Recent content by daver2

I am more concerned about where the existing voltage to pin 24 is coming from now (if not from the battery) - hence the concern. I assume, with power ON to the machine, there is +5V across the MC146818 between pin 12 (negative) and pin 24 (+5V)? This voltage *** MUST *** come from somewhere...

If the keyboard is removed, you should not have any signals at all on the keyboard PIA port B! All the inputs should be high... Dave

Did you also check to the nearby 4-pin header? It may be that selected 0.1" jumpers are required on the 4-pin header also? Dave

Everything that the CPU 'executes' are instructions - even if they 'look' like data! A lot of PDP-8 programs can contain self-modifying code - just to make life more fun :)! The problem occurs when a subroutine (JMS instruction) has been executed (e.g. as created by the macros NERROR and...

Pin 24 of the MC146818 should conduct to the banded ends of BOTH diodes D2 and D3. The positive rail should conduct to the none-banded end of ONE of the diodes (D2 or D3). The +5V rail of the PC should conduct to the none-banded end of the OTHER diode (D2 or D3). By "header" I am assuming you...

I would chase the circuit... Here is the device data sheet: https://www.nxp.com/docs/en/data-sheet/MC146818.pdf Here is the device pin-out: And here is a possible battery back-up circuit from the datasheet: If they haven't fitted the battery, did they fit the blocking diode? If not...

>>> It looks like an early C compiler. It is definitely "K&R-like". Dave

Agreed! But the design is still on paper - which means you have made the necessary decisions with all of the facts at your disposal. Dave

I was just posting a similar thing when your post came in... I was going to suggest a nice round number of a 1k resistor in series with each output pin. This would limit the current (at +5V) to a nice round 5 mA per pin. You may want to 'double up' on your 8255 ports though... Use an output...

>>> This still leaves the question of what safety features I can add to avoid shorting Vcc to ground when an output on one side is high and the other low. Can you give me an example? >>> but have never tried this on an 8255-style chip. The datasheets I have looked at do not define whether an...

The very last frame of the video (pin 16 = PB6) looks very dubious. The pin appears to be 'strangely' LOW... If you remove the keyboard, what do you obtain on pin 16 then? Dave

Makes sense. A SMPS produces way more noise than a linear one. Check for 50/60/100/120 Hz ripple (indicating lack of smoothing) and then increase the timebase to look at high-frequency switching noise. Don't forget to set your oscilloscope to AC coupling and increase the gain util you actually...

That looks strange. That looks to me like J6/6 = Cassette #2 switch to J2/3 = IEEE488 /SRQ IN. Unusual (and strange) but not the cause of your keyboard problem... Dave

So that is useful information. Perhaps decoupling or power supply noise then is to blame? Dave

What we are left with then is a fault within the keyboard 6520 PIA (but less likely if you have tried two of them and got the same result) or a really obscure fault (something like two devices being enabled simultaneously - one of which is the keyboard PIA - resulting in a logical AND of...