Harlequin 48K Rev. G - First Time Power Up

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Tiger
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Re: Harlequin 48K Rev. G - First Time Power Up

Post by Tiger »

I made such a "wired" modification (as I described above) on an 128K Harlequin and all banks where accessible with dip-switches. In the case of an 48K Harlequin it must also be possible to switch with the jumper J12. It will not work with the solution from 1024MAK (only If you make modifications on the PCB-wires).
But in your case: you have a working 48K clone - congratulation!
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1024MAK
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Re: Harlequin 48K Rev. G - First Time Power Up

Post by 1024MAK »

valentin wrote: Sun Jan 02, 2022 1:31 pm
1024MAK wrote: Sat Jan 01, 2022 2:43 pm [...]
Because pin 1 on a 28C256 EEPROM is address line A14, the ROM image has to be programmed in to the ‘top half’ of 28C256 EEPROM at address 0x4000 upwards. Note this is the address as seen by the 28C256 EEPROM and programmer, not the address from the Z80. If you can’t do this, combine two copies of the ROM image (firmware) so that the total file is 32k bytes long exactly. Then program as normal.

Jumper J12 needs to be in the position that links to +5V/VCC so that 28C256 EEPROM pin 27 (/WE) is connected to +5V/VCC.

Mark
From the above, my understanding is that, when using a 28C256 EEPROM, and not a 27C256 EPROM, and due to the differences in the pinouts, the 'lower half' is never accessible. As such, if I intend to have two ROM images programmed on the same 32K EEPROM, switching between the two is not possible by shifting the jumper in J12. Is this correct?
Yes, that’s correct. If you want to access the lower half of the memory in a 28C256, you need to either cut the PCB track for pin 1 (assuming the PCB track can be got at and it does not feed any other circuitry) or bend pin 1 out. Then add a 4.7kΩ to 10kΩ ‘pull up’ resistor between pin 1 and pin 28. And wire a SPST (on/off) switch between pin 1 and any suitable 0V/GND connection (e.g. pin 14).

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valentin
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Re: Harlequin 48K Rev. G - First Time Power Up

Post by valentin »

Now I can say I have correctly understood both options - to make use of the full capacity of a 32 KB EEPROM, a hardware mod is required. I may use this EEPROM for another project (a Harlequin 128K maybe) and replace it with a 27C128 EPROM I have - but first I need to find a way to program it (either buying a TL866II+ programmer, or finding someone who could program it for me). Or even get a used ZX Spectrum ROM recovered from another computer.

Again, thank you both for your help, it is much appreciated.
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Re: Harlequin 48K Rev. G - First Time Power Up

Post by zx-heinz »

If you can't burn a 27C128 and want to use an EEPROM instead of this, I would like to recommend the W27C512 wich is pin compatible with the 27C512 or 27C256, 27C128.
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Re: Harlequin 48K Rev. G - First Time Power Up

Post by valentin »

Thanks for the info, zx-heinz.

In my circumstances, I don't think it will work, though. Acording to the data-sheet, the voltage for erasing the W27C512 EEPROM is +14V and for programming is +12V. My very basic EEPROM programer, knows +5V only.
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Re: Harlequin 48K Rev. G - First Time Power Up

Post by valentin »

I hope everyone agree, the EEPROM are Read Only Memeories. Fact.

As mentioned at the beginning of this thread, I had some issues with putting my Harlequin 48K into service. It works fine now.

But!..

I'm curios, and I like to find out "what would happen if". I am using a 28256 EEPROM, with both halves (upper and lower) written with exact the same image: ZX Spectrum 48K ROM (16Kb in size). I am also aware that to select the top half of the addresses, pin 27 of the EEPROM should be connected to +5 (VCC).

In theory, I would think that, to select the bottom half of the addresses, I can connect pin27 of the EEPROM to GND. So I did it. The result: same screen as there was no EEPROM inserted, or the EEPROM was blank, or wrong image etc. (see below) Then I moved the jumper back to pins 1 and 2 of J12, to connect pin 27 to VCC. Again, no "@ 1982 Sinclair Research Ltd" message on the screen, but the same garbage on the screen. It looks like, somehow, the EEPROM was corrupted. Is this even possible? It is a Read Only Memory after all.

To fix it, I had to rewrite the ROM image to the EEPROM using the programmer.

Image
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1024MAK
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Re: Harlequin 48K Rev. G - First Time Power Up

Post by 1024MAK »

valentin wrote: Mon Jan 17, 2022 7:23 pm I hope everyone agree, the EEPROM are Read Only Memeories. Fact.
WRONG!

A ROM <> to a EPROM, or to a EEPROM (or E²PROM).

And a EPROM <> to a EEPROM (or E²PROM).

Some types of EEPROM (or E²PROM) when in ‘unprotected’ mode can be written to in a system. Hence EEPROM standing for Electrically Erasable Programmable Read Only Memory.
valentin wrote: Mon Jan 17, 2022 7:23 pm I'm curios, and I like to find out "what would happen if". I am using a 28256 EEPROM, with both halves (upper and lower) written with exact the same image: ZX Spectrum 48K ROM (16Kb in size). I am also aware that to select the top half of the addresses, pin 27 of the EEPROM should be connected to +5 (VCC).

In theory, I would think that, to select the bottom half of the addresses, I can connect pin27 of the EEPROM to GND. So I did it. The result: same screen as there was no EEPROM inserted, or the EEPROM was blank, or wrong image etc. (see below) Then I moved the jumper back to pins 1 and 2 of J12, to connect pin 27 to VCC. Again, no "@ 1982 Sinclair Research Ltd" message on the screen, but the same garbage on the screen. It looks like, somehow, the EEPROM was corrupted. Is this even possible? It is a Read Only Memory after all.
Ahh, but your theory breaks down because on a 28C256, pin 27 is /WE (active low write enable). So when you connected pin 27 to 0V/GND, you were telling the 28C256 EEPROM to take the address from the address pins, and the data on the data pins and write that data to the internal memory at the appropriate address that is on the address pins…
valentin wrote: Mon Jan 17, 2022 7:23 pm To fix it, I had to rewrite the ROM image to the EEPROM using the programmer.
Glad you got sorted in the end :D

Note that with a EPROM, in order to change its contents, the VPP pin has to go to 12V, 13V, 21V or 25V depending on the EPROM type. With a 27C256 or 27256, VPP is on pin 1. Pin 27 is address A14.

EPROMs can be programmed electrically but cannot be erased electrically. EPROMs that can be erased are supplied in a ceramic package with a quartz window in the top. But keep in mind that you can get One Time Programmable (OTP) EPROMs, and these come in a normal plastic package.

EEPROMs (or E²PROMs) can be programmed electrically AND erased electrically. And the 5V only types can be programmed and erased in a system.

Both are very different to a mask ROM chip which has its data permanently encoded at the time of manufacture.

Mark
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valentin
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Re: Harlequin 48K Rev. G - First Time Power Up

Post by valentin »

Mark, what a shame there is no "Thanks" button implemented in the forum's platform.

I knew the differences bewtween all types of ROMs; what i didn't know was that some of the EEPROMs (like the one in my Harlequin) could be written while being part of a system, and not only when inserted in a programmer.

Although I have enabled the write-protection, even when /WE is LOW - by using the programmer's Windows app - it apears it has no effect on the chip I'm using - ATMEL AT28C256-15PC.
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Re: Harlequin 48K Rev. G - First Time Power Up

Post by 1024MAK »

The datasheet for the AT28C256 is here ;)

Note that Microchip bought ATMEL, that’s why the current datasheet is on the Microchip web site.

As you can see from the datasheet, the chip has hardware and software write protection systems.

There are some chips available from trading sites labelled as AT28C256 but which may not be genuine parts. Or which may be used and remarked (relabelled) parts. These may be fully functional, or may not be fully functional.

Mark
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Re: Harlequin 48K Rev. G - First Time Power Up

Post by valentin »

Got the EEPROM on eBay, so it wouldn't surprise me if this is not a genuine ATMEL chip.

This is the feature I am using in the programmer:

Image
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