TRS-80 IDE/Model IV Technical info
Rog Blake
I figure SOMEONE has to have hooked up an IDE drive to a TRS-80 and
patched LDOS/NEWDOS/whatever to run on it. Please don't refer me to TCJ
or GIDE, since neither has any specific info on this (aside from the
hardware interface, which is really quite trivial).
Also, I was wondering if there's a technical manual for the Model IV,
including schematics. I believe this computer used some PALs, so the
schematics may not be too useful. What I really want is information on
the video controller. Programming information is good enough (where
video was mapped into memory, how bit-mapped graphic/80 column mode was
entered, etc) since I can figure out the hardware from that.
TIA,
Rog Blake
hu...@ripco.com
Frank Durda IV
Rog Blake (hu...@ripco.com) wrote:
: Also, I was wondering if there's a technical manual for the Model IV,
: including schematics. I believe this computer used some PALs, so the
: schematics may not be too useful.
Tandy was good about this. There was a technical reference and a service
manual prepared for nearly every computer they sold and at least one (usually
both) contained schematics. In the early Model 1000 days, they even included
PAL equations. PAL equations for the Model 4/4P systems were not published
at the time, but they still exist and hopefully will be available soon.
: What I really want is information on
: the video controller. Programming information is good enough (where
: video was mapped into memory, how bit-mapped graphic/80 column mode was
: entered, etc) since I can figure out the hardware from that.
The Model 4/4D/4P used a masked version of the Motorola MC 6845 Video
controller, a device eventually copied by just about everyody else.
It is the basic video controller in the original IBM PC, and all PC video
cards since then mimic the 6845 registers (zero thru 15) to provide PC
compatibility. Getting information about The 6845 should therefore be
incredibly easy. All of the Tandy 1000 and up technical reference manuals
have programming information on these registers.
However, Tandy elected to use a masked part in the Model 4 series, called a
68045, where some of the registers contain hard-coded values and cannot be
changed. In particular, the registers controlling the number of rows and
columns, horizontal and vertical sync, are preset. By raising and lowering a
pin on the 68045, you can select between two hard-coded sets of values,
which Tandy used for the 64x16 and 80x24 screen dimensions the Model 4
supports.
I assume there was a hardware cost savings to using the 68045, because
the creedo of Tandy R&D management at the time was to do everything possible
in software to avoid having to pay for hardware, even if it required
more hardware (like more ROM) to actually perform the function in software.
Somewhere around I have the masked settings that Tandy ordered for these
devices if you are really interested.
Trivia note: You can replace the 68045 with a real 6845 and TRSDOS 6 and
LSDOS 6 will work fine. To get Model III OS-es to work, you need the
XROM/XDROM from M.A.D. Software that correctly initializes all of the
registers for 64x16 operation (knowing that if TRSDOS/LSDOS6 shows up, the
settings for 80x24 will get loaded).
With a real 6845, you can run the Model 4 monitor in 80x25 mode for text and
minor patches to the DOS make things work pretty sane. You have to switch
back to 80x24 for high-res graphics, but that isn't that common an operation.
The real 6845 also allowed a hardware cursor and hardware scrolling. It seems
like one of these things was not available in the Model 4 series because
of the pre-programmed values. In any event, TRSDOS 6 and LSDOS 6 can't use
80x25 anymore anyway because LSI used the then-unused video RAM as a keystroke
buffer.
(2048 bytes of video RAM - (80x24 = 1920)) = 128 bytes.
Hardware scrolling was used in Model II/12/16/6000 systems, particularly in
all releases of XENIX. Hardware video cursor was used in all
Model II/12/16/6000 operating systems.
Frank Durda IV - only these addresses work:|"The Knights who say "LETNi"
<uhclem.may98%nemesis.lonestar.org> | demand... A SEGMENT REGISTER!!!"
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This Anti-spam address expires May 30th |"LETNi! LETNi! LETNi!" - 1983
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