Model 3 Overview
The Model 3 was Tandy's second personal computer. One of the
reasons it was developed was because the FCC instituted regulations about
the RF emissions generated by computers (and other electronic devices).
The Model 1 was completely unshielded (a fact many owners who also like
TV and/or radio discovered the hard way), and was unable to pass the emission
The Model 3 was somewhat better for new users because the monitor and
keyboard were all in one unit. That got rid of the problems the Model 1
had with funky connections between the keyboard and expansion interface.
The serial port was also usable. :-) The cassette interface supported both
the 500 baud speed of the Model 1, as well as a much faster 1500 baud.
Using a cassette based system was much less painful with the data screaming
along at that speed.
The Model 3's BASIC ROM was 2k larger (at 14k) than the Model 1, added
the 1500 baud cassette speed, allowed for left and right shift keys to
be at separate addresses in the keyboard memory area, and added a 25 or
30 HZ real time clock routine (country dependent) which allowed for TIME$
to be added to ROM BASIC.
Model 3 Operating Systems
As in the case of the Model 1, there were quite a few different
disk operating systems available for the TRS-80 Model 3.
Below, the major operating systems for the TRS-80 Model 3 are listed, along
with some comments from myself and others.
In MY opinion, the Model 3s TRSDOS was its one unredeeming quality.
Thank goodness for LDOS, NEWDOS/80 and DOSPLUS, and whatever other third-party
operating systems were available.
Instead of keeping things as compatible as possible with the Model 1,
Tandy decided to use a different directory track layout. Also, the directory
records were extended to be 48 bytes long, instead of 32. They supplied
a CONVERT utility which would copy files from a TRSDOS 2.3 disk to a TRSDOS
1.3 disk, (why was Model 3 TRSDOS at a lower version number, anyway?),
but it had a few problems when files on the Model 1 disk were password
In contrast, most (or all) of the other operating system vendors made
sure that their Model 3 systems could read disks from their Model 1 versions
without any trouble.
Eventually there will be a link here with more Model 3 TRSDOS information.
NEWDOS and NEWDOS/80
Apparat made NewDos/80. This was much more powerful than their first
release of NewDos 2.1 for the Model 1. NewDos/80 could do just about anything, and did
it rather well.
If you would like to see it, there is a page of NewDos/80
Commands that gives a rundown of the commands available on both the
Model 1 and Model 3 versions of NewDos/80. Thanks to Sander
Berents, who sent it to me. All I had to do was convert it from LaTeX
format to HTML.
Micro Systems Software made DOSPlus for the Model 3. I think most of
the Model 1 comments apply.
Again, the Model 1 comments probably apply to the Model 3 version. I
really need more MultiDos information.
Logical Systems wrote LDOS for the Model 3. Tandy even sold it as their
Model 3 Hard Disk Operating System, since TRSDOS was unable to support
those. For the most part, all the comments in the Model 1 section of the
page apply to the Model 3 version.
If you're interested, here is another link to that page full of LDOS
Internals. This has lots of good programming information for both the
Model 1 and Model 3 versions of LDOS. The entire LDOS 5.x manual is available
on the web at
Tim Mann's TRS-80 page, along with disk images of LDOS 5.3.1 for the Model 3 (and
Model 1). These have been released by Roy Soltoff, who holds the copyright on them, as
freely available. See Tim's page for the full details.
Model 3 Internals
In order to keep the size down a little, these are separate pages.
Here is a page of Model 3 (and 4) I/O Ports
Here is a page of Model 3 (and 1) RAM addresses
Here is a page of Model 3 (and 1) ROM addresses
Other hardware and stuff
This is still being worked on. There were a number of hardware
additions you could make to the Model 3, most notably a hard disk
controller and hard disks.
The TRS-80 Home Page created and maintained by Pete Cervasio
Copyright © 1998 Pete Cervasio