Tandy Video Mod

[Chuckphd53]

I have been reading as much as time allows to come back up to speed on the jr...

I have found some info on the Tandy mod, but to be honest I cannot understand the entire need for this mod.
It is suppose to allow 16 color mode?

Next, how does the mod affect the Jr and working in its standard mode, simply, does it still behave like a Jr or does it now become a differnt animal?

thanks
chuck

[OldComputerGuy]

The Tandy graphics vary a bit in the way memory is allocated from the PCjr graphics and has more software support.

[Chuckphd53]

so is it recommended to make the mod, and still have the Jr video remain workable for most other original programs?

[OldComputerGuy]

You'd have to ask someone who's actually done the mod. I also have a Tandy 1000 for those games, so I don't need to do it.

[Trixter]

The mod is recommended, as it allows more software to use the 320x200 16-color mode. IIRC It does not affect PCjr compatibility, so it's almost always a recommended thing to do.

[Chuckphd53]

Perfect, on my list od todo's

[alanh]

The only thing the Tandy mod does is fix the upper 16KB of the 32KB window at B8000 when mapping odd page numbers. Without it, you get:

Selected page 0: B8000 = page 0, BC000 = page 1
Selected page 1: B8000 = page 1, BC000 = page 0 (should be page 2)
Selected page 2: B8000 = page 2, BC000 = page 3
Selected page 3: B8000 = page 3, BC000 = page 2 (should be page 4)
Selected page 4: B8000 = page 4, BC000 = page 5
Selected page 5: B8000 = page 5, BC000 = page 4 (should be page 6)
Selected page 6: B8000 = page 6, BC000 = page 7
Selected page 7: B8000 = page 7, BC000 = page 6 (should be page 0)

As you can see the problem is when an odd page is selected, you don't see a linear 32KB at B8000 like on a real Tandy 1000. I did a small circuit board that replaces ZM31 with a 16V8 SPLD and dip switch that allows switchable PCJr or Tandy compatibility. However there shouldn't be any real issue keeping the Tandy mod active. A lot more information can be found in my public repo here:

https://www.retrotronics.org/svn/jride/trunk/tandymod/

[alanh]

Actually I should read my own notes (in the PLD file in the linked repo). I was describing what I call TandyMOD+. Without any modifications, the BC000 page frame is just an alias of B8000. With TandyMOD, you get a +1 for even starting pages and bad aliasing for odd. With my change (MOD+) you get correct linear behavior even for odd pages.

See the notes in the PLD file for a complete description of the PCJr lower 1M memory space with notes on how it's controlled and what can be mod'd.

[Chuckphd53]

Alan...

are the Mod boards available ?

I have three Jr's I am working on and would like to get them up to speed with this mod

thanks
Chuck

[geoffdaddy]

I am wondering if anyone else has problems with their mod on some PCjr programs. Notably, I have noticed that Colorpaint doesn't display properly on my jr with the mod performed. Of course, there could always be an issue with the mod I did, but it seems that most everything else I've tried with it works.