Sinclair ZX80 / ZX81 / Z88 Forums

Here is the 'poke' enabled circuit...

OOPS , realised I made a boob in the 'poke enabled' circuit - the link from the NAND gate output to the junction of the base of the transistor and the 22k is incorrect, the NAND output should go to the other side of the 22k resistor linked to the enable of the 74HCT541 - connect it wrong any you'll probably kill the NAND gate and transistor!

Been thinking about this a little if you change the memory address decoding, to decode $0F46 you only need force 1 data line high (D7) this changes the operand of the LD A,$7F to LD A,$FF which will result in no keyboard row been read.

You could probably get away with wire-(n)oring (using diodes and a pull-up/pull-down) 3 (maybe more but need to watch the capacitance) address lines together before feeding into a ttl logic chip. (obvioulsy only address lines that all need to be 0' and address lines that all need to be 1' can be wire-or'd)

I'll draw a picture.... Erm no i won't i'll send you to http://en.wikipedia.org/wiki/Diode_logic

Don't forget to include /mreq and /rd into the decoding too, i would used that to tri-state the drive to D7 (which if you connect directly to the CPU pin you can drive it high)

Andy

Hmm...maybe ...but I wanted to make an external unit - at least at the moment - but may change my mind if the size of the unit can be minimized, so 'forcing' a line would be a no-no for the moment. I do intend to try one of the circuits out as and when I get time, also have a circuit idea for a similar unit to make a US zx81/ts1000/ts1500/microdigital TK85(!) run at UK 50Hz speed(need to change 2 bytes in the ROM, but as it is in the keyboard/display loop, may affect timing--?? ) Maybe could combine both functions in one unit! Several other ideas on the go, so I don't know what will get tried first !
As for the diode logic, were you thinking of something like this...(need to change 0F47 not 0F46, according to the disassembly I have..?) .

One other idea is to permanently disable the normal break key, but this time by changing the port address completely and decoding in an additional key on that port..??

Though I see all of you getting the problem solved in hardware, I just now have a SW to do it...

It disables the BREAK unless it is pressed together with SHIFT. So you can enter SPACE with INKEY$.
Give it a try.

Matthias

Interesting! How does this work?

That's all very well, but what if you want to be able to enter '£' - which is shift +space..???!!

What about making a modified ROM, moving the '£' to the spare/missing function on the 'V' and then its ok having shift+space as break, as in the spectrum!

Hi gozzo,
I can generate a version to trigger BREAK on any combination of keys. Might be SHIFT+NEWLINE+SPACE or anything else.
Or wait for SPACE pressed longer than 5 seconds.
Just a piece of software.
Matthias

Hi sirmorris,
I just copied all the necessary machine-code from ROM. Reassembled it with mods (here only the BREAK function).
When running the machine-code it substitutes the interpreter main-loop of the Basic-ROM.
In this way I am able to patch the ROM code.
This is pretty much it.
Matthias