1200XL Jumper W1 Mystery

[+kheller2]

For as long as I can remember, when I first obtained a 1200XL and the 1200XL OS Manual Technical Reference Notes supplement.... I was always curious about this:

 

 

3.8 OPTION JUMPERS

The 1200XL is provided with a set of four hardware jumpers which are designed to tell the operating system how the system is configured As of the date of this writing, only one of the four jumpers has been assigned. specifically Jl. This is specified in the table below. During the power-on sequence, the 1200XL operating system reads the state of these jumpers and stores this state in the OS database variable JMPERS, location 030E.

The bit assignments for each of the four jumpers is as specified below. The bits are all active low, meaning that if a line reads a digital zero. the jumper is installed.

BIT FUNCTION HARDWARE NAME

o Self test enable (will run self test if low) J1 (pot 4) 1-3 Reserved for future use

4-7 Unused

(Granted, I was even more curious about section "3.13 Deleted Features" in the table of contents that didn't actually exist as a page...)

There it was on the schematics:

Today when looking over the different versions of 1200XL boards I noticed it in a completely different place... Rev 11A (a PILOT run board)

This area looks nothing like the production model Rev 13A and A which has W1 in a different location:

The 11A board always posts to the diagnostic screen too. Confusingly, and probably because I'm still getting over a cold and are missing something simple, I can't figure out why all boards don't boot directly to diagnostics since all 4 pots are tied to ground.

Karl

[+DrVenkman]

 

For as long as I can remember, when I first obtained a 1200XL and the 1200XL OS Manual Technical Reference Notes supplement.... I was always curious about this:

 

 

3.8 OPTION JUMPERS

The 1200XL is provided with a set of four hardware jumpers which are designed to tell the operating system how the system is configured As of the date of this writing, only one of the four jumpers has been assigned. specifically Jl. This is specified in the table below. During the power-on sequence, the 1200XL operating system reads the state of these jumpers and stores this state in the OS database variable JMPERS, location 030E.

The bit assignments for each of the four jumpers is as specified below. The bits are all active low, meaning that if a line reads a digital zero. the jumper is installed.

BIT FUNCTION HARDWARE NAME

o Self test enable (will run self test if low) J1 (pot 4) 1-3 Reserved for future use

4-7 Unused

(Granted, I was even more curious about section "3.13 Deleted Features" in the table of contents that didn't actually exist as a page...)

Oddly enough, I could never find J1/W1 on any 1200XL board, yet there it was on the schematics:

1200XL W1.jpg

that is until today when looking over the different versions of 1200XL boards. And here it is.. clear as day... Rev 11A (a PILOT run board)

IMG_3775.JPG

This area looks nothing like the production model Rev 13A and A:

IMG_3777.JPG

The 11A board always posts to the diagnostic screen too. Confusingly, and probably because I'm still getting over a cold and are missing something simple, I can't figure out why all boards don't boot directly to diagnostics since all 4 pots are tied to ground.

But I am happy to have solved another, perhaps useless, mystery.

Karl

 

 

Very interesting stuff.

 

I do wonder, however, what that jumper is right below R32 in the photo of the production version, right next to the two inductors.

[+kheller2]

That's W1 on the prod board, which is tied to R32. I was just too lazy to highlight it. LOL I try not to think too hard, because I would start wondering where all the caps on the 11A board went on the prod boards.

[Gunstar]

W1 is like a keystone, it doesn't seem to be any more important than any other component, to the untrained eye,but remove it and the entire board will crumble.

 

And all computers run on magic smoke, let the magic smoke out and it's OVER... bricked!

Edited January 13, 2018 by Gunstar

[Kr0tki]

Confusingly, and probably because I'm still getting over a cold and are missing something simple, I can't figure out why all boards don't boot directly to diagnostics since all 4 pots are tied to ground.

The OS boots straight to Self-Test if the value of POKEY's POT4 register is 0. I don't know what that W1 jumper does, but it apparently causes the value of POT4 no be non-zero. I guess that the 1200XL OS manual is simply incorrect in that regard. (For starters, they call the jumper J1 not W1, suggesting that this section was written before the 1200XL design was finalized.)

 

Could you boot BASIC on your working machine and post the values of PADDLE(4) to PADDLE(7)?

[phaeron]

The documentation is confusing but correct. It is referring to the bits in JMPERS that come from bit 7 of the POT4-7 registers, not the physical POT lines going into POKEY. WIth the jumper in place, POT4 never reaches threshold, the POT4 counter counts all the way to 228, and bit 4 in JMPERS is 1. With the jumper removed, the POT4 line is always above threshold, the POT4 counter stops quickly at 0 or near 0, and JMPERS bit 4 is 0.