squelch41
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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Wed Sep 21, 2022 9:02 am

mahjongg wrote:
Mon Sep 19, 2022 11:11 am
squelch41 wrote:
Sat Sep 17, 2022 3:14 pm
mahjongg wrote:
Sat Sep 17, 2022 12:23 pm
I have not looked at that yet, so I can't even give you an estimate.
Also, do you only want the ZX81+38 board + parts, or do you also want the keyboard PCB and buttons.
I'm going to create a BOM that has the prices for 5 pieces.
Probably the complete set.
I just dont want to waste your time if it turns out to be more than I can afford :)
I have time to waste, and anyway I already had planned to create a BOM with price information, I will start with prices from Reichelt only.
I just want to know this for myself too.
hehe, well then I look forward to seeing what you come up with :)

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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Thu Sep 22, 2022 3:54 pm

Well, I have calculated that if you buy all parts from Reichelt you pay something like 40,- euros (including the 40 buttons, which together cost euro 6,40)
The price of the PCB's depend on where you buy them, and often it doesn't matter price-wise if you buy single sided or double sided PCB's, only the dimensions count. the zx81+38 PCB is 100mm high, and 142mm wide, and the keyboard PCB is 65mm high, and 159mm wide. Prices also depend on if you want the PCB to be gold plated (for the edge connector or not). That depends on if you plan to use an expansion card. I'm planning to create a PSG expansion card, (PSG = sound card, but mine will also have an SD-card interface) but that will use the (optional) Pin header connector, which has gold plated contacts, you could use a female edge connector for the expansion connector but that will return the quite infamous "wobble" problem, but as the 16K Ram is built in that might not be a big problem. you can use tin plating, and if you want to use the pin header connector you can mount it later. The edge connector (and thus also the pinheader connector) is a dual 23 pole connector. Using an edge connector is free, as part of the PCB, a angled 23x2 pinheader connector costs euro 0.47 cent (its 25 pole angled and gold plated, order code SL 2X25W 2,54 (https://www.reichelt.com/de/en/2-x-25-p ... 19495.html), the equivalent female header costs euro 0.95.

Note that these prices are excluding any form of enclosure, and so some people have opted to ignore my keyboard solution, and use much larger buttons (for example real sherry keyboard keys) than the standard 6x6mm buttons I'm using. instead they want something they can print the labeling on, instead of using my free to use keyboard overlay. But the price of such buttons is much higher, and they need to create their own keyboard PCB, and, also a much larger enclosure, with a 3D printer, or from wood with a laser cutter, or with the classical wood saw (fretsaw).
an alternative can be to use MCDTS-2 12 x 12mm keys with KTSC-21 (square white) buttons, which are flat, so they can be lettered easily with stickers. I used these buttons for my Rhococo color home cumputer prototype see https://revspace.nl/Designing_the_RhoCo ... r_hardware

Or if you own a defective ZX-81 you can build my replacement PCB that is designed to fit into an old (or 3D printed) ZX-81 enclosure. they can then fit my keyboard with real buttons, or buy one of the replica keyboards, there are many options.

Hope this helps estimating the cost of building your own ZX-81 clone, its about 50 euro's. If you consider that expensive, note that you get many extra's like 16K built in RAM, composite video out with a back porch signal, a keyboard with real keys, a cassette pre-amplifier making loading files much easier, but mainly the joy that comes from building something yourself. And note that at its launch the ZX-81 in kit form was 50 pound (about Euro 280,- in 2022 prices)

It is also clear for me now that I can indeed afford to buy a limited edition of kits, although with reichelt it doesn't seem the price drops when I buy 5 of a part instead of just one. I can then sell a kit with programmed 27C256's and with or without a keyboard.

squelch41
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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Thu Sep 22, 2022 8:28 pm

mahjongg wrote:
Thu Sep 22, 2022 3:54 pm
Well, I have calculated that if you buy all parts from Reichelt you pay something like 40,- euros (including the 40 buttons, which together cost euro 6,40)
The price of the PCB's depend on where you buy them, and often it doesn't matter price-wise if you buy single sided or double sided PCB's, only the dimensions count. the zx81+38 PCB is 100mm high, and 142mm wide, and the keyboard PCB is 65mm high, and 159mm wide. Prices also depend on if you want the PCB to be gold plated (for the edge connector or not). That depends on if you plan to use an expansion card. I'm planning to create a PSG expansion card, (PSG = sound card, but mine will also have an SD-card interface) but that will use the (optional) Pin header connector, which has gold plated contacts, you could use a female edge connector for the expansion connector but that will return the quite infamous "wobble" problem, but as the 16K Ram is built in that might not be a big problem. you can use tin plating, and if you want to use the pin header connector you can mount it later. The edge connector (and thus also the pinheader connector) is a dual 23 pole connector. Using an edge connector is free, as part of the PCB, a angled 23x2 pinheader connector costs euro 0.47 cent (its 25 pole angled and gold plated, order code SL 2X25W 2,54 (https://www.reichelt.com/de/en/2-x-25-p ... 19495.html), the equivalent female header costs euro 0.95.

Note that these prices are excluding any form of enclosure, and so some people have opted to ignore my keyboard solution, and use much larger buttons (for example real sherry keyboard keys) than the standard 6x6mm buttons I'm using. instead they want something they can print the labeling on, instead of using my free to use keyboard overlay. But the price of such buttons is much higher, and they need to create their own keyboard PCB, and, also a much larger enclosure, with a 3D printer, or from wood with a laser cutter, or with the classical wood saw (fretsaw).

Or if you own a defective ZX-81 you can build my replacement PCB that is designed to fit into an old (or 3D printed) ZX-81 enclosure. they can then fit my keyboard with real buttons, or buy one of the replica keyboards, there are many options.

Hope this helps estimating the cost of building your own ZX-81 clone, its about 50 euro's. If you consider that expensive, note that you get many extra's like 16K built in RAM, composite video out with a back porch signal, a keyboard with real keys, a cassette pre-amplifier making loading files much easier, but mainly the joy that comes from building something yourself. And note that at its launch the ZX-81 in kit form was 50 pound (about Euro 280,- in 2022 prices)

It is also clear for me now that I can indeed afford to buy a limited edition of kits, although with reichelt it doesn't seem the price drops when I buy 5 of a part instead of just one. I can then sell a kit with programmed 27C256's and with or without a keyboard.
That comes in about what I was working around - the £50-£70 (which due to the mess things are in here is about equal to euros! :)) -- as you say, other than the fun of building (and 3d printing the case etc), the extras like the back porch signal and not having a membrane keyboard that is about to fail make it compelling.

Was originally debating just buying an old zx81 but these projects in the end looked more interesting - was just stuffed by the lack of available components!

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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Thu Sep 22, 2022 8:35 pm

mahjongg wrote:
Thu Sep 22, 2022 3:54 pm
It is also clear for me now that I can indeed afford to buy a limited edition of kits, although with reichelt it doesn't seem the price drops when I buy 5 of a part instead of just one. I can then sell a kit with programmed 27C256's and with or without a keyboard.
Any chance you would be interested on selling one of those kits on to the UK? I'm quite happy to build it myself and willing to pay postage & packaging and a couple of Grolsch for the privilege.
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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Fri Sep 23, 2022 11:57 am

wildfire wrote:
Thu Sep 22, 2022 8:35 pm
mahjongg wrote:
Thu Sep 22, 2022 3:54 pm
It is also clear for me now that I can indeed afford to buy a limited edition of kits, although with reichelt it doesn't seem the price drops when I buy 5 of a part instead of just one. I can then sell a kit with programmed 27C256's and with or without a keyboard.
Any chance you would be interested on selling one of those kits on to the UK? I'm quite happy to build it myself and willing to pay postage & packaging and a couple of Grolsch for the privilege.
If I do manage to create some kits I will post that fact here!
It may take some time, even If I bought the parts today. Sending a kit to the UK is not a problem (I think).
I'm not doing this for the money, (as you might realise by now) but I do like beer.

p.s. my intention is to deliver the parts (IC's) on a piece of PE-Foam the same size as the PCB, Rechelt sells electrostatic foam in sheets of 30 x 30 cm 5mm thick, order code ESD PE-MOS 300.

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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Fri Sep 23, 2022 12:56 pm

Thanks mahjongg,

I look forward to any updates.
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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Wed Sep 28, 2022 12:40 pm

I'm busy testing cassette loading, for now I'm not really content.
Initially I was considering to convert the .P files to .WAV files with an specific app, but when I tried it seemed to be a much more complex program than I would have expected, and as it was a program written in JAVA it only partially worked, and it crashed when I tried to use it to convert the .P to .WAV file, probably because the program was written for Windows, and I'm using Linux (mint) on my laptop.
In the end I booted my laptop to Windows 7, and after waiting half an hour for the obligatory updates (the main reason I stopped using Windows) and clicking away half a dozen programs that wanted me to update them, I managed to install the ZX-81 emulator "EightyOne", because it has built in cassette conversion and playback routines.
The main problem with it is that the audio it outputs is too soft, even for my pre-amplifier.
resulting in very thin lines visible on my ZX81+38, in "inverted K on a white screen" mode (so without giving a LOAD """ command) and with volume turned all the way up, the displayed lines were far thinner than when I had uses my TZXDuino (and sent a spectrum file), as can be seen here: viewtopic.php?t=254492&start=150#p2039563 in this case the screen was mostly white with only a few thin black lines.
Thin lines mean too soft a signal, and the lines only started to appear when I turned up the volume above 80% of maximum.
Its possible that EightyOne is not sending out as loud as possible, so I probably might be better off with playing .WAV files with audacity.
But anyway after many attempts I finally managed to load a game called "rock-crush", see below.

Image

after pressing a key, I got the instruction menu, but when I started the game it crashed into an all white screen. :cry:

Probably because something still went wrong with loading. I will try to find if I can make the pre-amplifier even more sensitive, following the signal with an oscilloscope. I will also try if I can use EightyOne as a .P to .WAV converter and play the .WAV files with audacity, I hope I can then send a louder signal.

I noticed that the monitor switched to black, and to "no signal" immediately upon receiving a cassette signal, so this monitor is unsuitable to view the loading bars while loading, I need a CRT monitor for that.

squelch41
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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Wed Oct 05, 2022 2:33 pm

Looking at the BOM - the SRAM chip is 50ns on the reichelt and farnell part listings but 12ns on the mouser listing -- does it matter if the 12ns is used rather than 50ns? (I presume as long as it isnt slower than 50ns, it's fine?)

Also, the github info suggests not mixing HCT, HC and LS logic, just stick to one or the other.
The mouser part listing for the 74hc11n is for a 74ls11ne4 - is that because a LS chip here is fine or is it an error and a 74hc11n should be used?

Thanks!

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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Thu Oct 06, 2022 2:22 pm

squelch41 wrote:
Wed Oct 05, 2022 2:33 pm
Looking at the BOM - the SRAM chip is 50ns on the reichelt and farnell part listings but 12ns on the mouser listing -- does it matter if the 12ns is used rather than 50ns? (I presume as long as it isnt slower than 50ns, it's fine?)

Also, the github info suggests not mixing HCT, HC and LS logic, just stick to one or the other.
The mouser part listing for the 74hc11n is for a 74ls11ne4 - is that because a LS chip here is fine or is it an error and a 74hc11n should be used?

Thanks!
as long as the RAM chip used is faster (or the same) speed as the original RAM used in the ZX-81 it should work fine. Ill try to find out how fast that one was, but I'm guessing that all modern SRAM chips are at least as fast as the old RAM's were. One specific thing you should look out for is not to make the fault to buy a "skinny dip" version, the layout is made for the regular PDIP package, with is 600mil wide, like the EPROM. The version from mouser in the old BOM (the 972-71256SA12TPG) was a 300mil wide chip, so it is UNSUITABLE for the ZX81+38, I will repair the BOM so that it uses a 600mil version,. The correct mouser product is the 913-AS6C62256-55PCN which is 55ns
The versions from farnell and reichelt were correct (600mil PDIP) with 55ns speeds.

the mouser parts listing for the 74HC11 was indeed faulty it should be the HC type of chip

I will repair the BOM, and re-upload the repaired version to GitHub.

I'll investigate the ZX-81 RAM speed issue, and report back if I know more.

Thanks for the corrections.

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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Thu Oct 06, 2022 6:12 pm

Done, uploaded a corrected BOM to GitHub!

It is hard to find the speed of the RAM's used in the original ZX-81, none of the many FAQ's and other sources I have read to reverse engineer my ULA replacement bother to mention the speed of the RAM chips used.

finally I manged to discover this fact by looking at real photograps of the ZX-81 board and found that most used two Motorola MCM21L14P30 RAM chips (4-bit x 1K) and the P30 indicates a speed of 300nS, so as I told earlier any modern RAM should be more than fast enough!

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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Fri Oct 07, 2022 9:19 pm

mahjongg wrote:
Thu Oct 06, 2022 6:12 pm
Done, uploaded a corrected BOM to GitHub!

It is hard to find the speed of the RAM's used in the original ZX-81, none of the many FAQ's and other sources I have read to reverse engineer my ULA replacement bother to mention the speed of the RAM chips used.

finally I manged to discover this fact by looking at real photograps of the ZX-81 board and found that most used two Motorola MCM21L14P30 RAM chips (4-bit x 1K) and the P30 indicates a speed of 300nS, so as I told earlier any modern RAM should be more than fast enough!
Thanks - that's very helpful.
About to order the bits from Mouser.
Assuming it all works, can update the BOM with the full mouser part lists too
For others playing along, Mouser sell all the parts needed and do ship to uk

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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Sat Oct 08, 2022 2:22 pm

I don't think you can upload spreadsheets to this forum.

The BOM on my GitHub page does contain all of the relevant parts for all three sources, Farnell, Mouser and Reichelt, the part codes that are not inserted in the BOM are low cost parts like resistors and capacitors. I have created a second private bom that has all parts, even the resistors and capacitors, WITH PRIZES but only for Reichelt, so I could do an estimate of the costs to create a set of all parts. Which as I said is around $40.

Still what you could do is to make a screenshot of your modified BOM, so I can insert your order codes for mouser into my BOM, so I can further complete my public BOM. Especially the reistors from Reichelt that I have in my private BOM are priced a bit ridiculous, far higher than I had estimated, so if the prices mouser offer are far lower that would be interesting to know. I had expected that resistor prices would be a few cent for 1 to 10 numbers, but reichelt sells them for about ten times as much, which might be reasonable, considering the overhead, but I remember they were much cheaper from Farnell, it also seems that Reichelt only has one price level, and does not offer prices for 5, 10 or 20 of the same part.

if you want to e-mail me, the address is at the bottom here: https://revspace.nl/Mahjongg

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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Sat Oct 08, 2022 3:42 pm

your BOM is more than adequate - no criticism intended! :)
was just to make it faster for others if needed :)

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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Sat Nov 05, 2022 4:52 pm

from various sources I heard that EPROM's (OTP's) are almost unobtainable. I have done a small research, and yes, it seems OTP ROM's are very hard to get. I found however one option that will work, you can buy an 28C64 for about ten euro. from Reichelt its an 8K x 8 reprogrammable (EEPROM), you can buy it from here: https://www.reichelt.nl/nl/nl/eeprom-64 ... ol_2&nbc=1

yes, you need to change a wire bridge, as the 28C64 has a different pinout than a 27C64 but that isn't a very big deal.

Here is a diagram showing the differences between a 27C256 and an 28C256

Image

For a 28C64 the only real difference is that pin 27 must be tied high so that /WR is inactive, so switch jp10 to tie pin 27 high, not low.

It is expected that 27C256's will become available again beginning next year (2023).


Another solution is to buy a new old stock EPROM , the ones with a glass windows from a Chinese reseller these have the exact same pinout as modern OTP's,as OTP's simply contain the same DIE (chip) as UV erasable EPROM's, so they have the exact same pinouts as OTP's. they just use the much cheaper plastic packaging.

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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Sun Nov 27, 2022 3:51 pm

I have bought 10 pieces of the ZX81+38 revision 1.9 PCB's, and 10 pieces of the new keyboard PCB.
I have also bought 5 TZXDuino pico PCB's.
I expect delivery in about two weeks. (longer than most other options, because without PayPal (long story don't ask) and a creditcard, the only option was to use the option of paying through Aliexpress, which is cheap (china post) but slow..

One PCB + keyboard I will built up for myself, using the IC's in my current patched revision 1.6 prototype, so I have one that is 100% compatible with the latest version on GitHub. I will also try if the alternative ROM, the 28C64 will work, on the old prototype.

I will buy parts for about 5 Kits.

I will mention it here when the kits are ready and what price they are (I don't plan to earn any money from this endeavour )
Perhaps some will want a built up ZX81+38, if there are people interested in buying one.

I will also buy some 28C64 and program them for the kits and pre-built ones. Not everybody has an Eprom programmer. If you are considering buying one, I can recommend the Chinese TL866II plus programmer, which is simple, cheap and supports a very large list of devices it can program, erase and test, It also handy to find defective logic IC's such as the 74HCxxx family.
These days its the canonical choice for an EPROM programmer,.

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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Mon Nov 28, 2022 10:08 am

Thanks for the update mahjongg, I'm still definitely interested in buying the kit.
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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Mon Dec 12, 2022 3:43 pm

Today I have received the PCB's. (10 x ZX81+38, 10 x keyboard and 5 x TZXDuino Pico PCB)

I will start with the ordering TZXDuino Pico, Parts, and the sockets and new connectors for two ZX81+38's then I will built up one ZX81+38, using parts of my older revision 1.6 build, and the new connectors and sockets, and for the keyboard new low profile 6 x 6mm buttons so that they remain behind the keyboard overlay,

I will also buy a 28C64 EEPROM, program and test it.

I will build my new rev 1.9 it into an original ZX-81 case. I will also put together a KIT. (with black antistatic foam). Both the built up ZX+81+38 and the kit I will photograph, and will publish the photo's here, so potentially interested people will see what they can get.

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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Tue Jan 10, 2023 4:30 pm

As I mentioned before I designed a cassette replacement device that can read files from an SD-card and convert it to audio for the cassette input of a ZX-81 or Sinclair Spectrum, the TZXDuino-pico .

Previously I wrote about it in my ZX81+38 posts, but as it is actually a separate project I split it off into a new project, you can also find in the off topic section.

As with my ZX81+38 project, my TZXDuino-pico is fully open source and you can find everything in my GitHub page, even the KiCad files. here: https://github.com/mahjongg2/TZXDuino-Pico so you can easily order PCB's and cheaply built this cassette replacement device.

My version the TZXDuio-pico is much smaller and cheaper than the much larger and more expensive TZXDuino kits you can buy online.
It is actually meant to be built INTO a home computer such as the Spectrum, and therefore is simplified and miniaturised. If you solder the Arduino-nano directly into my PCB it is also so low that it fits into a Spectrum 48K.

The main disadvantage of my version is that it doesn't have a display, end only two buttons (next and play) . But it is possible to add all six buttons, and either an 20 x 2 LCD panel or a miniature OLED display if you want a display and all buttons.

If you put one or more .TZX Files (for either a ZX-81 or a Spectrum) on an SD-card the TZXDuino-pico will play them in the order you place the files on the card, no pushes on the next button plays the first (or only) file, one plays the next file etc. if you press the play button or external button, then it will start playing the chosen file, it is that simple.

A large number of .TZX files can be found online for various computers including the ZX-81 and Spectrum.

Obviously the TZXDuino-pico also works with my ZX81+38 ZX-81 clone.

There are versions of the TZXDuino software for other home computers, like the Oric, and the Amstrad CPC, (and many others) and my pico version should be compatible with those too.

For the amstrad version, with its built in cassette deck I also added the "start" input option, so that the playbackk can start when pressing play on the cassette recorder (if you connect the start button to the electrical contact of the play button of the cassette deck).

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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Tue Jan 10, 2023 5:31 pm

mahjongg wrote:
Mon Dec 12, 2022 3:43 pm
I will build my new rev 1.9 it into an original ZX-81 case. I will also put together a KIT. (with black antistatic foam). Both the built up ZX+81+38 and the kit I will photograph, and will publish the photo's here, so potentially interested people will see what they can get.
I'm looking forward to it, and yes I'm still interested... Please put me down as customer #1 on your list :D
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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Wed Jan 11, 2023 1:36 am

wildfire wrote:
Tue Jan 10, 2023 5:31 pm
mahjongg wrote:
Mon Dec 12, 2022 3:43 pm
I will build my new rev 1.9 it into an original ZX-81 case. I will also put together a KIT. (with black antistatic foam). Both the built up ZX+81+38 and the kit I will photograph, and will publish the photo's here, so potentially interested people will see what they can get.
I'm looking forward to it, and yes I'm still interested... Please put me down as customer #1 on your list :D
I will contact you when I have anything to sell, do you want only the ZX81+38 main PCB, or also the keyboard?

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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Wed Jan 11, 2023 3:52 am

The whole package.
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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Wed Jan 11, 2023 10:52 am

I am trying to run software that uses WRX high res mode but none seems to work.
I *think* this is because WRX mode requires ram between 8k and 16k but in the zx81+38 the rom is mapped in 0k -16k so the 8-16k memory area is unavailable?
I've run a programmed called CHECKMEM and attached a screenshot

Using a rev 1.9 board, I have tried switching JP1 from it's default to the 8k.
The ZX81+38 boots but I cannot use keys Y-P

I am using a 28C256 EEPROM as a rom chip - I have disconnected the socket's pin 1 and patched pin 1 of the EEPROM (A14) across to pin 27 of the socket so it matches where A14 should be on a 27C256. I disconnected pin 27 of the EEPROM (WE) and tied it high.
The 28C256 is written with 4x the 8k zx81.rom

Oddly, also when I change the jumper to enable 32k ram rather than 16k, I get video sync but no video signal.

With all the jumpers in default position, the system seems to work perfectly.

I'm stuck!
Can anyone offer any advice on how to solve it?
(The specific hires program I want to get working is ZXTERM-80 but I have tried other hires software too and it doesnt work. ZXTERM-80 runs and then crashes when it tries to initialize the high res mode)
Attachments
ZX-Term 80 v2.5 (1987)(Fred Nachbaur).zip
ZXTERM80
(8.04 KiB) Downloaded 1 time
2023-01-11 10.33.39-1.png
CHECKMEM screenshot
2023-01-11 10.33.39-1.png (121.42 KiB) Viewed 456 times
CHECKMEM.zip
CHECKMEM zx81 file
(815 Bytes) Downloaded 2 times

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mahjongg
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Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Wed Jan 11, 2023 1:49 pm

the checksum programs shows the correct memory organisation.
Memory is divided in two sections, either the memory space is reserved for ROM, or for RAM, in fact the address space for ROM is selected using U24A U23A and U23B. per default it happens if A14 and A15 are both low, so the lower quarter of memory space , if you switch over JP1 that becomes the lower eight (8K) of memory space. ROM select happens when this memory segment is selected (or the first byte of an instruction is happening) and either a memory requests or a refresh happens.

If the memory space for ROM is not selected, then automatically it is used for RAM space. In a normal ZX81 the rom space is just the first 8K, but to expand it I added JP1, so ROM space can be extended to 16K.

so when you add A13 in to the selection for ROM space, it is the normal situation for a ZX81.

As for the keyboard, for the row of keys starting with Y and ending with P they are indeed selected when A13 is low, so I assume there is something wrong with A13. My educated guess is that there is something wrong with JP1, and that A13 is now tied low permanently.

I suspect that you have added the connection from a13 to pin 1 of U23A, but forgot to break the connection from A13 to GND.

alternatively, but I don't know how this could happen, you somehow managed to tie A13 high permanently, it would explain the keyboard problem.

squelch41
Posts: 19
Joined: Wed Mar 21, 2018 6:10 pm

Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Wed Jan 11, 2023 2:22 pm

mahjongg wrote:
Wed Jan 11, 2023 1:49 pm
the checksum programs shows the correct memory organisation.
Memory is divided in two sections, either the memory space is reserved for ROM, or for RAM, in fact the address space for ROM is selected using U24A U23A and U23B. per default it happens if A14 and A15 are both low, so the lower quarter of memory space , if you switch over JP1 that becomes the lower eight (8K) of memory space. ROM select happens when this memory segment is selected (or the first byte of an instruction is happening) and either a memory requests or a refresh happens.

If the memory space for ROM is not selected, then automatically it is used for RAM space. In a normal ZX81 the rom space is just the first 8K, but to expand it I added JP1, so ROM space can be extended to 16K.

so when you add A13 in to the selection for ROM space, it is the normal situation for a ZX81.

As for the keyboard, for the row of keys starting with Y and ending with P they are indeed selected when A13 is low, so I assume there is something wrong with A13. My educated guess is that there is something wrong with JP1, and that A13 is now tied low permanently.

I suspect that you have added the connection from a13 to pin 1 of U23A, but forgot to break the connection from A13 to GND.

alternatively, but I don't know how this could happen, you somehow managed to tie A13 high permanently, it would explain the keyboard problem.
Thanks - that gives me a good new jumping off point. I'll check A13 and do some probing!

squelch41
Posts: 19
Joined: Wed Mar 21, 2018 6:10 pm

Re: Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

Wed Jan 11, 2023 5:30 pm

mahjongg wrote:
Wed Jan 11, 2023 1:49 pm

I suspect that you have added the connection from a13 to pin 1 of U23A, but forgot to break the connection from A13 to GND.
You are right - with the jumper set to default, A13 is not held to ground and there is a small amount of activity on the line.
With the jumper set to 8k, A13 is held to ground with 0 resistance.

Checking the jumper - the lower (default) pin is ground, the middle pin goes to U23 pin 1 and the upper pin goes to A13

Looking at the schematic, it seems that just changing the jumper should break the ground connection between U23 pin 1 and ground - is that right (and so I have messed up the soldering, eg there could be a solder bridge under U23)? Or does another jumper need changing to break the U23-pin1 to ground link?

Just wanted to ask before I start desoldering or cutting pins!!

Thanks again for you help.

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