Zx81

[electronicskip]

Yes sorry meant the spectrum.

[Timbucus]

Originally Posted by John Earland

I never quite got PEEK (still don't to be honest) but as I'm going to be working from home for some time, I might get out my old manuals and start!

My diary from Thursday 11th June 1981 says:

"I also found out why the system crashes it has a 33rd column containing CHR$ 118 (NEWLINE) and POKEing this puts it off balance and crash"

Agreed on the new thread for Spectrum but, I will leave you with the note that the Spectrum Next contains the Farrow ZX81 emulator so you can play the included .p file of 3D Monster Maze - it works under the increased speeds as well...

[SiriusHardware]

Originally Posted by emeritus

My recollection is that the ZX81 manual encouraged you to PEEK into the memory and see what was in the various locations. A good way of getting an insight into to what was going on. Data for machine language programs usually had to be stored in REM statements.

In common with all computers of that era you got a very decent manual with the ZX81 which encouraged you to try to program the machine, at least in BASIC. Once the 16/32 bit home computers faded away and the PC became dominant that whole culture very nearly died altogether and has only been kept alive by a few hardcore enthusiasts who continue to write demos and come up with new hardware for their beloved machines. The resurgent 'Maker' scene owes a lot to Eben Upton and his Raspberry Pi, and other programmable platforms like Arduino and the Micro:Bit.

I think if I was trying to write assembly language code on the ZX81 now I would come up with some system of injecting the assembled code directly into the RAM through the rear edge connector - getting the Z80 to then run it automatically would need some extra trickery. No doubt someone has already done it.

[Slothie]

Use your MK14 loader to type in POKE commands ....

[SiriusHardware]

Yes, in theory that approach can be used in conjunction with any computer with a row / column key matrix. I don't know if there isn't an upper limit on the number of characters one REM line can contain though.

Then again, if you first poke RAMTOP to be just above what is needed for the OS to minimally run, then you can poke the RAM beyond that address with whatever you like and jump to the start of it with a BASIC RANDOMIZE USR address... statement.

It might be quite fun to watch a ZX81 'type in' and then run ~16K worth of BASIC program. From memory though the longer the program is, the slower the screen update after each line entry, so the entry speed might have to scale slowly downwards. (You can tell I am actually thinking about doing this).

[Slothie]

I was just thinking POKE commands if they can be used on the command line but entering them as program lines would mean you could save it. But taking over the bus and writing direct to memory would be quicker if the problem of running it could be solved.

[SiriusHardware]

I think that on the ZX81, like its later big brother, you can switch out the onboard ROM and replace it with an external ROM just by pulling one line on the edge connector.

If you knew that the entry / execution address for your direct-injected code was going to be address nnnn, then you could clone the normal ZX81 ROM and just make a few changes to the code it runs at reset, so that it would jump straight to your code instead of initialising the RAM, which of course is the last thing you would want it to do.

This would also leave the majority of the original ROM content as found so that your code could make use of the OS subroutines, character set, etc, if required.

[SiriusHardware]

Actually, I think the ULA needs to be able to see parts of the ROM code at all times, not least the character / graphics set, in order to be able to maintain generation of the display.

[John Earland]

I think I need to do a whole lot of learning!

[Slothie]

Originally Posted by John Earland

I think I need to do a whole lot of learning!

Well the good news is that although ZX81s are stupid prices these days, the books are plentiful and cheap on auction sites. Also many are available as free downloads along with loads of other pages and forums full of info!

[SiriusHardware]

...and anyway John, first you need a working ZX81. So how's that CCU PCB installation coming along?

[John Earland]

Originally Posted by SiriusHardware

...and anyway John, first you need a working ZX81. So how's that CCU PCB installation coming along?

I haven't fitted it to be honest. I initially bought the ZX81 for my brother who is 50 this year and he wanted a vintage computer to tinker with. So, I thought I would either fit the mod myself or let him have the fun - so I've decided upon the latter!

I have ordered the vLA81 you mentioned though and will replace the faulty ULA. (Draws breath) - I may attempt the installation myself! I will practice on a small amplifier pcb to hone my soldering and desoldering skills and have a go. On the other hand I might wait until I can meet my friend again and ask him!

[Slothie]

Here's a,summary of desoldering techniques.
https://m.youtube.com/watch?v=77JgIqraX_I
I've got a soldering pump but for years I used th flux and desolder braid technique. I desoldered 40 pin chips this way with no problems, as long as you do diagonally opposite pins to stop overheating the board. Once the solder has been sucked out wiggle each pin to break any adhesion - if it doesn't free up use more flux, more heat & braid. Practicing on an old board that doesn't matter is an excellent idea.

[John Earland]

Originally Posted by Slothie

Here's a,summary of desoldering techniques.
https://m.youtube.com/watch?v=77JgIqraX_I
I've got a soldering pump but for years I used th flux and desolder braid technique. I desoldered 40 pin chips this way with no problems, as long as you do diagonally opposite pins to stop overheating the board. Once the solder has been sucked out wiggle each pin to break any adhesion - if it doesn't free up use more flux, more heat & braid. Practicing on an old board that doesn't matter is an excellent idea.

Thank you for this. I like the braid technique. What sort of braid is that? Can any wire be used?

[John Earland]

Quick question - not entirely sure it's for this thread but there is a link:
I had modified the ZX81 using the transistor and two resisitor method-still awaiting the diode. I know this works for the ZX81, would this also work for the ZX Spectrum? I'm assuming it will.

[nigelr2000]

Not ordinary braid but a product like this https://uk.farnell.com/chemtronics/8...ING-TEST-23MAR

[John Earland]

Originally Posted by nigelr2000

Not ordinary braid but a product like this https://uk.farnell.com/chemtronics/8...ING-TEST-23MAR

Thank you!

[Slothie]

Originally Posted by John Earland

Thank you!

The soldering braid is coated with flux so ordinary braid won't work. Use extra flux to maximise the amount of solder removed. In the video they used gel flux but you can use liquid flux it just is a bit messy, you can even "draw" on the braid with a flux pen (that's what I did). Rosin flux is a bit messy for desoldering as it leaves a hard brown deposit that needs scraping or washing off with isopropyl alcohol.

[SiriusHardware]

I don't really like to be the *Voice Of Doom* but I think it might be a bit early for John to try to remove a 40-pin IC intact from an antique double sided PCB, especially when there is absolutely no need to attempt that.

As with the keyboard decoder IC which was replaced in his PET, by far the better approach at this stage would be to cut all of the ULA pins high up beside the body of the chip, remove the body of the chip, desolder each pin individually and then use the desolder tool(s) to clear the holes ready for the replacement ULA (or the socket for it) to drop into.

Needless to say this should only be done when the replacement ULA is already physically in front of you - you don't want to chop up the old chip and then find that oops, there is a problem in the supply chain and you aren't going to get one after all.

John, if you do decide to try to get the original IC out in one piece as some sort of challenge, the one thing you absolutely MUST NOT do is remove 90% of the solder and then try to lever the chip straight up off the PCB with a screwdriver or similar wedge between the body of the chip and the PCB. If you do, some of the top side PCB pads and probably some of the through hole plating will come off with it. The sharp end of the screwdriver / lever may also cause damage to any PCB tracks under the ULA.

If you get to that stage the correct way to release the chip is to use a blunt wooden object like the square end of a small brush handle to push the chip from side to side, back and forth, until the little traces of solder which are keeping the chip in place crack and let go.

Let me put it this way: The PCB is irreplaceable, damage that and the whole machine may be lost. The chip is almost certainly duff, so it's better to destroy the chip and remove it in pieces than it is to destroy the PCB.

[Slothie]

I would cut the old chip out but I wasn't sure if John wanted the option of refitting it. The best option is a desoldering pump, they're much less expensive than they used to be. I have one of the ZD915 ones that are around £100 and it is excellent. The plunger style ones at £5 work well if your not doing too many chips or you have the patience of a statue, just heed Sirius's advice about not forcing anything; if you've removed enough solder and freed the pins as described the chip will almost fall out.