MEGA ST2 - "random" characters on the screen

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exxos
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Re: MEGA ST2 - "random" characters on the screen

Post by exxos »

Just to be sure.. With no hard drive connected and blank floppy in the drive.. Does it still act up ?

As Steve mentions, if it behaves while holding down the keyboard connector.. Have you continuity check the PCB traces out ? Also tried pressing down on the PCB near the connector ?
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DominoTree
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Re: MEGA ST2 - "random" characters on the screen

Post by DominoTree »

With no hard drive booting from a blank floppy about half of the time the keyboard will act like it’s disconnected until I put my finger on/near the connector, at which point the issue goes away

While the keyboard is in the weird state or if there is no keyboard attached I get unexpected characters in non-GEM apps but everything else seems fine
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Re: MEGA ST2 - "random" characters on the screen

Post by exxos »

There is a pull up resistor I think on one of the pins ? Is that OK ? Might be near the acia.. Also check gnd and 5v is OK on the connector..
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Re: MEGA ST2 - "random" characters on the screen

Post by DominoTree »

exxos wrote: Sun Feb 21, 2021 1:33 am There is a pull up resistor I think on one of the pins ? Is that OK ? Might be near the acia.. Also check gnd and 5v is OK on the connector..
Signals and connections at the connector are good, there wasn’t an obvious pull-up in the area but I’ll check the schematic - maybe it was in RP7

Found a bit more desoldering alloy buried on my bench and got the MFP and YM2149 removed and socketed. Found one more issue - pin 24 on the MFP was shorted to the via directly next to it - that probably wasn’t a good thing.

Curious to see how this all behaves when I get it back together. Still haven’t lifted a single pad - and haven’t broken any traces - at least, as far as I’ve gotten in testing :P
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Re: MEGA ST2 - "random" characters on the screen

Post by DominoTree »

Dug into this more this evening and I think at least part of the issue is that there's enough noise on the bus (or maybe an inconsistently weak pulse) that the ACIAs are being activated when they shouldn't. This is what the CS1/CS2 lines on the relevant chips look like. CS0 from the Glue is clean.

Removing the second ACIA completely solves the issue (albeit disabling keyboard input), and swapping the first ACIA in its place brings the issue back, so I doubt it's the 6850s themselves.

Checked damn near every pullup/pulldown I could find and everything seems good across the board.

It's still just a guess that those spikes on the CS lines are what's doing this, but I don't see anything similar on my STf's lines. I also see similar spikes across the rest of the address and data bus, and my guess is that the ACIAs are just more sensitive (or responsive) to the voltage than the other components might be.

I did swap in a couple CPUs, DMAs, and Shifters with no change. Also pulled the Blitter, YM2149 and FDC for good measure. All of the standard logic ICs on the board tested fine with basic truth tables as well. Tomorrow I'll do the MMU and Glue, and I might be able to dig up an MFP to try as well.

Funny enough, it actually fired up with a 68HC000 without any other changes, and the signals got a whole lot cleaner - except for those pronounced spikes in the middle of them.

157400966_10226991228873050_4687170869144789229_o.jpg
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Re: MEGA ST2 - "random" characters on the screen

Post by exxos »

All signals tend to look like that :roll: try lifting cs pin out of the socket, tie it high .. Or low .. Can't remember which way it is so check the date sheet... Also try the same on the IRQ pin... Acia trigger IRQ to the mfp..which then tells the CPU to enable the acia to read data..
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Re: MEGA ST2 - "random" characters on the screen

Post by DominoTree »

Disconnecting /IRQ works as well as forcing /CS2 to high and leaving /IRQ connected, which would both effectively disable the ACIA. Glad I socketed these :D

Finally bit the bullet and hooked up a logic analyzer, and I got a few decent captures but I want to add a couple more signals and then I'll post what I end up with along with a look at each of the signals. So far I've seen a couple situations where the ACIA will pull /IRQ low and stay there forever, but I've gotten a couple captures as well where I *think* everything works as expected.
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Re: MEGA ST2 - "random" characters on the screen

Post by exxos »

I assume you have continuity checked all the ACIA pins are connected to where they are supposed to be ?
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Re: MEGA ST2 - "random" characters on the screen

Post by DominoTree »

Yeah they're all good and all of the relevant resistors and caps have good continuity and values as well
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Re: MEGA ST2 - "random" characters on the screen

Post by stephen_usher »

DominoTree wrote: Fri Mar 05, 2021 4:49 am Dug into this more this evening and I think at least part of the issue is that there's enough noise on the bus (or maybe an inconsistently weak pulse) that the ACIAs are being activated when they shouldn't. This is what the CS1/CS2 lines on the relevant chips look like. CS0 from the Glue is clean.
Looks totally normal for an ST to me.
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