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Apple IIgs Text mode doubled
- Thread starter Pitou
- Start date
I had to review again a few times the video as the first few seconds as comes up scrambled on the old OSX 10.4 QuickTime. Now I see the start up screen come up scrambled but not rest of the video where it boots. VLC made me see the beginning.
This is an interesting error.
I think your Apple ROM is either Bad or a couple of pins rotted, not necessarily the Mega II Chip. It would be also why the BASIC from IIgs desktop also fails.
Like I said before, time to get that multi-meter and start checking address lines on the chips. Got schematics?
This is an interesting error.
I think your Apple ROM is either Bad or a couple of pins rotted, not necessarily the Mega II Chip. It would be also why the BASIC from IIgs desktop also fails.
Like I said before, time to get that multi-meter and start checking address lines on the chips. Got schematics?
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I already checked all pins with an oscilloscope. Continuity is good. Signals seem to be on all pins except for 1,2,4 and 6 if I remember correctly. Again, I'm not sure what to expect on these lines.
As for schematics, yes I have them, I even did post them on post #12 of this thread if you want to take a look.
I tried with an emulator to get familiar on how the Apple IIgs boots/works/behaves. It's definitely different from the real machine I got.
I'll work on the rom again and double check everything. I might order another ROM to test.
Thank you.
Pitou!
As for schematics, yes I have them, I even did post them on post #12 of this thread if you want to take a look.
I tried with an emulator to get familiar on how the Apple IIgs boots/works/behaves. It's definitely different from the real machine I got.
I'll work on the rom again and double check everything. I might order another ROM to test.
Thank you.
Pitou!
I'm looking at the schematics, and pins 2 (MD7), 4 (MD5), and 6 (MD3) on the Mega II Chip are MD lines (memory data?). They head to the MDBus and to UI5, which is a 74HCT245 on pins 18,16,14 respectively. These pins are labeled "B".
From the 7HCT245, it goes to, well, that part is snipped. from the image. But the 245 looks like a buffer chip, so the lines goes across from a B-side to the A-side and is bi-directional.
From the B-Side pins 18, 16, and 14 goes to A-Side pins 2, 4, and 6 respectively.
If the A-side of the chip are address lines, then this area is likely to be the problem:
Lines from the Mega II to the 74HCT254
The 74HCT245 itself
The A-side of the 74HCT245 address (?) lines.
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The address/data bus to the MegaII VLSI is dead on, otherwise you wouldnt be able to generate any legible characters.
The chip very well could be bad. But it would be odd, but not out of the question. Also that chip has a speaker output for some weird reason, even though the IIgs has its own audio hardware.
Go figure.
The chip very well could be bad. But it would be odd, but not out of the question. Also that chip has a speaker output for some weird reason, even though the IIgs has its own audio hardware.
Go figure.
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The Ensoniq 5503DOC does not produce "classic" 8-bit Apple II sound. The clicker speaker logic is located in the Mega II. This is why when you use a stereo sound card, you don't get 8-bit Apple software's beeps and boops on the output since those cards connect directly to the DOCs output.
It's OK. Pin 1 on most ROMs (up to 64K) is either ground or +5v, and about 24 pins. 128K, 256K and 512K ROMs Pin 1 is usually an address pin, depending the ROM size. They are to up 28 pins in size. Usually.
I'll check the schematics when I get home. Right now I'm at the doc office with my travelling Mac and do not have the schematics with me.
I'll check the schematics when I get home. Right now I'm at the doc office with my travelling Mac and do not have the schematics with me.
The "ctrl-reset" is what you use if you're booting with no bootable media in any drive. Are you saying that you're doing crtl-reset (power key on an ADB keyboard) while looking at the moving boot-bars and it's not dropping you to the prompt? That's... unusual.
Anyway, given the evidence here I'm totally going to venture that your Mega II chip is bad. The close-up photo of your display shows that the top four rows of pixels in each character are being doubled into the bottom four lines of each 8 pixel tall character cell. The text display on an Apple II (and most other machines with "character-based" display modes) is generated with the aid of a counter which is connected to some of the pins on the character generator ROM; for instance, in the case of a "real" Apple II with discrete components a 3 bit "row counter" is connected to A1-A3 of the 2513 character generator while the state of remaining lines is generated by the value fetched from the video buffer RAM. Meanwhile, the output lines from the character generator are connected to a parallel-in->serial-out shift register which is connected to the pixel clock. The dance that happens when generating a line of text characters on the apple II thus looks like this:
1: A "Video Address Generator" circuit walks through 960 memory locations in 24 blocks of 40 linear addresses. (or 1920 cells in 80 column mode, 24x80.) Each block of 40/80 addresses is repeated 8 times (controlled by the 3 bit row counter) before moving to the next block of 40/80.
2: A byte is fetched from the RAM location pointed at by the VAG, and is fed into the high address pins of the character generator, which the low bytes are slaved to the row counter. The output lines of the CG (which is of course just a ROM) produce one line of bits making up the character and shoves them into the shift register.
3: The bits are clocked out as individual pixels. When enough pixels for the character have gone out the VAG updates and the next byte is fetched.
4: When the end of the line of pixels is reached the row counter is incremented and the VAG counts back to the start of the line addresses; once the horizontal blanking interval is over and the electron beam is back at the beginning of the row the same block of characters is run through again, this time displaying the pixels for the next line down.
5: When the row counter rolls over the VAG moves to the next line, allowing the next line of characters to be painted.
6: Wash, rinse, repeat, until all 24 lines have been walked through; reset the VAG and row counter, enter the vertical blanking period, and wait for the next frame.
I think your MEGA II is fouled up so the 3rd line on the three bit row counter is stuck. (Or, perhaps more likely, the address line on the character generator ROM is.) The reason I believe it's inside the Mega II and not elsewhere is because there doesn't appear to be a dedicated "character ROM" address/data bus on the Mega II, which means that either the character ROM *is* internal to it or the data is fetched from the general purpose ROM over the regular bus, and if *that* had a stuck bit the machine almost certainly wouldn't work at all. Meanwhile the graphics modes work fine because in all graphics modes other than the low-res "blocky" 40x48/80x48 ones don't rely on a row counter or otherwise replaying the same block of addresses over and over again.
(Part of me wonders if the "Low Res" graphics are also corrupted, since they intrinsically also involve cutting the character cell in half horizontally; this would point to a different root-cause failure in the Mega II, but likewise almost certainly unfixable.)
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CTRL-RESET doesn't drop me into BASIC. It just restart over again (like a power on)
Thanks very much for the explanation Gorgonops. I found a ROM replacement on ebay. I'm gonna try that and if it doesn't work, then the Mega II is the culprit. I'm gonna swap the main board.
Thanks very much for your help.
Pitou!
Thanks very much for the explanation Gorgonops. I found a ROM replacement on ebay. I'm gonna try that and if it doesn't work, then the Mega II is the culprit. I'm gonna swap the main board.
Thanks very much for your help.
Pitou!
If I thought there were a chance in heck I could desolder the surface-mount keyboard controller IC from the bad board and put it on my bad IIgs' I'd PM about it, but I'm only moderately competent with through-hole rework, I'm sure attempting to swap the keyboard IC would totally end in tears.
(This is why I love my "new" Apple II+; it's all generic ICs, and a lot of them are even already in sockets.)
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