CB>William Levak wrote: CB>> At least that was a coherent description of the circuit. CB>> CB>> I don't see where there is a problem. We have 3 TTL level chips connected CB>> together. Typically a TTL chip output can drive 10 devices. There is no CB>> potential for chip failure here. Remember, these are not unlimited output CB>> drivers. for instance, a 74LS20 will output 0.4 milliamps, maximum, CB>> whereas it's input current is 0.1 milliamps. The 6522, on the other hand, CB>> can handle 1.6 milliamps. If the 6522 is set to input, it will clearly CB>> pull the signal to it's level, whether high or low. The signal will then CB>> either be close to zero volts or five volts. This signal is fed to a ramp CB>The Problem is that if that would be true, then the poke would not even CB>work on the original PET. As I said in the description, in the original CB>PET the Sync signal (produced by an LS08) and read by VIA PB5 is fed CB>to a 74LS20. This LS20 blanks the video signal during off-screen areas. CB>Would the VIA, if set to output, draw the level to close to 0V, then CB>the screen would always be completely blank. CB>Therefore I had a look at the 74LS08 datasheets (see for example CB>http://www.fairchildsemi.com/pf/DM/DM74LS08.html) and it indeed states CB>that the chip can source 0.4mA on high output. But when the VIA draws CB>up to 1.6mA, the LS08 would surely think this is a short circuit, and CB>happily source up to 20-100mA short circuit current [1]. I guess that's TTL is current sourcing logic, isn't it? So, the current would be limited to what could flow through the internal pull-up resistor from the +5vdc rail to ground. CB>more than the VIA can handle. As I said, not a healthy situation. CB>From the fact that the LS20 enables the screen as it should we can then CB>draw the conclusion that the voltage level on the line is CB>still above approximately 2V, which is the minimum "safe" voltage CB>for a "1" (could be less, depending on tolerances, etc. max "0" voltage CB>is 0.8V). But it may be close. CB>> generator. The actual voltage level is not relevant. all that is CB>> necessary is that it is high enough to trigger the ramp generator. The CB>> ramp generator controls the output to the deflection yoke by a feedback CB>> network. You cannot overdrive it. The only potential problem is to drive CB>> it at a frequency higher that it is intended for, and thus supplying a CB>> higher effective power output to the deflection yoke. But, the feedback CB>> circuit limits this also. CB>I don't know much about ramp generators either. But some plausibility CB>checks: CB>The VDrive input of the early 8032 schematics (see CB>http://www.funet.fi/pub/cbm/schematics/computers/pet/8032/321448.gif CB>expects 5V for onscreen and 0V for flyback. CB>As long as the Vdrive input is (much?) higher than 1.93V CB>a current is simply integrated to get the ramp voltage (see oscilloscope CB>point (4)) A diode (D602) lets the TTL output draw the charge of C601 and th CB>ramp generator goes to 0 when it is time (see oscillscope picture (3) & (4)) CB>The DC voltage at (4) is around 1.93V (so the schematics says). CB>Now apply the poke to set Via PB to low output. If CB>the ramp goes a bit above the 1.93V, and then probably above the CB>voltage of the TTL level (that might be lower than 5V because the VIA CB>draws it) the flyback could be triggered earlier, because the diode CB>drains C601 much earlier than it should. CB>> This is about as much as I know about ramp generators. All I can do is CB>> repeat what several engineers have said to me. If you try to run the ramp CB>> generator at too high a frequency, it will simply not trigger and you will CB>> get no output. CB>Don't forget that we have to handle pretty old stuff, no CPUs that CB>analyze the video signal and check the timings or so, just simple CB>analog electronics. Probably you could ask those engineers to CB>analyze this analong circuitry with the schematics? CB>Later (see CB>http://www.funet.fi/pub/cbm/schematics/computers/pet/8032/8032034.gif) CB>the analog electronics has been replaced by an integrated circuit, CB>a TDA 1170. This could probably handle the reduced Vsync voltage. CB>Andre CB>[1] Only one pin and not longer than a second, otherwise.... CB>-- CB>Email address may be invalid. Use "fachat AT physik DOT tu-chemnitz DOT de" CB>------Fight SPAM - join CAUCE http://www.cauce.org------Thanks, spammers... CB>Andre Fachat, Institute of physics, Technische Universität Chemnitz, FRG CB> http://www.tu-chemnitz.de/~fachat CB>- CB>This message was sent through the cbm-hackers mailing list. CB>To unsubscribe: echo unsubscribe | mail cbm-hackers-request@dot.tcm.hut.fi. - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail cbm-hackers-request@dot.tcm.hut.fi.
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