1350A vs 1351A

Tony Duell ard at p850ug1.demon.co.uk
Mon Apr 24 17:10:15 CDT 2006

> This is a little strange!
> A HP1350 will plot vectors from 0 to 1022.

Hve you tried that for both coordinates?

I've not played with mine much, but from the manual, it appears there's 
10 bit wide memory in there. Each endpoint/character takes up 2 memory 
locations (hence the 2048 endpoints, it uses 4K RAMs). For endpoints, the 
2 locations are simply the X and Y coordinates. For a chracter, one of 
them (I forget which) is set to all 1's, the other is the ASCII code (7 
bits) + attributes.

My guess, therefore, is that it should take from 0 to 1023 in one direction.

> A HP1351 will not.  It will plot vectors from 0 to 1020.
> I need to retest this up to 1021 to see exactly where the
> limit is.

Very odd. I suppose I'd should look at the service manual on hpmuseum.net 
and see if I can sport the circuit that engorces this.

> The program I'm using for this testing draws a box around
> the limits of the display, and then two diagonal lines from corner
> to corner.  The result is a large box with an X through it.
> When the 1351 'fails' to plot the box, only the left hand Y axis
> vector is drawn.  No letters appear.
> When a 1350 tries to draw off the edge of the screen one
> of two things happen.
> Either a vector is drawn that wraps around, or a letter or
> symbol is drawn in place of the out of bounds vector.

MY guess (and it is a guess) is that it truncates the value to 10 bits. 
If they're all ones for the appropriate coordinate, the other coordinate 
gets turned into a character. Otherwise it just wraps round.

I am not sure if there's any limit on the number of characters that the
BCD-bianray converter will handle. It may do odd things if given more than
4 digits. 


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