Some DG 1200 progress

Jay West jwest at
Tue Aug 17 12:37:04 CDT 2010

Christian wrote....
>Yep.  Each lamp draws about 10ma in order to keep them warm and to avoid 
>large surge currents when they're driven on.  The supply should actually be 
>15V unfiltered and unregulated coming directly off a bridge, not 11V.
Ok, I'm confused then. The schematics (and the theory of operation verbiage 
{long}) all clearly state 11v for the lamps. Is it possible you're thinking 
nova 2 or 3 or some other model? The schematics and technical manual I have 
specifically state nova 800/1200. This system in particular is a 1230.

And I had written....
> I can deposit and examine correctly to all 4 cpu registers, but memory is 
> a different story. There are a few spots in memory that I tested (at 100o 
> for the one 8K stack and at 20000o for the other 8K stack) where I can 
> deposit and read back as expected. However, there are a fair number of 
> spots where if I deposit a value, the machine goes into run mode when I 
> hit the deposit switch. Doesn't that sound more like a cpu card issue than 
> memory or front panel?

To which Christian replied....
>It's decidedly not memory.  It *may* be a CPU issue but it actually sounds 
>more like a noisy bus (i.e., no terminator) or a wonky front panel.

>Do you have anything in the topmost slot of the chassis?  If not, do you 
>have a bus terminator installed on the big-ass connector on the back of the 
>machine?  This is less of an issue for a short chassis, but a jumbo will 
>pretty much do entirely random things without the bus having some sort of 
>termination on it, particularly with respect to the front panel.

Well, funny you should mention that. I definitely plowed into this machine 
before really digesting all the docs in detail, and am not familiar with the 
termination setup. This machine had cpu in slot 1, dg 8K in slot 2, cassette 
I/O in slot 3, a plessy 8K board in slot 4, and then at the very top (slot 
17) had a home brew interface (using one of the DG prototyping boards). I 
left that board out, was trying to start with as small of a setup as 
possible. From the sounds of it, I really need to look up the termination 
requirements/methods :)


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