Problem(s) Solved

[Tom Droege <tdroege@veriomail.com>]

As you all know, I have been going nuts trying to solve the junk data 
problem.  As if often the case in such situations, there were two problems 
with similar symptoms.  This makes it really tough to solve.

Problem 1.

The untied connections on the scanner board would sometimes drift to the 
right state and cause the scanner to keep scanning during readout of the 
memory.  This often did not happen until long after the system was turned on.

Problem 2.

This is the problem I have been having with the test setup for the last 
couple of weeks.  Sometimes it worked, sometimes it did not.  This gave 
very confusing symptoms since I was trying to test boards of unknown 
quality.  This turns out to be caused by AC common mode between the place 
where the test set up was plugged into the wall, and the place where the 
computer was plugged into the wall.

Arne take note about problem 2.  There are two ways to solve this 
problem.  You can put the Mark IV into an extension cord that is then 
plugged into the same outlet as the computer.  You can plug the computer 
into an extension cord that runs out to plug into the same outlet that is 
used by the Mark IV.   Note that the serial cable, the DB-25 cable, and the 
extension cord should all be run together.

Note that when I say "same outlet"  I mean exactly that.  This does not 
mean the same wall duplex outlet.  Often a wall duplex outlet will be wired 
to two different circuits.  This could cause big trouble.  A cube tap (if 
you can find one) will do the job.  Better is to plug an outlet strip with 
a switch into the wall, then plug the Mark IV and the computer into the 
same strip.

Note that I have long worried about such problems.  I just forget from time 
to time.  TOM in the tower has an extension cord that comes down the pipe 
and plugs into the same outlet strip as the computer controlling it.  I was 
not so careful running MIKE on the porch, so I had problems.

I once designed a computer room with a dozen large computers and 75 or so 
smaller ones- this back when a "small" computer required killowatts.  This 
was a 15,000 square foot room with 160 tons of air conditioning and a 500 
HP motor generator set providing isolated power.  The power to the 
computers was laid out in a big star.  We never had the type of problem I 
describe above.  It is possible to plug up a Mark IV system so it will work.

Tom Droege