29 Mar
2006
29 Mar
'06
2:58 p.m.
Tony wrote...
Most, if not all, fequency counters detect the number of times the input
crosses a given throeshold voltage in a particular time interval (or
alternatively measure the time between the crossings of said threshold).
If the input signal is noisy, or something like that you might get
spurious results.
However, I don't see how this can be the
cause of the problem. A 180Hz
signal would still trigger the DL11-W circuit. I doubt the machine would
get interrupt-bound either.
Nope, standard 110v singlephase outlet. I think
something is up with my test
cable. I'm going to double check it tomorrow. I do know that I set a
function generator on 50hz and connected my freq counter to it. It
registered right on 50hz. I moved the freq counter to the /45 and got about
189hz on CD1. That just seems wrong. I connected my scope to the function generator set at the same 50 hz and the scope picked
it up sharp at 50hz
(scope does measurement). I moved it to the /45 and got odd readings, around
189hz. Here's something else... on the function generator, the 50hz clock
was square and very clean. It also had a stable display. When looking at the
trace on the /45, the rise and fall of the square were noticeablely sloped,
not square. Also, it's like it was triggering right... because the square
That is actually correct. If you look at the PSU scheamtics in the
printset you'll see that the LTC L signal is produced by clamping the
sine-wave output from the transformer using a zener diode (and a couple
of resistors). The output will look like a sine wave with the top cut off.
The input stage on the DL11-W uses a schmitt trigger gate (part of a 7414
IIRC) which woould squeare up that signal.
image seemed to alternate between two traces 90
degrees out of phase (1/2
step back). Made it hard to measure the period.
What have you connected the ground lead of your 'scope probe to?
Have you tried fiddling with the trigger level (and other trigger
controls) on the 'scope?
Do you have the DL11-W prints (e.g. in an 11/34
printset)? If so, I can
talk you through some simple tests on the board itself.
I do not have DL11-W
prints alas.
Maybe I should start from the beginning and re-explain the problem as I
percieve it. The machine boots xxdp+ v1.0 and 2.5 just fine. However, when I
try to boot RT11, the lights flicker and the disk flickers for about 10
seconds, and then the system just halts. Because of the fact the system can
read and write full disk packs using VTserver, I kinda doubt there is a
major read/write problem with the drive. I am GUESSING that what is
happening is that xxdp does not use interrupts for anything, nor does
vtserver. My GUESS is that RT11 does, and that there is something wrong with
interrupts on my machine. I am GUESSING that at some point in the boot
process of RT11 it tries to use interrupts and thus my machine just dies
there. I centered on the LTC as a possible route to investigate for two
reasons - one, I was under the impression that the LTC signal had to be
present for interrupts to work, and if it wasn't, I'd get this behaviour,
Not really. The LTC signal _geenates_ an interrupt once every mains
cycle. It's used for a software real time clock, for things like task
switching, etc.
But there are many other devices that can cause interrupts. Just about
any I/O device can, including your console port (separate interrupts for
input and output), yuor RL11 disk controller, etc. Those interrupts will
work (assuming you have a working machine!) whether the LTC L signal is
there or not. The interurpt logic does not require LTC L to work.
-tony