Search results

... And for laughs, if you prefer DIP here's a version of the GAL code that provides a "HI" and "LOW" MEMCS signal for directly driving two readily available DIP 74HC138s. Simply put the 3 demux inputs for the '138s on A16-A18 and use the appropriate MEMCSHI/MEMCSLOW on one of the E inputs...

Why not use an HC '244 instead of LS? They're a completely standard part readily available in both DIP and surface mount. 

You'll just be making the CMOS chip work unnecessarily hard when it asserts a "low". Here is a quick rundown. TL;DR, you need the pull-up when going TTL->CMOS, when going CMOS->TTL the main limitation is fanout. Adding a pull-up resistor would just make fanout *worse*, but fanout doesn't...

The resistor I was questioning is the one you have on the outputs of your 74HC244 and the inputs of the 154 and the GAL (RN3 on your diagram). That's an HC driving LS; CMOS has a wider voltage swing than LS for a given supply voltage so CMOS outputs should basically always be valid inputs for...

No questioning, man. ;)  I was momentarily confused trying to figure out what was the deal with the single "WE" pin, but then I realized you were using 16 bit wide memory chips and using a buffered version of /LDS and /UDS for byte selection.

I checked Aliexpress and they appear to have CMOS in DIP, but of course you're kind of making a bet at what you're going to get. (And be waiting for it for two months.) Personally I'd go with the pair of 74HC138s, that'll take less space than the '154 plus all those resistors unless you go...

That's what the 74154 or a pair of 74138s is for, decoding within the 1MB window that's defined by the GAL (in the single GAL version; the GAL provides one chip select that's cascaded to the demultiplexers). The 2-GAL idea would need the GALs both listening to A16-A19; a 22v should provide just...

Why do you think you need a 20v10, IE, which inputs did I miss? Remember, A16-A19 don't need to go to the GAL because it's not demultiplexing addresses within the 1MB window, it only needs to produce a chip select to signal the demultiplexer to output its selection and the shared read/write...

I'm curious about this, were GALs really a problem? Microchip still produces the ATF16v8B that should be compatible with the original. (Older GALs are also readily available on Aliexpress and whatnot; got a whole sack of GAL20v8s on the shelf behind me right now.)

(Note of course all of the above comes with the usual disclaimers, IE, I probably have no idea what I'm talking about, don't do it, you'll melt your computer, etc.)

There is a copy of the "Guide to Macintosh Family Hardware" scanned on Archive.org. The section about the Portable's memory configuration starts on page 205. Here is a relevant quote from page 206: On page 208 there's the diagram of the card's pinout. The description of pin 32 is: So if...

From reading what few docs there are about to what extent A/ROSE was actually leveraged by that card it sounds like the 68000 mostly acts as a sort of caching substitute for a DMA controller, there isn't really any protocol offloading per se, but considering how some of the structural defects in...

There was a thread about this pretty recently; TL;DR, the 68000 is for running A/ROSE, an experiment with trying to make Nubus cards easier to build by making them fiendishly complicated and overbuilt. (Yes, seriously, not joking.) While in theory at least this ridiculously powerful blob of...

One comment per the proposed solution for providing the "decoding" of A21-23; the diode thing might work, but a 3-input NOR gate like a 74HC4075 would probably be more kosher. (Or you could use a triple-input NOR gate; attach its input to the "high" enable line on the '154 instead of "low...

No. The direction line on a '245 is to tell *it* whether it's passing data from "left to right" or "right to left"; the other control line on it, OE, tells it to actually pass the pin state in the chosen direction or to tri-state and stay quiet. It doesn't *control* anything. In short, its DIR...

So, devil's advocate here: instead of trying to make a replacement for the dead ASIC for this card (if all you want is 1MB) would it make more sense to just cook up a new card using a pair of AS6C4008s and send it off to a PCB puppy mill? You only need to generate two chip selects instead of a...

Do you happen to have lying around one of those A9M0106 "universal" floppy drives that were sold for both Macintoshes and the Apple IIgs? Since you say the machines boots from a FloppyEMU but doesn't work with the 400k drives my wild guess would be that you did something that wrecked the PWM...

That's certainly true enough, IE, it's pretty unlikely someone's going to buy one to toss into their bookbag every time they leave the house anymore. That said, if you happen to have one that works and want to keep it in your collection "forever" I'd probably recommend giving it "desk queen"...

The original 500/600mhz IceBooks, IE, the ones with Rage 128 video instead of Radeon, mostly didn't suffer from the motherboard issues of the later ones. I'm not sure that follows. My understanding of the problem was the root cause of the failure of those machines was largely mechanical, IE...

I have Goteks with FlashFloppy installed in a couple Tandy 1000s and they do a great job, pretty much zero complaints. I do *highly* recommend the OLED Display Mod, it's literally plug-and-play outside of having to hack a larger hole in the case.