Harlequin 48K Rev. G - First Time Power Up

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valentin
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Location: Faversham, UK

Harlequin 48K Rev. G - First Time Power Up

Post by valentin »

Hello,

I have finished soldering all the components on the Rev. G PCB I bought of eBay.

Image

Only the RGB video out is going to be used, so I haven't installed any of the Composite Video components. I power up the board using my lab power supply:

Image

For the +5V line, the tester shows a constant 4.96 - 4.97 V

Image

However, there must be something wrong with the way I assembled the components, or one or more may be faulty - irrespective of the jumper position on J12, all that is shown on the monitor is:

Image

After removing the EEPROM and powering up, I see this:

Image

"Reset" seems to work only sometimes - there is not always a "feedback" on the screen after shortening the two pins on J13.

I have a logic TTL tester on order, but I wonder if there is something else I could do before it arrives. Any help will be greatly appreciated.
Valentin
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Tiger
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Re: Harlequin 48K Rev. G - First Time Power Up

Post by Tiger »

Something with the ROM? I think you have to burn the 16K ZX48K ROM into each bank of the EPROM (#00000, #04000, #08000 ...), so more copies into an 27C256 than an 27C128.
On the other hand - are all the IC-pins perfectly fittet in the sockets? All soldering points checked?
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1024MAK
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Re: Harlequin 48K Rev. G - First Time Power Up

Post by 1024MAK »

The section that does the ‘ULA’ screen generation system is working.

As the border is not white, it looks like the Z80 is unable to execute instructions from the ROM.

Follow Tiger’s advice and check that your EPROM is programmed correctly. On the Harlequin 48K board that I have, I have a 27256 EPROM with BASIC ROM image in one half and a diagnostic ROM image in the other half, with pin 27 (address pin A14) controlled by a switch. I can’t remember which image is in which half anymore! :lol: :oops:

Anyway, I had a dry solder joint on one of the EPROM connections, and the results were very much like yours. So check all EPROM and Z80 connections, including visual checks for any solder splashes.

If none of that helps, the next step is to check that the Z80 is doing something.

Do you have any test gear that can test the 3.5MHz clock on pin 6 of the Z80?

Using a multimeter, test also these pins: (best done with a logic probe, but if you don’t have one use your multimeter, that will give an average voltage which may give us some clues):
Pin Function
16 /INT (should be high but pulsing low at approximately 50Hz for PAL or 60Hz for NTSC)
17 /NMI (should be high)
18 /HALT (should be high)
19 /MREQ (should be pulsing rapidly)
20 /IORQ (should be pulsing)
21 /RD (should be pulsing rapidly)
22 /WR (should be pulsing)
24 /WAIT (should be high)
25 /BUSRQ (should be high)
26 /RESET (should be high, except for a very short time after power is applied or go low then high as the reset button is pressed and released)
27 /M1 (should be pulsing)
28 /RFSH (should be pulsing)

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
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valentin
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Location: Faversham, UK

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

Post by valentin »

Thank you both for your feedback.

I had a quick look and it doesn't seem to be any cold joint on the board, nor any short-circuit, but I took a few hi res pictures and I will check again.

My cheap, DIY frequency counter (reviewed here) shows 14.060 MHz on pin 4 of U13, and 3.5583 MHz on pin 6 of the CPU; the same frequency counter has a "quartz crystal oscillator" tester feature which showed 14.0065 MHz for the 14MHz crystal I soldered on the board. Mind you, I am not saying this measurements are accurate as the frequency counter / crystal tester is far from being a reliable device.

The Logic Probe I ordered on Amazon has just been delivered and I swiftly made the suggested tests. Here are the results:

Code: Select all

pin 16:	HIGH	HIGH
pin 17: HIGH	HIGH
pin 18: LOW	LOW	should be HIGH
pin 19: pulsing	pulsing
pin 20: HIGH	HIGH	should be pulsing
pin 21: pulsing	pulsing
pin 22: HIGH	HIGH	should be pulsing
pin 24: HIGH	HIGH
pin 25: HIGH	HIGH
pin 26: HIGH 	HIGH	(LOW when RESET is pressed)
pin 27: pulsing	pulsing
pin 28: pulsing	pulsing
The first column shows the values with the EEPROM off-the-board, and the second column with the EEPROM inserted int its socket (no difference, though)

So, 3 out of 12 tests have a different output than the expected one. Could these be the issue?

For programming the EEPROM I used the ARDUINO MEGA programmer described here, and the ROM image downloaded from here.
Valentin
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Tiger
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Re: Harlequin 48K Rev. G - First Time Power Up

Post by Tiger »

OK - you use a 28C256 instead of an 27256. They are not pin-compatible! The WE-connection on pin 27 at the 28C256 is not used - in the Harlequin on pin 27 must be the A14 signal which the 27256 has here. You can fix it: cut pin 27 at the 28C256 and make a connection from pin 1 (there is the A14 at the 28C256) to pin 27 in the IC-socket. you have to bend pin 1 from the IC out of the socket and solder a little wire to pin 27 in the socket. At the 27256 is pin 1 Vpp wich is used for programming (you don't need it after programming). Simply described it is as WE which will be used for programming the EEPROM, also you don't need it after you programmed the IC...
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valentin
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Re: Harlequin 48K Rev. G - First Time Power Up

Post by valentin »

Thanks for that! I will try it next year ;) Until then, all the best for 2022!
Valentin
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1024MAK
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Re: Harlequin 48K Rev. G - First Time Power Up

Post by 1024MAK »

Tiger wrote: Fri Dec 31, 2021 8:11 pm OK - you use a 28C256 instead of an 27256.
We should have seen it in the photo he posted :oops:

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.

If you board has the same PCB connections as per the schematic, you don’t need any track cuts or to bend out any chip legs.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
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valentin
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Joined: Mon Nov 22, 2021 2:57 pm
Location: Faversham, UK

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

Post by valentin »

Cheers to the New Year! May 2022 be a much better one!

Guys, it's working! :D

With help from Kris (the designer of the Simple EEPROM Programmer), I managed to write the same ROM image into both halves of the 28C256 chip.

Image

And the proof ;)

Image

Many thanks for your help with this one. More questions will follow. :)
Valentin
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1024MAK
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Re: Harlequin 48K Rev. G - First Time Power Up

Post by 1024MAK »

Ahh, good progress, that’s what I like to see :D

Well done 8-)

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
User avatar
valentin
Posts: 35
Joined: Mon Nov 22, 2021 2:57 pm
Location: Faversham, UK

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

Post by valentin »

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?
Valentin
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