Spectrum Computing

As for the pure communication protocol, ULA and S80 are masters and RAM is a slave, so the problem with timings is unlikely. Unless ULA is doing something unusual. Unfortunately, knowledge of the ULA communication protocol seems to have been lost to history. Actually, ULA was completely reverse eng...

Are they both same issue?

On the other hand, there is EMI from nearby traces, which will drive floating lines as well.

When CPU is in reset, it's not driving address lines, so they are floating. Floating inputs change between 0 and 1 randomly.

Try to pull A15 up with 10K resistor

Another option is MFM modulation, like in floppies and early hard drives. It would double the speed.

berarma wrote: ↑Tue Dec 12, 2023 1:24 am It's a fundamental design feature.

Indeed. They even have patent for this.

Memory is one reason. Very first Spectrums only had 16K.

Also, completely eliminating color clash would increase traffic between ULA and lower RAM - more delays for CPU

8BitSC wrote: ↑Wed Dec 06, 2023 11:14 pm You mean this? http://oldmachinery.blogspot.com/2012/0 ... -mini.html

LOL

Toastrack. If you can get it for a reasonable price.

I've got 48K Spectrum first.

Later got C64 as well. But the only thing which impressed me was the keyboard

It could be real RAM chips, but (as already mentioned by Mark) may have different access time. Sometimes they are just remarked to put a new date and sell them as "new". People who do it have no clue they are no longer manufactured. Are there any markings on the bottom side? How does the s...

Make sure you don't leave unused address pins floating.

Another option is to duplicate ROM image (8KB 4 times or 16KB 2 times) and burn it to fill entire EEPROM.

You get it every time on power up?

Maybe mark8bit is using different memory on his 2 layer board? And it has different timings?

Another option. May work on Amstrad models as well.

Cut the trace going to the ROM A14 pin and insert inverter (for example transistor) between the trace and ROM pin. If ROM is socketed, can do it without cutting

I did wonder at the back of my mind if swapping the _Select lines (not sure of the exact wording sorry) between the two IC's would do this, along with probably a couple of other changes. Although vaguely (unrelated) I am reminded of the BBC's ROM sockets where you could simply attach a flying lead ...

1. Make your own ROM with 48K ROM starting at address 0. Replace internal ROM with this one. 2. Same but use your own ROM with external ROM interface. Can DIY very cheap 3. Connect A14 pin on the ROM permanently to +5V 4. Remove IC31 (LS174) and replace it with custom board. This board must replicat...

You can use ULA from 4B board in 3B board I believe. But not ULA from issue 2 board.

All ROM and CPU are the same so you can reuse them as well.

One decoupling capacitor might be enough if you have 4 layer pcb with solid ground and power planes. But it's better to have them anyway. It's no fun trying to fix power supply issues.
Capacitors cheap small and easy to solder.

Should work.
When no key pressed, you probably need to set your outputs to high-impedance state. Check if Pico supports it

When RGB cable is plugged into the DIN socket, the noise is there. When a self-made composite cable (only pins 1 and 2, cut off phono plug and soldered wires to a DIN plug) connected to the DIN socket, there is no noise. When nothing is connected to the DIN socket there is no noise. This means some...

I remember someone made pcb for real mechanical keys (the standard ones used in PC keyboards) to fit inside 48K rubber case

That was quick.
Congrats!

Can you check all the power rails (+5V, +12V and -5V) ? Just do it one by one, by turning the spectrum on for less than one second at a time

KeplerOfMars wrote: ↑Sat Jul 01, 2023 11:04 pm As the ULA is the only IC that becomes hot over time, I concluded that it is the most likely candidate. Fortunately, I had access to another ULA for testing and the replacement seemed to solve the problem. My machine was certainly not crashing after a few minutes.

Hot ULA is normal. How hot is it?