ZX81 - save without tape plus memory check

[1024MAK]

Trying the newer program from Mark gives:

0 68
17408
1024

That’s showing the available RAM as only 1k byte…

Did you run this program after doing other stuff on the machine, or after a power down / up?

Mark

[jarweb]

Hi

Since the results were obviously not as expected I decided to double check my homework.

The upgrade involved running link wires from 4 of the RAM chip pins to 4 of the diodes at the bottom of the board.

It looks like when I put the board back in the case and put things back together one of the wires had disconnected from one of the diodes.
Hard to see even when looking closely due to the wire being so thin and it still looked as if it was connected but the end had actually broken off.

Anyway - re-doing the joint and trying the program again with the starting value of 17764 now comes back with 16K !!

Probably didn't think of this initially as I assumed it wouldn't work at all if one of the wires wasn't connected but I suppose it depends what that particular one does. It was Pin 21 of the chip to diode 2 on the board. The others are P1-D1, P23-D3 & P26-D5

Thanks guys for the pointers. It helped to know that I wasn't getting expected results and the programs should have worked.

I now need to find a way to save programs as losing all this typing every time is getting tedious !

Cheers

[1024MAK]

The broken wire is the A10 address line. Without this, the RAM chip will look like a 1k byte memory chip to the ZX81 BASIC ROM code when it tries to work out how much available RAM there is.

That does not mean that the system could not see more RAM than this, but with A10 missing, pin 21 on the RAM chip was electrically ‘floating’, it would act like an antenna and randomly flip between logic low and logic high. Thus it would cause the system confusion.

Think along the lines of a post man or post woman trying to delver mail to a large block of flats. But because half the flats have lost one digit of their flat number, how to decide which flat to deliver the letters to. You can still post a letter through the letter box, but you are likely to get the wrong flat…

Mark

[jarweb]

Thanks Mark

On the subject of how to save programs. Does anyone know or think a small headphone amp might work to amplify the signal from the ZX81 enough for me to record the output on a pc ?

I'd need to buy something but don't want to waste money if there's no chance of it working.

e.g. something like this

https://www.amazon.co.uk/eSynic-Profess ... 08G8KCQJM/

Not specifically that one but something similar.

Cheers

[1024MAK]

I recommend something like this: 2PCS LM386 Module 20 Times Gain Audio Amplifier Module for loading. I have not tried using this item for recording, but it may be good enough.

I can’t promise being able to test, but if I can find the time, I will give it a try (I have two of the modules that I linked to above).

Mark

[jarweb]

Hi Mark

Certainly worth a punt at that price. Looks like it's coming from China so I might be waiting a while.

Will need to check what other cabling will be needed as well. Does this need a separate power source ?

Thanks

[1024MAK]

You will need a stereo splitter cable like this one 3.5mm STEREO Jack Plug to 2 x MONO Jack Plugs SPLITTER Converter Cable Lead 1m.

That amplifier needs a 9V DC supply. You can either use a suitable battery pack (six AA cells) or take a 9V feed from the ZX81 or from the ZX81 PSU.

You will also need various other connectors. E.g. mono 3.5mm connectors.

Mark

[jarweb]

Sorry - stupid question. Why the splitter cable ?

My thought would be cable from Mic->Amp then another cable from Amp->PC LineIn (mic) - for saving

And same cable reattached from PC speaker out -> Ear - for loading

Also - is there definitely enough juice available to take it from somewhere on the system board ?

Thanks

[1024MAK]

The stereo splitter cable is required because the PC or laptop will have a stereo 3.5mm socket. But the ZX81 is mono, and uses mono 3.5mm sockets.

You can of course use other arrangements as long as you read and understand the following:

By convention, when using a stereo device to play back a mono sound channel, only the left channel is used. Do NOT connect both left and right stereo channels of an output together.

Also, a mono 3.5mm plug does not work in a 3.5mm stereo socket. A 3.5mm stereo plug does not work in a 3.5mm mono socket. The signal contacts between mono and stereo plugs and sockets don’t align.

A standard UK700 Sinclair PSU is rated at 9V at 700mA. A typical ZX81 (without expansions) takes less than 350mA. So there is 350mA spare capacity from the PSU. That’s more than enough for the amplifier I linked to earlier. The 9V on the ZX81 board is just the 9V passed through from the 9V PSU. Remember the ZX81 board is designed to support a ZX Printer (which takes far more power).

Mark

[jarweb]

OK, understood

Thanks again Mark