Path: utzoo!attcan!uunet!cs.utexas.edu!rutgers!sun-barr!texsun!letni!merch!cpe!hal6000!trsvax!uhclem
From: uhclem@trsvax.UUCP
Newsgroups: comp.sys.tandy
Subject: Re: Model 4 ref manual
Message-ID: <193300150@trsvax>
Date: 30 Jun 89 15:18:00 GMT
References: <108@konkord.UUCP>
Lines: 50
Nf-ID: #R:konkord.UUCP:108:trsvax:193300150:000:2455
Nf-From: trsvax.UUCP!uhclem    Jun 30 10:18:00 1989


<>
B>	I've recently gotten my hands on a tandy model 4. One problem, turn
B>the thing on and all you get on the screen are 80 X 24 @'s, and the machine
B>does nothing but sit there. 

This means the video was never initialized to 0x20's.  Most common cause
is a problem with the AROM or the CROM.  Both must be present for the
machine to give the faintest hint of life.  Both must be version-matched,
although you will get beyond clearing the screen before that fails.

Next check the video ram chip, usually has a number like 6116 in the part
number somewhere.  For some reason in the Model 4 design, a missing video
RAM chip or an uninitialized one nearly always displays '@'s.

If this is truly a Model 4 (not a 4D), then you will see three ROMS
near the lower left corner of the circuit board.  From left to right,
they are "A", "B" and "C".  The A part number should have a 64 in it,
the B a 32 and the C a 16.  (These are 64kbit, 32k and 16k respectively.)

If you are able to leave it alone for a while and stuff on the screen
changes, that is a good sign that at least the CPU is running.  If
nothing happens, it is a negative test and doesn't tell you anything.

Since a Model 4 starts in Model III mode with the video ram existing
in the address space starting at 0x3c00, it is unlikely that the
CPU failed to map the video in since this is automatic.  The CPU does have
to map video out to go into Model 4 mode.

B>	I do have a model III, so I said, HEY! I could pull some of the IC's
B>out of the III and put them in the IV. Well, out of the 15 or so socketed
B>roms, I've been able to replace about 13 of them but to no avail.

True Model III ROM's were not meant to work in a Model 4 and may not be
able start at all.  I would get Model 4 ROM's in there first.  They are
required to initialize some ports on the CRTC used in the Model 4/4D/4P,
although this would not prevent text from appearing somewhere on the screen,
it just may not start in the upper left corner.


<This information is provided by an individual and is not nor should be
 construed  as  being  provided  by  Radio  Shack or Tandy Corp.  Radio
 Shack/Tandy Corp has no obligation to support the information provided
 in  any way.  As they say, "A Model 4 what?  Is that an XT or AT bus?"
 And I say, "No, its a working one.">
						
					"Thank you, Uh Clem."
					Frank Durda IV @ <trsvax!uhclem>
				...decvax!microsoft!trsvax!uhclem
				...sys1!hal6000!trsvax!uhclem