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VTL-2 for Altair 680b


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  VTL-2 for Altair 680b by Bill Degnan - 07/10/2007 21:04
Just a quick note...I have been working with Grant Stockley to build and test ROMs for VTL-2. I helped test. Grant sent me a copy of the three PROMs needed to load Very Tiny Language 2 (VTL-2). Here is the current status.

I put all three PROMs in place. I was not able to boot to BASIC, instead I get various inconsistent results and no "OK" prompt. Sometimes a series of 10 or so " . " characters, scroll down the screen, one per line before the system halts; sometimes just a single letter will appear, sometimes nothing happens at all. There are more details, but that's the long and the short of it. I can't get to the ACIA monitor prompt if all three PROMs are in place.

I was able to use the toggle switches to check memory. Working backwards from the end of VTL-2, I found that all of the memory was intact/correct (spot checking a few dozen locations). The program is loading into memory, or appears to be.

Grant suggested that it could be the motherboard. Maybe using 1200 baud is preventing the screen output to be OK. Have anyone ever tested the VTL at 1200 baud? ACIA seems to be loaded in memory correctly as well. Could be that there's not enough memory to execute the program.

  (was) VTL-2 NOW: Diagnosing PROM issue by Bill Degnan - 07/14/2007 22:35
VTL-2 fits in three 1702 Proms on the MITS Altair 680b (the 6800 Processor Altair)

PROM 1 and 2 fine.

Insertion of the VTL-2 Prom #3 in the U slot causes ACIA to become unresponsive (can't use terminal). Very occasionally a character or two will appear on the screen.

Fortunately I can use the toggle switches, and with them determine that the PROMs are all OK, their intended memory is in proper places.

I need to figure out what is causing ACIA to shut down and therefore can't "J" (Jump) to the VTL-2 start address. Basically I am stuck unless I remove the 3rd PROM.

First, note PROM slots are T, U, V, W. ACIA goes in T, Proms # 3-2-1 go in U-V-W.

Here is a pic of the actual PROM slots (ACIA installed in slot T, VTL PROMs not pictured)

ACIA section motherboard

Here is the pinout diagram for the PROM's
Prom Pinout Diagram for all Altair 680b / 1702A Proms

New pics of the motherboard underside:


NEXT: This is a job for Oscillosope Man!

  VTL BASIC for 680b - Working by Bill Degnan - 06/25/2009 23:21
I hate to simplistically say "OK Now the Proms work" - but that's about it. Either I was doing something wrong/stupid or the computer just fixed itself. Whatever the case I am now able to load VTL (at FC00), create programs, and save (via serial port to a terminal program on my laptop) both VTL machine language code listings at the VTL BASIC or monitor prompt.

I have saved a HEX dump program (from the ACIA Monitor Manual), and a VTL language program called Hurkle.

Next - Work on the cassette interface board and install more memory. I would like to start writing programs now.



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