From: uhclemLOSE.jul06@nemesis.lonestar.org   
      
   The 16K Model 4 was a base-level "entry" system, effectively to get a   
   foot in the door.  It was a ROM BASIC-only environment, with tape as the   
   only storage medium.  Customers of the day who bought the 16K model   
   invariably found it hopelessly constrained and purchased memory and disk   
   drive upgrades as they could afford them, but it was cheaper to just   
   get the 64K disk system in the first place.   
      
   The 16K chips were discarded when the upgrade was performed, or   
   ended up in the repair shops supply of "tested and new" replacement   
   16K parts used to repair other systems.  (They hated it when you asked   
   for the removed parts back.)   
      
   16K systems normally did not have a disk controller, since no OS was   
   available that would fit in 16K and leave much room for applications.   
   TRSDOS 6 used 0x0000-0x2fff (12K) for itself nominally, leaving   
   0x3000-0x3fff (4K) of RAM for stack and applications, and you had to   
   hack the OS to have the stack pointer that low anyway.   
      
   The ability to use 16K RAM chips was dropped after a board revision   
   or two, because the 16K units were not selling (not even useful as   
   education network stations), and eliminating the capability to use   
   the 16K chips meant dropping several jumpers and some circuitry for   
   the three voltages the 16K chips required.  The 64K chips only used   
   a single voltage.  Somewhere along in there, the pads for the expected   
   Z-800 processor were also removed, as we gave up waiting on Zilog.   
      
   The Model I/II/III/4/4P systems used an off-white phosphor.   
   Gradually Tandy switched to green in models (including the 16B/6000),   
   and there were a few rogue units around with an amber phosphor.   
   I had heard at the time that the green was cheaper, but there was   
   also a bit of pressure to copy what others were doing, and the IBM   
   Monochrome monitor that came out in the fall of 1981 was green too.   
      
   It is important to remember that Tandy Merchandising really didn't   
   come up with many ideas on their own - they were far more likely to   
   react to and copy others in the marketplace, at least up to the point   
   where it cost something to copy exactly.  So dim impressions of what   
   others were offering was more common than precise duplication.  (Over   
   time, as everybody got on the IBM-clone bandwagon, that behavior   
   altered somewhat.)   
      
   With few exceptions, the Z-80-based TRS-80 systems used a video system   
   based around a Motorola 6845 CRT controller chip, which IBM later used   
   as well in the earlier CGA and EGA IBM cards.  A lot of text terminals   
   of the day also used the 6845.   An emulation of the 6845 register   
   set lives on in most modern VGA/XGA/SVGA video cards to provide hardware   
   compatibility back to the stone age of IBM-compatible systems.   
   Today, a "megacell" functional copy of the 6845 can found buried in the   
   video chips used in PC/IBM-compatible video cards.   
      
      
   Frank Durda IV - only this address works:|"Recently, I noticed someone did   
     | a search on Google for 'slot   
   You must remove the "LOSE" to mail me.   | machine source code' and it took   
   	http://nemesis.lonestar.org 	 | them to the LS-DOS source code."   
   Copr. 2006, ask before reprinting.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)