8085 RESET

[Adrian Graham]

On 19/03/2017 19:59, "cctalk" <cctalk at classiccmp.org> wrote:

> On 2017-Mar-18, at 5:21 PM, Adrian Graham via cctalk wrote:
>> 
>> 8085-based phone system weirdness continues and I'm beginning to wonder if
>> the PSU rails are all coming up in time for RESET to go high - given there's
>> 4116 DRAMs in there isn't there supposed to be a proper power up order?
>> 
>> While I look at using a 20-pin ATX PSU to run this machine temporarily I
>> need a safe way to reset the CPU rather than constantly power cycling. The
>> RESET line comes from an ICL7611 op-amp via an MC14081B through pins 1-4 of
>> a 74LS04 and I need to pull it low for longer than 3 clock cycles.
>> 
>> I wish I had a schematic to show!
> 
> Recalling the messages from December, it looked like proper operation of the
> reset circuitry at power-up
> expected the presence of the battery to power the reset circuitry with 12V
> prior to the 5V bus coming up.
> 
> It looks like you could add a reset switch to the unit by paralleling R406
> (11K) with a NO pushbutton switch and ~ 4.7 uF cap (all 3 in parallel).
> Closing the pushbutton switch simulates the absence of 5V. The cap provides a
> little debounce delay, but may not even be necessary.

I've just checked the image you're referring to and the resistor numbers are
offset so it looks like R406 is the top one of three where it's actually the
middle one. Here's an up-to-date version:

http://www.binarydinosaurs.co.uk/STCExecutelResetOpamp.jpg

Cheers!

-- 
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?