----- Original Message ----- From: ""André Fachat"" <afachat@gmx.de> To: <cbm-hackers@musoftware.de> Sent: Sunday, May 22, 2011 6:43 AM Subject: Re: PET 2001 fix Part 3 - RAM/ROM board etc. > Here (C9 pin 8, as well as pin9) it would be interesting to see where zero > potential is. Both signals show a slower delay when going down, with a > noticeable slope, compared to the working PET. The question is what do the > receivers of that signal see. At what voltage level does the slope start? > If that voltage is high enough, the receivers may see a delayed or spiked > signal. Note the reason for the slope could be a weak driver in C9, but > also strong pulls from aging receivers on that line (although, as the > slope appears even in the unconnected pin 9, C9 may be the cause). ------- When we started this I noticed that pulling C9-9 high in a working PET replicated Philip's symptoms (even though it's not connected to anything) so I also suspect C9. Philip and I have spent a little time privately getting his scope and probes set up so we could look at the signals and especially the baseline voltages on pins 8,9 and 10; looks like he's finally got everything adjusted and connected properly and they do inded look a little suspicious. A word of caution: don't sub the 'LS93. Not only are its power pins non-standard, but different versions have a different pinout. ****************************************************** > > I support Ruud's question " about D5 pin 9 and E6 pin 23"? > > You seem to have two probes (to display old and new PETs on one screen). > Do you have a decent trigger on the 'scope? Then you could trigger on > DIS.ON going high (i.e. start of a raster line), and look for example at > the address line signals. With a time base of 1us/cm you can see each > cycle, and should easily see for example A0 (D5 pin 5) staying low 2us as > the screen display suggests - or going high after 1us and going back > possibly due to a spike in DIS.ON connected to the reset line of the D5 > flipflop. You should be able to get a stable trigger on DIS.ON even if is > has a spike if you vary the trigger voltage - but not getting a stable > trigger is one more indicator for a spike problem on DIS.ON. > > André > > > > -------- Original-Nachricht -------- >> Datum: Sun, 22 May 2011 16:28:28 +0900 >> Von: Philip Lord <random6000@mac.com> >> An: cbm-hackers@musoftware.de >> Betreff: Re: PET 2001 fix Part 3 - RAM/ROM board etc. > >> So I've busted out the old scope that was given to me many years ago. >> I've >> never used a scope before, so with the help of MikeS I think I have it >> somewhat calibrated. >> >> I've posted some videos showing the working board (top) compared to the >> non-working board (bottom). Presently I have only tested C5 pin 5, C9 pin >> 8, >> 9 and 11. >> >> I've posted video's here again: >> >> http://web.me.com/lord_philip/other_computers/20012.html >> >> Phil >> >> >> >> >> On May 21, 2011, at 11:45 AM, Philip Lord wrote: >> >> > Ok...I looked at the archived list here: >> > http://www.softwolves.com/arkiv/cbm-hackers/15/date.html >> > ...and found the message I missed (very strange indeed). >> > >> > Here they are if others have also missed them: >> > >> > >> > I admit I have to speculate a bit. The schematics of the PET clearly >> remind me of what's going on inside the 6545 (see for example here >> http://www.6502.org/users/andre/hwinfo/crtc/internals/index.html >> > where I've analyzed the CRTC internals). I immediately recognized the >> 74100 as the feedback register ... the purpose of this register is to >> store >> the memory address of the first character in the line. >> > >> > While displaying a line, the video circuitry first loads this memory >> address into the registers D6/D7, which get incremented every cycle. When >> 40 >> characters are shown, the "DIS ON" (from C5 in the horizontal timing >> circuitry) signals goes off, but the counter still counts on. >> > >> > The counter is reloaded with the DIS ON signal. I.e. every time a new >> raster line starts, the memory address counter is loaded new from the >> 74100 >> latch. If the latch would not be reloaded, the screen would show a single >> line over and over. >> > >> > The characters are made by using the same memory address, but selecting >> different raster lines addresses in the character ROM A2. >> > >> > Only if the three raster line counters (output of A1, input into the A2 >> 6540 charrom) are all ones (7 = last rasterline of a character), AND the >> DIS ON signal is on (not sure about the exact timing though) the buffer >> 74100 >> is loaded with the next address (triggered via the C7 NAND - shown as OR >> with inverted inputs) - the last character address in the current line + >> 1, >> which is the first address in the following character line. This is how >> lines are counted in memory. >> > >> > In the current problem we see the following: >> > - the character lines still show 40 characters >> > - For each line, the character address is advanced only 36 characters. >> So the 4 last characters of a line are (somewhat) repeated in the first 4 >> characters of the next line >> > - It seems the first character of each line is repeated >> > - This happens per character line, not per raster line >> > >> > So the memory map in screen coordinates would be: >> > >> > 0 0 1 2 3 4 5 6 .... 35 36 37 38 >> > 36 36 37 38 39 ... >> > 72 72 73 74 ... >> > >> > As I said this happens per character line, which is interesting, as the >> "mixup" of memory addresses must happen each raster line. >> > >> > Now I see that the 74100 and D6/D7 counters actually loads the _upper_ >> > 8 >> bits of the 10 bits video memory address. The lower two bits are handled >> by the D5 dual-JK-flipflop and the following multiplexer at D2. >> > >> > Looking at the fact that the lower two bits show a strange behaviour in >> the memory map: >> > >> > 00 00 01 10 11 00 ... >> > 00 00 01 10 11 00 ... >> > >> > my assumption is there is a spike in the DIS.ON line within one cycle >> after it becomes active that clears the lower two address bits (so the >> "00" >> is repeated). This would also explain the 36 character line length. Due >> to >> the repeat of the first cycle, the last character of a line has an >> address >> of 38, the one after that (which is stored in the latch(!)) has 39, which >> is >> binary 0000100111. Of that the upper 8 bits are stored in the latch for >> the next line, and the lower two (in the D5 flipflops) are cleared (as >> they >> are always 0 at the start of the line for 40 chars/line). So after >> clearing >> the lowest two bits is 00100100 = 36 - the starting memory location for >> the >> next line! >> > >> > So the summary is: look for a spike in the DIS.ON signal. Maybe even >> > try >> to suppress it (temporarly) with a small capacitor to ground to see if >> the >> problem goes away. >> > >> > Why it is there I don't know. The signal is being produced by C5, a >> 74LS107. >> > >> > Hope this helps >> > André >> > >> > ____ >> > >> > Excellent analysis! >> > >> > I'd also gotten to the point of suspecting the DIS ON signal that loads >> the >> > latches and especially D5 since it holds the two low bits, but it >> > hadn't >> > occurred to me that the first character is at offset 39 instead of 40 >> and >> > that resetting D5 effectively subtracts 3; brilliant! >> > >> > I was also trying (unsuccessfully) to find some logical fault that >> > would >> > create an extra pulse on DIS ON, but maybe it is just a spike as you >> > suggest. >> > >> > It would be nice to know for what diagnostic procedure the TPs are >> > intended... >> > >> > Time for Philip to get out his 'scope and look at C5 pin5 (and compare >> to >> > the good PET). >> > >> > m >> > >> > >> > >> > >> > >> > On May 21, 2011, at 8:31 AM, Philip Lord wrote: >> > >> >> Hi again, >> >> It seems that I've haven't gotten a couple of emails in this thread. >> They contain the messages: >> >> >> >> "Excellent analysis!" >> >> >> >> and >> >> >> >> "So the summary is: look for a spike in the DIS.ON signal. Maybe even >> try >> >> to suppress it (temporarly) with a small capacitor to ground to see if >> the >> >> problem goes away." >> >> >> >> Sorry, I'm not sure who these mails came from, but unfortunately I >> never received them and only know of their existence from the response >> from >> André. Can someone send please forward the missing mails. >> >> >> >> Thanks again >> >> >> >> Phil >> >> >> >> >> >> On May 21, 2011, at 7:09 AM, André Fachat wrote: >> >> >> >>> >> >>> >> >>>> Excellent analysis! >> >>> >> >>> Thanks, that was a tricky one. I missed the important point only >> >>> until >> I realized that the 74100 actually does NOT store the lowest two bits >> because it handles the upper 8 bits and not the lowest 8 bits of the 10 >> bit >> video memory address. >> >>> >> >>> I only hope it helps. I'm looking forward to the 'scope analysis. >> >>> >> >>> André >> >>> >> >>> -- >> >>> NEU: FreePhone - kostenlos mobil telefonieren! >> >>> Jetzt informieren: http://www.gmx.net/de/go/freephone >> >>> >> >>> Message was sent through the cbm-hackers mailing list >> >> >> > >> > >> > Message was sent through the cbm-hackers mailing list >> >> >> Message was sent through the cbm-hackers mailing list > > -- > Empfehlen Sie GMX DSL Ihren Freunden und Bekannten und wir > belohnen Sie mit bis zu 50,- Euro! https://freundschaftswerbung.gmx.de > > Message was sent through the cbm-hackers mailing list Message was sent through the cbm-hackers mailing listReceived on 2011-05-22 16:00:20
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