RC2014 ZED reset issue

[Mark Pepper]

I have built the basic RC2014 ZED edition and am having a reset issue. The system does not reset on a power up and only resets if I press the back plane reset button, and not the reset button on the dual serial board. I replaces the 47uF capacitor on the serial board with a 100nF one and now both resets work. I still get no power on reset however.

Should I get a power on reset and if I should, any ideas why I aren't?

Mark

[Steve Cousins]

Hi Mark

The dual clock module should provide a power up reset. My Pro system resets reliably at power up.

You said "I replaces the 47uF capacitor on the serial board with a 100nF"

I assume you meant the dual clock module, not the serial module.

Did you really mean 100nF? (or perhaps you meant 100uF)

The 47uF capacitor forms part of the power up reset and would need to be in the microfarad range, not the nonofarad range.

Steve

[Marten Feldtmann]

No, actually on my pro-system the power on reset is not always working. I always press reset after power on

Marten

[Michael Cullen]

On my pro system the backplane’s reset button works, whereas the one on the module doesn’t. A bit of probing around suggests the one on the module produces quite a lot of bouncing whereas the one on the board is pretty close to a square wave. It’s entirely possible I assembled it wrong, but interestingly, the scope probe I was using to test this actually made it work, so I guess all it needs is a little capacitance on the reset line...

Michael

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[Mark Pepper]

Well, I agree with the bigger capacitor idea (yes I fitted 100nf) but other reset circuitry uses the smaller cap and 10K resistor (see Dr Scott Baker's reset circuit). My scope also shows a decent reset low with either button press, on the backplane reset line. I suspect maybe a 74HCT14 Should be used instead of the 04 to get the hysteria, just a thought. There is a problem though.

[Andrew Instone-Cowie]

I had something very similar on my full monty - the backplane reset (which is just a switch and a resistor) worked fine, the reset on the dual clock board didn't. I found that although the RC circuit on the dual clock gave a nice smooth voltage rise over ~0.5s, the output of the inverter oscillated wildly in the middle of the range instead of the Schmitt trigger switching cleanly. Result, lots of short pulses on the reset line and the CPU locking up. Replaced the 74HCT04 and it all worked. The replacement IC was the last thing I bought from Maplin!

Andrew

[Mark Bramwell]

This thread has some photos of how I solved the reset issue.

I soldered a small cap across where the reset button is connected.

My system now starts every time I power up.

No resistor.

https://groups.google.com/forum/#!searchin/rc2014-z80/mark$20bramwell$20reset/rc2014-z80/00pJOKgdiwU/p3Hy17cyAgAJ

[Mark Pepper]

I have ordered some 74HCT14s to try in place of the 04s. It makes more sense to me that the Schmitt triggered inverter should be used in a reset circuit. I think that it should be OK for the oscillator part of the circuitry but we will see. The following is the circuit type that I am more familiar with (the inverter type not the MAX811 type).

[Mark Pepper]

Here is the link I referred to http://www.smbaker.com/z80-power-on-reset-circuits

[Bill Shen]

MAX811 is relatively expensive and a surface mount part, I use MCP130 voltage supervisor which is TO92 package and about 60 cents from Jameco.
  Bill

[Виталий Головач]

Good day!
Did you get a replacement for 7404 by 7414, did the reset work better?

среда, 24 октября 2018 г., 11:53:05 UTC+3 пользователь Mark Pepper написал:

[Mark Pepper]

Ahh yes... Sorry, I did replace with a 74HCT14 but no, it didn't work at all because the clock didn't work (I seem to remember). Because of course the inverters are also used in the crystal oscillator circuit. So, I have no answer at this stage I am afraid.

Mark

[Mark T]

You could try adding hysterysis to the 74HCT04 circuit to stop it oscillating with a slow input rise time. Possibly a 47K resistor from U1 pin 4 to U1 Pin 1.

Mark

[Christopher Mallery]

I just put together a Zed and have the same problem.  I will try the capacitor swap.  Thanks

[Tom Plano]

Did anyone get a definitive answer on a fix for this? I am experiencing the same power up, no reset and reset failures on dual clock mod.

[Christopher Mallery]

I never found a fix, but I discovered on accident that the reset only doesn't work if either of the clocks is jumpered the maximum.

On Mon, Sep 6, 2021 at 2:52 PM Tom Plano <tom.p...@gmail.com> wrote:

Did anyone get a definitive answer on a fix for this? I am experiencing the same power up, no reset and reset failures on dual clock mod.

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[Tom Plano]

After playing around and swapping some components if did get something to work. First, I swapped out the 47uF cap with a 1uF cap. This fixed the reset button on the clock module, but did not give me reset on power up. I was then poking at the inverters with a DMM as a basic sanity check during boot, and found that measuring voltage across either U1p1->U1p4 OR U1p3->U1p4 got me reset on power up. I guess the 10Mohm impedance of my DMM was enough to get the desired result. This lines up with Marks suggestion. I stuck a 10k resistor (what I had handy)  across U1p1->U1p4 and it works fine, resets on all buttons and on power up. I haven't tested all clock speed permutations yet but 7MHz for both work, so it should work for all.

[Phil G]

A DS1233 is only a pound from RS , its  a TO92 (like a small signal transistor with 3 legs).

Conveniently on mine, /wait is wired to +5 so access to pos, neg and reset is really convenient under the Z80: