What kind of oscilloscope? It looks legitimate, so if you can't do 2MHz (with the harmonics for a square wave) on it, I wonder if you're not using x10 probes. With x1, any 'scope will be severely limited, and the probes will load down your circuit badly too. It should have come with a good set of probes with a x10 switch position.

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

I just got it from a friend. It's an OWON PDS5022S. It does have (in the menus) x1, x10, x100 and x1000 modes. I was told that this scope is a bit cheap and won't go that low.

[edit] I've found this youtube video: http://www.youtube.com/watch?v=tFeUuT2Udo and it mentions it's 20MHz...?

[edit] After watching a youtube video or two I've managed to get a square(ish) wave (used x10 mode, Trig Level and Autoset) and can see the clock. it doesn't look exactly square. I even disconnected the rest of the circuit - this is straight from the 2MHz osc can
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BTW, the probes say x10 / x1 60MHz on them

There should be a switch on the probe itself to change from 1x to 10x. The 10x in the menu only corrects the voltage readout on the display (50V/division, but should be 5V/d.) to match the probe setting.

The point is, that the impedance of the probe changes from 1M to 10M and even more importantly the load capacitance of 20pF is reduced substantially. You should see less ringing on your signal with the probe set to 10x, provided it is calibrated correctly.

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6502 sources on GitHub: https://github.com/Klaus2m5

You're quite right - I've switched that over to x10.
I've still got some ringing/distortion, so i removed the entire power circuit + oscillator can from the breadboard and put it on a smaller on.
This is what I've got in effect:

Schematic:
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Breadboard:
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Scope output:
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[edit] I've now changed oscillator and the power regulator. I do have a half metre cable running from my 9V battery to the board. Would this have any effect?

Your scope output is quite O.K. for 2 MHz on a breadboard!

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6502 sources on GitHub: https://github.com/Klaus2m5

Okey dokey . I expected a perfect square wave. In your experience is the ringing that you see there typically enough to cause issues? I have heard the breadboards carry capacitance which causes this problem.

A square wave carries a lot of odd harmonics. 2 MHz would have harmonics at 6 MHz, 10 MHz, 14 MHz and so on with decreasing amplitude. Yours and probably anybody elses scope will not be able to show a square wave absolutely clean of any harmonic distortion.

At 2 MHz a breadboard should still be O.K., although it adds a bit of distortion due to various physical effects. Just keep wires short and direct.

edit: of course harmonics of a square wave are odd, not even!

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6502 sources on GitHub: https://github.com/Klaus2m5

Here's how I see it, responding to the general question of what you should expect to see:

The important qualities of waveforms are edges and levels. For a clock or strobe signal, the edges matter most: so the waveform should cross the middle voltages rather decisively. The only problem with ringing or other disturbances is if they threaten to bring the voltage towards the threshold of some chip input which might respond as if there were an extra edge. The input might conceivably be on a chip which is experiencing ground bounce or power noise, so the threshold is correspondingly raised or lowered. For signals which are not clocks or strobes, what matters is the signal level around the time of the relevant clock or strobe: it should be comfortably within a logic high or logic low level, as seen by a chip input, again as modified by the local power and ground conditions.

In other words, a completely clean square wave such as you'd see in a diagram is not necessary for a reliable circuit, and you won't normally see one. Rounded corners, sloping edges, ringing and bouncing are all expected and normal - what matters is the clearance, in voltage and in time, from the signals to the logic thresholds and from the events to the clock and strobe edges.

This is the nature of digital logic: each logic gate is a high gain amplifier which only distinguishes high from low. Each clocked element is a filter in time which samples the inputs only for short periods in each cycle.

Cheers
Ed

The clock then seems ok and the address bus seems fine, too. However, the database seems to be going nuts. I expected variations in frequency, but not amplitude. Is my expectation wrong?
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[edit] Afer reading BigEd's post I think I'm right in saying that the data bus has something very wrong going on with it: it's go roughly 5 different amplitudes going on.

True. However, there is times when the data bus is not valid in a cycle. You should use the second channel to see when RAM or ROM is selected to output to the bus.

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6502 sources on GitHub: https://github.com/Klaus2m5

Looks a bit like there might be multiple drivers - is RnW being used in all cases to ensure there's only a single writer on the bus?

RnW is outputted from pin 34 of the 65C02S and goes straight to pin 27 of the HY62256 (the /WE pin). The VIA has yet to be added in and has no connection to the anything t all for the moment.

Hmm. Are those spikes getting towards 5V? Just wondering what voltages we have here.

Interesting. I removed the 62256 from the circuit and now I've got a reasonable output on the data bus....
[edit] Erm it was *ahem* a connection problem with the VSS line on the chip.
The levels on the data bus seem fine now.

WOOT! Detected 11101010 on the data bus: $EA = NOP!