> On 28 Sep 2017, at 03:47, dwight <dkel...@hotmail.com> wrote:
>
> Were does the powerup signal go?
> Dwight
Hi Dwight,
It goes to the output control (pin 15) of an LS257, the outputs of which look
to be chiefly in charge of ROM selection via an LS138, schematic here:
http://www.binarydinosaurs.co.uk/newbrainPWRUPSchematic.png
<http://www.binarydinosaurs.co.uk/newbrainPWRUPSchematic.png>
Looks like it’s giving a few seconds delay for everything to power up cleanly.
A
>
> From: cctalk <cctalk-boun...@classiccmp.org
> <mailto:cctalk-boun...@classiccmp.org>> on behalf of Adrian Graham via cctalk
> <cctalk@classiccmp.org <mailto:cctalk@classiccmp.org>>
> Sent: Wednesday, September 27, 2017 5:23:45 PM
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Strange grounding problem
>
> Hi folks,
>
> This is another Grundy Newbrain problem on a different machine that is
> probably easily explained by someone who understands current flow. This
> machine has two startup circuits that are simply a resistor and 10uF
> capacitor each, both feeding a Hex Schmitt Trigger (CD401068CM). When the
> caps have charged up sufficiently they activate both PWRUP and RESET signals
> one after the other thanks to the 220K and 560K resistors.
>
> Schematic for the circuits is here -
> http://www.binarydinosaurs.co.uk/newbrainPowerupCircuits.png
> <http://www.binarydinosaurs.co.uk/newbrainPowerupCircuits.png><http://www.binarydinosaurs.co.uk/newbrainPowerupCircuits.png
> <http://www.binarydinosaurs.co.uk/newbrainPowerupCircuits.png>> - top
> circuit is PWRUP and the bottom one is RESET that goes straight to the Z80.
>
> Only in this machine it doesn’t unless I have 3 sampling probes and a GND
> probe on my USB-powered logic analyser attached to pins 3-5 of the CD401068CM
> and the GND pin of any nearby chip. Less than 3 sampling probes and it won’t
> start so those probes must be facilitating current flow to GND?
>
> Cheers!
>
> PS Brent, I got the other Newbrain going by replacing the 74LS166 shift
> register, the one that provides the SOVSR signal. All good now!
>
> —
> Adrian/Witchy
> Binary Dinosaurs - Celebrating Computing History from 1972 onwards
>
