jar@konkord.UUCP (jar) (06/20/89)
I've recently gotten my hands on a tandy model 4. One problem, turn the thing on and all you get on the screen are 80 X 24 @'s, and the machine does nothing but sit there. I do have a model III, so I said, HEY! I could pull some of the IC's out of the III and put them in the IV. Well, out of the 15 or so socketed roms, I've been able to replace about 13 of them but to no avail. So, I'm in desperate need of a schematic for the mother board of the 4! Radio Shack wants (ugh! $50!!!). Needless to say, Radio Shack has been NO help! I've called up looking for info, and all you get is, 'We don't know, you'll have to buy the service manual' (yea, for $50!) So, PLEASE! Anyone out there who has had a heart breaking experience like this will understand! All I'm looking for is a nice photo copy of a model 4 schematic. Anything would be great! If you could FAX something or send mail back with some info would be GREAT! Info can be sent to (FAX) number: 508-481-9772. Thanks alot! john Roberts
paulm@ccicpg.UUCP (tmp Paul Moreau usenet acct) (06/29/89)
In article <108@konkord.UUCP>, jar@konkord.UUCP (jar) writes: > > I've recently gotten my hands on a tandy model 4. One problem, turn > the thing on and all you get on the screen are 80 X 24 @'s, and the machine > does nothing but sit there. > [...... Sorry to post this to the net but I haven't been able to get a hold of John using the mailer .... Hi John, I was just thinking of the problem that you're having with the TRS-80 Model 4. Before I went into Diagnostic Software, I was A hardware Engineer. From my experience with computers, my guess is that when you first power the system up it does the following: Clears ram by writing 00's to all locations (including screen ram) relocates the ROM to Ram. Swaps memory (turn off shadow ram). goes to town. Well you said that the screen is full of @'s. It would take the processor to write that. My guess is that you might have a data bit (bit 6) stuck high because of a short or a bad data buffer and instead of writting 0x00's, it's writing 0x40's (a @ character). After clearing ram it probably relocates the ROM into ram and jumps to Ram. If bit 6 is stuck High, then all the code is bad, hence crash. OK, now I'm looking at my schematics , lets see .... On a Rev.A-D board, U71 (74LS245) is the Data buffer tranceiver. All data EXCEPT ROM is passed through this chip. U71 (CPU side) A B (LOGIC side) _____ D0----4| 7 |16----D0 D1----6| 4 |14----D1 D2----8| L |12----D2 D3----9| S |11----D3 D4----7| 2 |13----D4 D5----5| 4 |15----D5 D6----3| 5 |17----D6 <-- check this puppy D7----2|__ |18----D7 |EN DR| ----- 19 1 | | | | The DR (direction) pin is high for a CPU WRITE to RAM, low for a READ. The Gate Array board has this same chip as U28 but the pinouts are different. CPU side d0-d7 are 2,3,4,5,6,7,8,9 Logic side are 18,17,16,15,14,13,12,11 Hope this helps, it make sence to me that this is probably it. ---- .===========================================================. | ####### ####### ### C O M P U T E R | =----------= | | ### ### ### C O N S O L E S | An STC Company | | ####### ####### ### Incorporated | =----------= | |-----------------------------------------------------------| | UUCP: ...ccicpg!dl2!paulm (Paul L. Moreau) | | or ...ccicpg!dl1!paulm (Diagnostics Software Eng) | | or ...ccicpg!paulm (Irvine, California) | `===========================================================' Avoid reality at all costs.
uhclem@trsvax.UUCP (06/30/89)
<> 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