How is the Pro backplane reset button different from the dual clock module reset button?

[Tom Stepleton]

Hi again,

I mentioned in my last message that I updated RomWBW on my Zed Pro from 3.2.0 to 3.2.1 (using RCZ80_std.rom). I was interested to find that 3.2.1 hangs after the second LED illuminates on power-up. Additionally, pressing the reset button on the dual clock module yields the same hang. But if you press the reset button on the backplane, 3.2.1 will boot successfully. The boot loader's reboot command and the one in the XModem flash updater also work fine.

I was interested to investigate why the two reset buttons should be different. I found that the clock module button will assert RESET and \RESET, but only one of these appears to go out to the RCbus. I'm afraid I don't understand why both reset buttons get different results, and while it's not so important for me to find out, I'm still very curious. Does anyone know why things should work this way?

Thanks,

--Tom

[Tom Plano]

So I recall having a similar problem about 4 years ago when I first put together my zed pro. At that time I had no tools other then a cheep DMM and was just starting my computer hobby journey so I knew next to nothing. Didnt have a scope yet so couldn't check timings of reset line rise/fall.

 

Anyway I was seeing very similar behavior as you describe, where the backplane reset worked fine, andthe dual clock reset would lock up the machine. I did what I could to try and debug this issue with just my DMM, which meant poking around to see if anything was dead. Not the ideal solution but it did reveal something anomalous.  I found that if I probed across U1pin1 and U1pin4 of the clock, the computer would just boot. At the time I just assumed that the U1B inverter was bad so I just whacked a 10k resistor across U1pin1 and U1pin4 and boom, card has worked since then. 

Couple of caveats. I now know that this is kind of a nonsense fix, that at best acts to pull the reset line low faster or in a less bouncy manner, but really a scope would have allowed a proper diagnosis.  I also seem to have swapped C1 to a 1uF cap at some point, so YMMV. Finally I have no clue why changing the rom would cause this effect when it wasnt happening before, so that's weird.

Dont know Id advise this fix explicitly, but at the same time it really shouldn't hurt anything. If you do try it, let me know how it goes.

- Tom P

[Mark T]

Not really a nonsense fix. Adding the 10k resistor is creating a schmitt trigger, which would help to avoid any oscillation of the reset output as the voltage across the capacitor rises quite slowly. Reducing the capacitor also has a similar effect by decreasing the rise time. You might not need both, maybe you changed the capacitor before you added the resistor.

Fitting 74hct14 in place of 74hct04 would possibly be better for the reset circuit, but would then probably stop the clock from running.

[Phil G]

I did a really simple mod to my Zed Pro as the buttons were contact-bouncing.  

Just take a DS1233 (a quid from RS) and bend the legs to fit around the backplane reset button.

This debounces both reset buttons and forces a proper power-on reset:

[Bill Shen]

The signal assignments of RC2014 backplane happens to match the pin assignment of reset supervisor, MCP130-450DI/TO (64 cents from Mouser).  So I can easily add a power-on reset supervisor by soldering a MCP130 on 4-pin header and plug it in a spare slot on RC2014 backplane.

  Bill

[Phil G]

Indeed, my point was that however you fit it, a cheap-&-cheerful reset chip fixes reset bounce and POR problems at minimal expense.
You can also fit a DS1233 to the unused backplane link positions:

[Tom Stepleton]

On Saturday, August 19, 2023 at 9:40:23 AM UTC+1 Phil G wrote:

Indeed, my point was that however you fit it, a cheap-&-cheerful reset chip fixes reset bounce and POR problems at minimal expense.

Thanks to the information you've all shared, I've made what might be one of the smallest expansion modules for the RC2014. Works great with RomWBW 3.2.1!

Thanks all,

--Tom

[Phil G]

You could easily add a DS1233 next time you do a backplane run Spencer :)    or just a space for the option of fitting it?
Cheers

Phil

[Spencer Owen]

It is included in the RC2014 Mini II.

I doubt that I will be adding it to the Backplane Pro (or 8 or 5) however the Dual Clock Module is due for a revision before too long and that seems to be the logical place to add it.

Spencer 

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