+batari Posted February 4, 2009 Share Posted February 4, 2009 Been kicking the 7800 version of the mod around. Seems the biggest issue is separating the 7800 and 2600 chroma lines from each other. The Luma lines go through a NOR gate (U3). I am assuming the NOR gate separates the 7800 and 2600 lines. When the 7800 mode is on what is being sent by the TIA? 0 or 1? When the 2600 mode is on what is being sent by the MARIA? 0 or 1? If it is a NOR gate then the "inactive" mode has to be set to low to even get a difference in the output but it is inverted. I guess I can head to the EE lab on Friday (have those off!) and figure it out if no one knows. Then I have to figure out how to detect when the board is in 7800 mode. There has to be a signal somewhere that is high when 7800 mode is enabled and low when 2600 (or vise versa?). If I can figure this out then it will be pretty easy to let the Mod know when to switch to 2600/7800 chroma line. Anyone with more knowledge of the inner secrets of the 7800 would like to share? edit// I guess one way for the mod to know is to figure out how to read when the 7800 kicks the bios screen into gear. I have found this before. I'm looking at the 7800 schematics now and I'm pretty sure it's the '174 at U11 that selects 2600 mode. I believe that pin 2 will be high in 2600 mode and low in 7800 mode. I think there are other pins you can tap into as well. Quote Link to comment Share on other sites More sharing options...
Longhorn Engineer Posted February 4, 2009 Author Share Posted February 4, 2009 I have found this before. I'm looking at the 7800 schematics now and I'm pretty sure it's the '174 at U11 that selects 2600 mode. I believe that pin 2 will be high in 2600 mode and low in 7800 mode. I think there are other pins you can tap into as well. Cool Beans looks like I have a starting place on Friday then! Quote Link to comment Share on other sites More sharing options...
Longhorn Engineer Posted February 5, 2009 Author Share Posted February 5, 2009 Well! I did some digging around on the Atari 2600 mod and realized that the Luma signal is delayed by 140ns because of the buffer chip! This is probably the reason why the composite image created by the FMS6400 had allot of bleed. I am guessing this delay caused a problem with the combining of the two signals. Reading a couple articles a delay of over 20ns causes issues with video quality. I found a replacement part and will be ordering it to replace the other Cd4050s. It is a direct drop in so no change to the layout of the board is needed. CD74HC4050E - Gate speed is 6ns @ 5V. I will be testing this theory on my Atari by ramping up the voltage to 15V for the CD4050. According to the spec sheet. The higher voltage will decrease the gate delay to 40ns. If there is an improvement in the image I will be changing the partslist and schematics to reflect this change. I will be going to the EE lab tomorrow to scope out the 7800. Quote Link to comment Share on other sites More sharing options...
Longhorn Engineer Posted February 6, 2009 Author Share Posted February 6, 2009 (edited) Ok I bumped the voltage and there was a noticeable difference in the composite (none for the S-video signal tho). I am going to return the old CD4050s and order the CD74HC4050E. They work exactly the same as the old CD4050s just faster. Edited February 6, 2009 by Longhorn Engineer Quote Link to comment Share on other sites More sharing options...
Ransom Posted February 6, 2009 Share Posted February 6, 2009 (edited) Awesome! This is going to be one great upgrade. Will this one still have the problem where for some objects in some games every other pixel is background colored rather than the solid line you get with a CRT? Or is that just unfixable? Edited February 6, 2009 by Ransom Quote Link to comment Share on other sites More sharing options...
Longhorn Engineer Posted February 6, 2009 Author Share Posted February 6, 2009 Awesome! This is going to be one great upgrade. Will this one still have the problem where for some objects in some games every other pixel is background colored rather than the solid line you get with a CRT? Or is that just unfixable? The only game I have seen that is like that is Asteroids. And its every other scan line. I haven't seen alternating pixels. Quote Link to comment Share on other sites More sharing options...
Longhorn Engineer Posted February 7, 2009 Author Share Posted February 7, 2009 (edited) Ok after scooping the board I figured somethings out. U3 is a OR gate not a NOR as I have read. Makes allot of sense now. When the 7800 is in 7800 mode the TIA sends 0 to the video and vice virsa for the 2600 mode. I checked every chip on the board and could not find a dedicated pin for reading whether or not the console is in 7800 or 2600 mode. So I plan on using the Sync line for the chroma switch. Anyone know of a way to do this "latch"? Unlike the 2600 mod the 7800 mod will work allot like the 8bitdomain mod. Where you remove the U3 chip and place a socket on the board. The 2600 version will stay with its "wires" because it has to fit in 3 different types of Ataris. Edited February 7, 2009 by Longhorn Engineer Quote Link to comment Share on other sites More sharing options...
+batari Posted February 7, 2009 Share Posted February 7, 2009 I checked every chip on the board and could not find a dedicated pin for reading whether or not the console is in 7800 or 2600 mode. So I plan on using the Sync line for the chroma switch. Anyone know of a way to do this "latch"? I'm 100% sure that pin 2 of U11 is it. Pin 9 of U12 will also work. Look at the schematic. When you write a value, with bit 0 set, to address $0001 (INPTCTRL) while in 7800 mode, the flip-flop at U11 will be latched permanently to whatever state you set. The 7800's BIOS writes $1D to set 2600 mode, which also disables the MARIA, enables the TIA, floats A12-A15 on the 6502, and disables all RAM except RIOT RAM (from other outputs of U11.) The flip-flop is fixed because its output at Q1 (pin 2) feeds an OR gate, which feeds the clock. If this ever gets latched high, there is no way to ever change the clock again. Quote Link to comment Share on other sites More sharing options...
Longhorn Engineer Posted February 7, 2009 Author Share Posted February 7, 2009 Well the sync lines will work just as well to. Plus the video mod will already have those signals on the board so thats one less wire. I will look at that pin again though. Quote Link to comment Share on other sites More sharing options...
+batari Posted February 7, 2009 Share Posted February 7, 2009 Well the sync lines will work just as well to. Plus the video mod will already have those signals on the board so thats one less wire. I will look at that pin again though. I thought I should mention that the pin won't be high during the 7800 BIOS splash screen, but only a few milliseconds before the 2600 game actually starts. Quote Link to comment Share on other sites More sharing options...
GroovyBee Posted February 7, 2009 Share Posted February 7, 2009 See DanBoris's blog for more information on the latch. Quote Link to comment Share on other sites More sharing options...
Longhorn Engineer Posted February 7, 2009 Author Share Posted February 7, 2009 Using both an oscilloscope and multimeter there is no difference between the signals coming off of the LS174 pin 2 chip when in either 2600 or 7800 mode. When in 7800 mode it does stay at 0V till the bios ends then jumps back to 5.5V. I need a constant state signal. The Sync lines provide this. Quote Link to comment Share on other sites More sharing options...
+batari Posted February 7, 2009 Share Posted February 7, 2009 Using both an oscilloscope and multimeter there is no difference between the signals coming off of the LS174 pin 2 chip when in either 2600 or 7800 mode. When in 7800 mode it does stay at 0V till the bios ends then jumps back to 5.5V. I need a constant state signal. The Sync lines provide this.Thanks for testing. The only time the BIOS sets Q1 on the '174 and disables its clock is if it enters 2600 mode. The only way 7800 mode could do that is if the game code itself does it. If 7800 games do that, the only way to get a reliable signal is a combination of two - If pin 2 and pin 5 on the 174 are both high, the MARIA is permanently disabled (2600 mode.) Otherwise, you're in 7800 mode or BIOS. If you don't have an AND gate handy, you can use a couple of diodes. Quote Link to comment Share on other sites More sharing options...
Ransom Posted February 7, 2009 Share Posted February 7, 2009 Awesome! This is going to be one great upgrade. Will this one still have the problem where for some objects in some games every other pixel is background colored rather than the solid line you get with a CRT? Or is that just unfixable? The only game I have seen that is like that is Asteroids. And its every other scan line. I haven't seen alternating pixels. Hmm. I always assumed it was due to something like goes on with the 8-bit computer line, with "artifacting" or blending of colors by careful selection of every-other-pixel coloring. Mebbe it's just my LCD TV. But it only happens when using the 8-bit-domain mod, not with RF. Has anyone else encountered this effect? Quote Link to comment Share on other sites More sharing options...
+batari Posted February 7, 2009 Share Posted February 7, 2009 Using both an oscilloscope and multimeter there is no difference between the signals coming off of the LS174 pin 2 chip when in either 2600 or 7800 mode. When in 7800 mode it does stay at 0V till the bios ends then jumps back to 5.5V. I need a constant state signal. The Sync lines provide this.Thanks for testing. The only time the BIOS sets Q1 on the '174 and disables its clock is if it enters 2600 mode. The only way 7800 mode could do that is if the game code itself does it. If 7800 games do that, the only way to get a reliable signal is a combination of two - If pin 2 and pin 5 on the 174 are both high, the MARIA is permanently disabled (2600 mode.) Otherwise, you're in 7800 mode or BIOS. If you don't have an AND gate handy, you can use a couple of diodes. I'm replying to my own message to say I'm not quite right, 2600 mode could be detected with certainty when pin 2 is high and pin 5 is low. When pin 2 is high, the '174 is fixed and cannot be changed, and when pin 5 is low, the MARIA is disabled, i.e. 2600 mode. Otherwise it's in 7800 mode/BIOS. What is this Sync line and how would that detect the state? Quote Link to comment Share on other sites More sharing options...
GroovyBee Posted February 7, 2009 Share Posted February 7, 2009 I'm replying to my own message to say I'm not quite right, 2600 mode could be detected with certainty when pin 2 is high and pin 5 is low. When pin 2 is high, the '174 is fixed and cannot be changed, and when pin 5 is low, the MARIA is disabled, i.e. 2600 mode. Otherwise it's in 7800 mode/BIOS. Why not use pin 5 on its own? I don't see the need to detect if the latch is "locked" as well. There are only two reasons pin 5 would be low :- 1) Briefly after power on reset (when TIA and MARIA are both disabled). In this case it doesn't matter which chroma is selected because the video output isn't active. 2) When 2600 mode is entered. In either case the signal is "known". The problem as I see it with using the syncs is to detect the difference between pulsed low and permanently low. My first thought would be retriggerable monostables with a time constants a fraction over the NTSC/PAL line frequency (allowing for suitable component tolerances). Quote Link to comment Share on other sites More sharing options...
+batari Posted February 7, 2009 Share Posted February 7, 2009 I'm replying to my own message to say I'm not quite right, 2600 mode could be detected with certainty when pin 2 is high and pin 5 is low. When pin 2 is high, the '174 is fixed and cannot be changed, and when pin 5 is low, the MARIA is disabled, i.e. 2600 mode. Otherwise it's in 7800 mode/BIOS. Why not use pin 5 on its own? I don't see the need to detect if the latch is "locked" as well. There are only two reasons pin 5 would be low :- 1) Briefly after power on reset (when TIA and MARIA are both disabled). In this case it doesn't matter which chroma is selected because the video output isn't active. 2) When 2600 mode is entered. In either case the signal is "known". It might work, but I don't know enough about 7800 games to be sure. I can say that I've looked at the BIOS, and it enters 7800 mode without locking the latch, so in theory it's possible for the 7800 to keep it unlocked and switch between the TIA and MARIA during the program, or to use the TIA exclusively if it wants to. I don't know if either ever happens or if it would adversely affect video if it does. But if pin 5 alone works, it would probably make things a lot easier. Quote Link to comment Share on other sites More sharing options...
Longhorn Engineer Posted February 7, 2009 Author Share Posted February 7, 2009 Scoped pin 5 today and its perfect. With a 7800 cart in the system it starts out low for about half a second then jumps to high. With a 2600 cart in the system it starts out low and stays low. Looks like this is the right pin. The video mod will not lock to 7800/2600 chroma so if a game decides to disable the MARIA and run the TIA it can and the video mod will just swap. Are there any games out there that do this? For the switching I was going to use something really simple like the Ti SN74CBT3306DR. Quote Link to comment Share on other sites More sharing options...
+Mitch Posted February 7, 2009 Share Posted February 7, 2009 The video mod will not lock to 7800/2600 chroma so if a game decides to disable the MARIA and run the TIA it can and the video mod will just swap. Are there any games out there that do this? I believe the 7800 diag cart is the only one that does it. Mitch Quote Link to comment Share on other sites More sharing options...
Longhorn Engineer Posted February 8, 2009 Author Share Posted February 8, 2009 The video mod will not lock to 7800/2600 chroma so if a game decides to disable the MARIA and run the TIA it can and the video mod will just swap. Are there any games out there that do this? I believe the 7800 diag cart is the only one that does it. Mitch Sounds good. Whats the easiest way to obtain a 7800 diag cart? I am going to bet they are pretty expensive to purchase. I think I may have access to a Rom burner in the EE lab. I should pick up a 2600 one while im at it. Quote Link to comment Share on other sites More sharing options...
+Mitch Posted February 9, 2009 Share Posted February 9, 2009 The video mod will not lock to 7800/2600 chroma so if a game decides to disable the MARIA and run the TIA it can and the video mod will just swap. Are there any games out there that do this? I believe the 7800 diag cart is the only one that does it. Mitch Sounds good. Whats the easiest way to obtain a 7800 diag cart? I am going to bet they are pretty expensive to purchase. I think I may have access to a Rom burner in the EE lab. I should pick up a 2600 one while im at it. I suppose it depends what you consider expensive. I think Best Electronics sells them for around $35, they also have the 2600 version. You should be able to build one yourself if you have an EPROM programmer though. Mitch Quote Link to comment Share on other sites More sharing options...
Longhorn Engineer Posted February 9, 2009 Author Share Posted February 9, 2009 Atari 2600 PCBs have been shipped from China. Two days ahead of schedule! Quote Link to comment Share on other sites More sharing options...
nathanallan Posted February 9, 2009 Share Posted February 9, 2009 (edited) I've been watching this thread and am now attempting my own AV mod on a 2600 I have here. Thanks for the inspiration, LE. Edited February 9, 2009 by nathanallan Quote Link to comment Share on other sites More sharing options...
Longhorn Engineer Posted February 9, 2009 Author Share Posted February 9, 2009 I've been watching this thread and am now attempting my own AV mod on a 2600 I have here. Thanks for the inspiration, LE. No problem Nathan. I checked UPS tracking and the PCBs will be here on Wednesday (2/11). So I will spend the weekend making 3 guides for the 6-switchers, 4-switchers, and Juniors 2600 models. There will also be a guide on how to put together the Unassembled kits. Then I will ship out the Bare PCBs and Unassembled Kits. Assembled kits will take a bit longer but I should have them ready in another weekend. I will post pictures of the PCBs when they come in. Quote Link to comment Share on other sites More sharing options...
Longhorn Engineer Posted February 11, 2009 Author Share Posted February 11, 2009 By the way if anyone can help out with this. It would be great! Quote Link to comment Share on other sites More sharing options...
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