11/45 RTC

[Tony Duell]

> 
> 
> Tony wrote...
> > 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 

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.

> 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.

I think somebody else has pointed them out on Bitsavers.

Assuming you can read thsoe, you should see the LTC L signal coming in on 
CD1 and going into a 7414 gate. My next test would be to look at the 
output of that gate with a 'scope. You should get a nice square wave at 
mains frequency there.

> 
> 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