a jumper that switched > output from a 74LS165 bit shifter from the Q to /Q pins but the 74LS166 > doesn’t have an inverted output, unless I’m reading the datasheet wrongly > of course :) > > > > http://www.binarydinosaurs.co.uk/newbrain-05.jpg < > http://www.binarydinosaurs.co.uk/newbrain-05.jpg> > > http://www.binarydinosaurs.co.uk/newbrain-14.jpg < > http://www.binarydinosaurs.co.uk/newbrain-14.jpg> > > http://www.binarydinosaurs.co.uk/newbrain.pdf < > http://www.binarydinosaurs.co.uk/newbrain.pdf> > > > Howzabout: > > On schematic pdf.pg4, what looks to be the video pixel stream SOVSR (from > IC436 on pdf.pg3) feeds an OR gate of IC433, thence inverter in IC450, XOR > in IC434, combined with some other stuff in OR gate IC464, resistive mixing > with the composite sync CMSNC, to produce the composite video CMPVD. > > The XOR of IC434 is the point of interest, the state of the other input of > that XOR gate will invert-or-not the pixel stream. > It is labeled as being fed from RVF (ReVerse Field or Frame?). > > I haven't looked at the schematic long enough to find the source of RVF. > Hardware configuration? Bit of an IO port from software configuration? > > But I don't know, going from your pic isn't their a lot more screwed up > than just reversed video? > > -- adrian/witchy Owner of Binary Dinosaurs, the UK's biggest home computer collection? www.binarydinosaurs.co.uk
