284[Altair Computer Club] Re: New Vintage Computer Forum and a question
- May 6, 2003Erik,
You're lucky to have two machines to compare, even if they aren't the
same models. Troubleshooting is a lot tougher with just one computer.
I think you're examining the correct starting address of the DBL boot
ROM, but you mistyped it in your answer. The correct address to
examine then run is 177400 or hex FF00. You said 177000, which is
the address of the MBL cassette loader. You were correct, though,
when you said you raised the 8 switches for A15 thru A8.
Are you using the same good I/O cable (the ribbon cable that goes
from the computer to the drives) with both computers? It's not
unheard of to have bad connections in them.
You know the CPU board, PROM board, and DBL are all probably good or
you wouldn't be getting the head to load on the floppy. On the other
hand, the problem you're having can indicate bad RAM.
I would try to eliminate as many variables as possible. That is, use
the same known good RAM boards, controllers, cables, drives, drive
interfaces, diskettes, etc. in both machines. You can't use the BT's
Turnkey board, serial port (you're using the one on the turnkey
board, I assume), or CPU board, but the other boards should be OK in
Have you checked the 3 system voltages on the 8800? Sometimes it was
easy to overload the 8800's -12V supply (the raw DC should be
about -16V on the buss and where it enters boards' -12V regulation
circuits). The supplies on the model B were relatively bulletproof.
Be sure none of the fragile wires going from the mother board to the
front panel have broken loose at either end.
PS to akessel- You should be able to leave or put the address
switches in any position after you have examined the correct starting
address of the boot ROM (177400). You shouldn't need to flip them
all down before hitting the RUN switch. Andy, are you sure your
computer really require all switches to be down before running?
By the way, Erik's method of raising STOP and RESET together is a
good one. Sometimes, but not always, the 8800/8800A would
get "stuck" and you couldn't stop or reset it to 0000 with just one
switch. The 2-switch method was the recommended action.
--- In email@example.com, Erik Klein <Lifo@p...>
> I've inserted my responses below. Hopefully the formatting isn'ttoo screwy.
>at 177000 so I turn the machine on, raise stop and reset
> alltare <firstname.lastname@example.org> wrote:
> These questions/comments come to mind:
> How are you booting the 8800? Are you directly addressing and
> executing the boot PROM, using a turnkey board with Turmon PROM, or
> [ESK] I am directly addressing the boot prom. I'm pretty sure it's
simultaneously, raise the first 8 switches (A15-A8) and raise
Examine. I then "run."
>the above (set after examine, before I run)
> [ESK] I've tried several variations of sense switch settings with
>one of which seems to match up to the 4.1 version in the 8800BT I
> What boot PROM are you using (DBL, MDBL, ...)?
> [ESK] I am using a DBL PROM. I have at least two versions of this,
> MITS had two 8" disk "formats"- the original and the later "NWD"
> Write Delay) format. There were several timing changes to theusing
> controller boards and therefore to the recorded data. I think you
> have a paper giving the various timing differences. You may be
> relatively incompatible controller/diskette combinations thattotally
> fail to work right in the 8800. I think all model B's weresupplied
> with NWD controllers, and almost all 8800s had controllers with theand the 8800BT. Both work on the drives and disk as attached to the
> original pre-NWD timing.
> [ESK] I have two sets of functional controllers between the 8800
8800BT, neither works on the 8800.
> How much RAM is in the 8800? Are you sure all RAM is good? You
> should probably have at least 32K of good memory (beginning at
> address 0) to load later versions of BASIC. Try readdressing your
> RAM boards or swapping with those known good ones from the 8800BT.
> On MITS' 16K boards, they had real problems with the 4-position DIP
> switches used for addressing (dirty contacts with resistance of a
> ohms)- often the switches would be replaced with soldered jumpersas
> a cure. Try restricting max RAM to no more than 48K.time when I've been doing testing. I'm pretty sure that all of the
> [ESK] I've got anywhere between 32 and 56K in the machine at any
RAM is good although I haven't tested every bit. I may swap over the
3 16MCDs from the BT to the 8800 and try that for giggles.
>contains the processor, the PROM board and a 32K SRAM card, the next
> Because of the noisy buss, put all RAM as physically close as
> possible to the CPU board.
> [ESK] I've got three 4-slot motherboards in the 8800. The first
has the SIO-2, 16K and 8K Static cards the last has the 2 drive
> The code in those old 1702 EPROMs can become corrupted after almost
> 30 years. Try using the apparently good boot PROM from the BT in
> 8800. If you're using chips that you bought fromaltairmanuals.com,
> they should still be good for a long time to come unless you removeI've moved the 4.1 PROM from the BT to the 8800. All PROMs have
> the sticker from the erasure window.
> [ESK] I've done both. I have new PROMS from Altairmanuals.com and
worked about the same.
> Even if you do have a bad serial I/O board for your terminal, you
> should still see the disk's head stepping from track to track as it
> boots. The boot ROM doesn't check for the presence of a terminal
> even an I/O board like modern PCs do. Since the head doesn't movethen one track before prompting on the 8800BT, but I can take the
> after loading, I think the problem is not the serial board.
> [ESK] I'm inclined to agree. I'm not sure if the heads move more
cover off and watch both machines try to boot to be sure.
>try these things for a while).
> [ESK] I'll post progress when I make any (I may not have time to
> [ESK] If y'all get any other ideas, please let me know!
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