TRS-80 Model 16B "Burst Mode" PAL Upgrade

[dlightman]

When Tandy released XENIX 3.x a "68000 DMA BURST Mode" feature was added that allowed the Z80 to grab and hold the 68000 bus (apart from refresh cycles) and do DMA directly to/from the 68000 memory to/from Z80 I/O ports, or to/from Z80 memory.

The 8Mhz 68000 CPU card in the 6000 came with this feature but the 6Mhz (short) 68000 CPU card in the Model 16B required a upgraded set of PAL's, one capacitor and few simple modifications detailed in TB 12/16B:44 to add it. Without this upgrade unmodified XENIX 3.x would display the message "Bughlt NewPal - Hardware Change Required" and refuse to run.

In addition to the new feature Tandy also used TB 12/16B:44 as an opportunity to fix some memory related issues and replace U48 with what was referred to internally as the "corrected PAL". Both of these issues were corrected at the factory on the 8mhz CPU board.

Unfortunately 30+ years later so few of the upgraded 6Mhz CPU cards seem to exist they are regarded as rare and seeing as how the Model 16B was the best selling UNIX workstation of 1984 I think we should fix that.

I lucked into some unprotected PALs so here is everything needed to perform the modifications: https://bit.ly/2EnMBzY

 

[Eudimorphodon]

I keep meaning to play with JEDutil, I'm curious if it'd be able to extract the equations from the .jeds in that ZIP file and allow them to be replaced with generic 16v8 GALs.

 

[bladamson]

So, I am probably too dumb to do it, at least by myself, but.....

I keep thinking that we ought to come up with some kind of board layout for a modernized 68k+RAM card to slot into Model II type boxen to turn them into 6000s. I mean, with all the bespoke PCB manufacturing houses nowadays, it would probably be cheaper to build our own than to buy old ones.

Or hell, we could do a whole Model II/16/6000 machine that way. That fits into an AT/ATX form factor enclosure. And with a video card that can drive a VGA monitor. I've been reading the Model II technical manual a little before bed each night, and I am still a little head-scratchy about the video and disk controllers, but the CPU and RAM cards are very straightforward once you get the DRAM refresh logic out of the way. And a clone wouldn't need all that, we could just use a 512k SRAM.

But maybe I am once again enjoying too many adult beverages and making yet more unrealistic plans that would never come to fruition. :3

 

[pjhacnau]

So, I am probably too dumb to do it, at least by myself, but.....

I keep thinking that we ought to come up with some kind of board layout for a modernized 68k+RAM card to slot into Model II type boxen to turn them into 6000s. I mean, with all the bespoke PCB manufacturing houses nowadays, it would probably be cheaper to build our own than to buy old ones.

Or hell, we could do a whole Model II/16/6000 machine that way. That fits into an AT/ATX form factor enclosure. And with a video card that can drive a VGA monitor. I've been reading the Model II technical manual a little before bed each night, and I am still a little head-scratchy about the video and disk controllers, but the CPU and RAM cards are very straightforward once you get the DRAM refresh logic out of the way. And a clone wouldn't need all that, we could just use a 512k SRAM.

But maybe I am once again enjoying too many adult beverages and making yet more unrealistic plans that would never come to fruition. :3

I think it's quite possible - if you look at the S100 boards that have been done in the last 10 years at S100 Computers I don't see why the same couldn't be done for the Model 2/12/16 bus. It's more a case of the right individual, or individuals with the skills, motivation and time (especially time) to do the design work.

Talking about S100 systems, from the boards above you can set up a 68000 + Z80 system working in a very similar fashion (Boot on Z80, set up and transfer control to 68000). . . I was actually wondering earlier this week how hard it would be to port Tandy XENIX to S100 . . .

PJH

 

[dlightman]

I keep meaning to play with JEDutil, I'm curious if it'd be able to extract the equations from the .jeds in that ZIP file and allow them to be replaced with generic 16v8 GALs.

My interest was making everything available again but if you have success moving the code to a pin compatible GAL please update the thread and let everybody know.

 

[dlightman]

So, I am probably too dumb to do it, at least by myself, but.....

I keep thinking that we ought to come up with some kind of board layout for a modernized 68k+RAM card to slot into Model II type boxen to turn them into 6000s. I mean, with all the bespoke PCB manufacturing houses nowadays, it would probably be cheaper to build our own than to buy old ones.

If you want to do this quickly new CPU cards and memory boards are sold by rpsi92 on eBay so you can add 68K support to your MII fairly easily. I myself would certainly be interested In a more modern 68K card with integrated memory and expanded MMU. There's been talk about doing something like that for a long time but IMHO it would be a labor of love sort of project rather than something profitable.