SWTPC 6800

[Pete Lancashire]

My 2cents

Some of this may have been mentioned. Your going to need an oscilloscope,
and a scope probe with a good ground. Your 'scope probe should
be less than 15 pF. Anymore and the probe will start to filter out any high
frequency noise.

Is the supplies output grounded ?, If so that point is where you put your
ground clip. Since this will be 90% of what you will find, I won't go into
non grounded DC sources.

Check what the output of your DC supplies looks like. The actual voltage
and the amount of ripple (60/120 or 50/100 Hz noise) and other noise
and what the ground at the exit of the supply looks like. Yes the ground.
If your happy with no ground noise you can use that point for your
ground clip.

Lets assume you have boards that plug in to a backplane. With all the
boards installed what do the supplies and again the ground(s) look like ?

Have an extender ? For each board, look at where the boards ground passes
to the backplane, if that point looks good, then you can move your
probes ground to that location. Now do the same thing, if just a 2 layer
board look at the ground pins on various devices, starting with the 'big'
chips,
then bus drivers, etc. Basically the devices that swing a lot of current.

Unique for memory boards. do the same for every device. Is the ground and
power pin clean ? One reason to look at every device is there maybe
a defective decoupling cap. Or sadly not enough of them. If the board is
only 2 layers and there is not a cap for each and every memory chip the fun
beings.

To keep things short, if the supply lines do not looks clean, your going to
be wasting a lot of time. What is clean, there is not short answer,
think about < 50-100 mV on every chip, specially memory devices.

Even the 'big boys' that should have know better designed and shipped a lot
of hardware that totally sucked.








On Fri, Aug 5, 2016 at 9:56 AM, Brad H <vintagecomputer at bettercomputing.net>
wrote:

> RAM could certainly be an issue.  I had *tons* of bad RAM dogging me with
> my Digital Group Z80.  That took two weeks of testing to get through it.  I
> might just test out all this RAM in the DRC boards first and see if I can
> get away with just using those rather than bringing the soldered MP-M
> boards into the mix right now.
>
> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Dave Wade
> Sent: Friday, August 5, 2016 2:42 AM
> To: 'General Discussion: On-Topic and Off-Topic Posts' <
> cctalk at classiccmp.org>
> Subject: RE: SWTPC 6800
>
> > -----Original Message-----
> > From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Brad
> > H
> > Sent: 05 August 2016 07:46
> > To: General Discussion: On-Topic and Off-Topic Posts
> > <cctalk at classiccmp.org>
> > Subject: RE: SWTPC 6800
> >
> >
> >
> > Yes.. I tried 7 bits.. different parity settings, speeds etc.
> > Couldn't quite nail it down.  In every tutorial online for the 6800
> > being used with PC terminal, they go 8 N 1.. nobody mentions
> > specifically if you are supposed to use hardware or xo/off or nothing
> > though.  So that's another thing.  I'm also confused because some docs
> mention baud rate settings for the cpu board?
> > I'm also not sure if bad RAM or bad TTL etc could be contributing to
> > just throwing out random junk too.
> >
>
> If the data received is inconstant then bad RAM is a likely cause, ROM
> usually fails consistanly.
>
> but as with all problem solving, Conan Doyle had it right when he said
> "after eliminating the impossible, whatever is left, however improbable,
> must be the solution"
>
> Dave
> >
> > Sent from my Samsung device
> >
> > -------- Original message --------
> > From: Dave Wade <dave.g4ugm at gmail.com>
> > Date: 2016-08-04  11:33 PM  (GMT-08:00)
> > To: "'General Discussion: On-Topic and Off-Topic Posts'"
> > <cctalk at classiccmp.org>
> > Subject: RE: SWTPC 6800
> >
> > "Random Stuff" on serial ports can be a speed, data-bits, or even
> > parity mis- match.
> > I assume you have tried tweaking these?
> >
> > Dave
> >
> > > -----Original Message-----
> > > From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of
> > > Brad H
> > > Sent: 05 August 2016 06:58
> > > To: 'General Discussion: On-Topic and Off-Topic Posts'
> > > <cctalk at classiccmp.org>
> > > Subject: RE: SWTPC 6800
> > >
> > > I think this 6800 is live but I am being dogged by my inabilities
> > > when it
> > comes
> > > to cabling.  I'm going to purchase a 'proper' db25 female connector
> > > and
> > the
> > > molex pins for the MP-S connector and solder it up.  I tried sort of
> > > using jumper wires to make it go.  When I turn the 6800 on, the PC
> > > terminal I'm using reacts by producing single or strings of random
> > characters.
> > > According to the SWTPC 'system checkout' stuff, if I get anything at
> > > all showing up on the terminal it usually means the 6800 is alive.
> > > Based on
> > the
> > > notes written on the MP-S, I'm confident I have the correct baud
> > > rate and
> > bit
> > > (8) and parity (1) settings, but alas, doesn't seem to work.  When I
> > > ran
> > into
> > > this sort of thing with MSI 6800 it turned out the baud rate switch
> > > on the serial card wasn't correct, despite factory labelling.  That
> > > might be the
> > case
> > > here but not really sure how to sort it.
> > >
> > > -----Original Message-----
> > > From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of
> > > Paul
> > Birkel
> > > Sent: Thursday, August 4, 2016 12:29 PM
> > > To: 'General Discussion: On-Topic and Off-Topic Posts'
> > > <cctalk at classiccmp.org>
> > > Subject: RE: SWTPC 6800
> > >
> >
>
>
>
>