Sinclair ZX80 / ZX81 / Z88 Forums

xubuntu wrote: ↑Sun Nov 06, 2022 5:30 am So we can't have an eprom with coral basic running after all.

No, it is possible. But there is more work that needs doing.

Some of this is partly covered by the posts by Paul and Martin.

Question is:

• When the Coral BASIC is relocated, does it try to write to any of the memory locations in the address range it’s been relocated to? There are a number of ways of working this out.

I won’t answer this, as I see Paul has just posted a reply.

So the remaining item, can a 16K bytes EPROM be ‘blown’ (programmed) and fitted to a ZX81? Yes, but a modification is needed. It’s easiest to modify the EPROM chip. Three wires need connecting and some pins bent out so that they don’t go into the socket on the ZX81 board (assuming your board has a 28 pin socket fitted).

If there is no socket and the ROM is soldered directly, then this needs to be removed.

Mark

Thank you so much Paul !! Thank you !!!

Ok Mister Mark

Three wires need connecting and some pins bent out so that they don’t go into the socket on the ZX81 board (assuming your board has a 28 pin socket fitted).

Which pins need bent out and which 3 wires need connecting ?

Yes my board does have a socket in every IC.

Is this sym file a circuitry help of how the changes should be done for this eprom to work? How could I open the sym file in your zip file Mr Paul ?

You can ignore the sym file. It's just a file to help debugging in eightyone.
For the pins, have a look at marks link.

This post is what you need to look at.

Not covered in that post is that pin 26 also needs to be bent up. Then a wire soldered between this pin (pin 26 is address A13) and a suitable place on the board where the Z80 A13 signal can be picked up. Normally the cathode of diode D5 is the easiest place. D5 is near the right hand keyboard connector. The cathode is the wire furthest away from the keyboard connector.

Mark

Pin 26 needs bending? is this the 2764 you're referring to? I only ask as it's not connected internally and like the photo in your linked post I've never bent that one out, if it's something else then sorry in advance.

No, this would be for the 27128 which is the 16K byte EPROM. A 16K byte EPROM being needed as ZX81 BASIC (with minor modifications) and Coral BASIC are both wanted in it.

Hence this additional change compared to using the 8K bytes 2764 EPROM or using a 27128 EPROM only for a ZX81 BASIC.

Sorry if this was not clear.

Mark

What you think of that Mister Mark ?

Will coral eprom work here or do we need to change things ?



I am helping a good friend to present an homebrew 8-bit computer as thesis in his university.

All-in!

xubuntu wrote: ↑Mon Nov 07, 2022 8:39 am Will coral eprom work here or do we need to change things ?

No idea!

That diagram needs improving in my humble opinion.

A circuit schematic should be as clear and easy to read as possible. So mainly black on white rather than bright colours for starters. Only using colour to highlight very important items such as polarity or dangerous high voltages.

Using bus lines for data and address busses, with each signal line labelled where it splits out to go to a chip or component.

Other signal lines are normally shown as lines connecting to each component. Or where this is not practical, or would make the diagram difficult to read, the label should give both the signal name and the identity of the destination where it is going (e.g. /MREQ to IC7 pin 5) so that the reader does not have to search the entire diagram looking for it.

The layout should be spaced such that there is enough room to show components and connections without them being too close together or overlapping or on top of one another.

Each chip should be clearly labelled with a recognisable part number, long strange numbers that mean nothing to me are of no use. I can guess what each chip (strangely labelled as SK3, SK1 etc.) is, but that diagram does not actually say.

With regards to the ROM, which type of chip is it? Is it a mask ROM, a EPROM, a EEPROM, a flash ROM? And which size/capacity/version/make?

I don’t see an address line for A13 there, so it looks like it may only be a 8K byte type.

Although this diagram does not use bus lines for data and address busses, it is clear enough that it is reasonable enough to read without difficulty:


Mark

Thank you for the suggestions, I appreciate, although I ain't the designer!

Well my problem is that I can't apply your changes from the link you gave me because the rom we use is not 24 but 32 pins.

https://www.alldatasheet.com/datasheet- ... 7C020.html

That is why I showed you our schematic.

But anyway, I will find the way.