56

There are many issues here. As it is already said in comments, decoupling capacitor is a must! 555 (non-CMOS) timer output is very much like the output of TTL ICs, however Z80 requires a firm logic one. When feeding Z80 clock pin from a TTL output, you should use pullup resistor of 200..500 Ohm. NMOS Z80 uses dynamic logic, that means it has some minimal ...


44

It's hard to know for sure with just that picture, but that's a Mersenne number and a prime one at that. When a mathematician writes a Mersenne number on a board they are not thinking about getting all the digits. By far the most interesting and likely question computationally speaking would have been "prove that the number is prime". That could be used as ...


41

It appears that A0 through A6 operate correctly, but A7 though A9 (I've not tested the rest of the upper bits) are only active on the clock edge. Doesn't that exactly look like refresh cycles? :)) Basic Z80 bus behaviour, here especially the M1 cycle: Z80 timing is structured in Machine cycles (M-states). A machine cycle consists of several Clock cycles (...


30

Update: It seems as if David Marshall has found the missing link: The footage used in the clip is not just from the Manchester Baby, but also from the Manchester Mark 1 which became operational about a year after. As noted its first (useful) program was in fact a proof for Mersenne primes, a perfect fit for the chalkboard scene. Looks like the combined ...


14

I'd just like to expand on a couple of points in lvd's excellent answer. Reset Circuit You might get by with just using a jumper wire to short the reset pin to ground for a brief moment after you've powered up the CPU. It's worked for me, but if you're having problems it's best to build a proper reset circuit. Many CPUs have a minimum length for the reset ...


9

The paddles and joysticks on the Apple work by charging a capacity through a variable resistance (potentiometer). For the first joystick, the X-axis corresponds to paddle 0, and the Y-axis to paddle 1, and similarly for the second joystick. Accessing address $C070 discharges all capacities, so they can now charge with a speed that is determined by the ...


8

The Omnibook has its own self-test which can be run by pressing Esc when powering it up. The best freely-available general-purpose memory checker is Memtest86+, and older versions are available as plain DOS executables (look for “Download - Pre-Compiled EXE file (Pure DOS - Obsolete)”). They are identified as obsolete but should be fine for your purposes; ...


7

PAGE and HIMEM are what you might call "system variables" in BBC basics, they mark the bottom top of the memory available for Basic use, they are intiialised from OS-provided values but can be changed by the user. On a regular BBC micro HIMEM will point to the bottom of screen memory, so it can change if you change screen modes (this BTW meant you couldn't ...


6

WARNING: Electricity is dangerous. Don't open up your computer without turning off the power and allowing the capacitors to discharge. If you need these instructions for anything other than knowing how this particular computer works, do not try. The voltages involved can be fatal. Before starting Ensure the computer is safe from sparks. Place the ...


5

Issue A is probably just the card waiting to detect a printer. (But I haven't disassembled the ROM.) Issue B is probably because you're trying to connect a serial printer to a (Centronics) parallel card. From the Grappler plus Operator's Manual, top left on page 3: GRAPPLER+ STATUS CHECK Before sending a character to the printer, the GRAPPLER+ will check ...


5

The 64KB of RAM for the 65C102 co-processor is provided by eight MB8264 64k x 1-bit chips. Each chip provides one bit of memory for every address, meaning that all eight chips are used for every memory location. In many instances, the failure of a RAM chip will affect all memory locations (including the 6502's page 0 registers), meaning that the co-processor ...


1

In the DiagROM "AddressError Detected" is displayed when the CPU enters exception vector #2 at address $0000000C. This occurs if a word or longword access is attempted at an odd address, which is illegal on the 68000 (but not 68020 and above). Unfortunately the debug data doesn't include the PC or stack contents, so I can't tell what code caused ...


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