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3421Re: 8800b front panel glitch

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  • steve
    Aug 25, 2013
    • 0 Attachment
      Kip,

      The 8800 and 8800a don't have a D/C Interface board, so your problems lie elsewhere. A common cause of trouble with your machine is the one-shot devices on the front panel. Their timing can drift.

      Steve
      ----------

      --- In altaircomputerclub@yahoogroups.com, "Kip Koon" <computerdoc@...> wrote:
      >
      > Hi Guys!
      >
      > I have an Altair 8800 with 3 - 4 slot motherboards wired together into one
      > motherboard to give you some idea of the type of system it is. The previous
      > owner said that it did not work, that the front panel did not work. After
      > following what you guys have been saying about the R IC being a 74LS04 and
      > needing to be a 7414, I wonder if that is the problem with mine? I will
      > have to check this out. If it turns out that this IS true, you all will
      > have solved a problem that I had no idea how to troubleshoot much less fix!
      > Once I verify this and if true fix it, I'll definitely have to check out the
      > power supply to see if it will work. It has not been turned on in literally
      > more than 3 decades, maybe more! Talking about resurrecting a computer! It
      > Will be interesting. I'll keep you guys updated as to my progress as soon
      > as I can get to it. My computer repair business is growing finally after 5
      > years of limping along on two broken legs, J so it looks like I will be able
      > to eventually restore my not so little Altair 8800! Take care guys!
      >
      > Kip
      >
      >
      >
      > From: altaircomputerclub@yahoogroups.com
      > [mailto:altaircomputerclub@yahoogroups.com] On Behalf Of deramp5113
      > Sent: Saturday, August 24, 2013 9:52 AM
      > To: altaircomputerclub@yahoogroups.com
      > Subject: [Altair Computer Club] Re: 8800b front panel glitch
      >
      >
      >
      >
      >
      > Interesting. My board has an 'LS04 in position 'R' but the computer has been
      > pretty solid for quite a while now. I'll have to pull out a scope and see if
      > my hardware is coming close to experiencing the glitch you noticed. One
      > subtle difference -- all of the chips on my DC interface board are socketed.
      >
      > Soon I'll be working on this 8800b extensively for testing of a floppy disk
      > controller I'm building. I definitely don't want random variables like this
      > problem thrown into the middle of hardware debug of the new board, so I'm
      > going to dig into this one. Thanks for the heads-up!
      >
      > Mike
      >
      > --- In altaircomputerclub@yahoogroups.com
      > <mailto:altaircomputerclub%40yahoogroups.com> , "mfeberhard" <eberhard@>
      > wrote:
      > >
      > > I tried several 74LS04's in this position, from several manufacturers. All
      > failed. I tried several 7404's, all worked fine. Same with 7414's.
      > >
      > > The particular chip in position 'Y' made a difference on how bad the
      > failure was with the 74LS04. IC 'Y' is the driver for the adjacent signal on
      > the ribbon cable. I would get a variety of flaky behaviors depending on
      > which particular chip I put in IC 'Y', so long as a 74LS04 was in 'R'.
      > >
      > >
      > >
      > > --- In altaircomputerclub@yahoogroups.com
      > <mailto:altaircomputerclub%40yahoogroups.com> , "deramp5113" <deramp5113@>
      > wrote:
      > > >
      > > > Was this the same, original 74LS04 moved between the two computers, or
      > were you able to make them both fail with a "new" 74LS04 as well?
      > > >
      > > > Mike
      > > >
      > > > --- In altaircomputerclub@yahoogroups.com
      > <mailto:altaircomputerclub%40yahoogroups.com> , "mfeberhard" <eberhard@>
      > wrote:
      > > > >
      > > > > I recently debugged an 8800b front panel, and I thought I'd share what
      > I learned with those of you who might also be working on 8800b's.
      > > > >
      > > > > The conclusion first: I strongly recommend replacing IC 'R' on the D/C
      > Interface board with a 7414. The manual calls for a 74LS04 here, which is
      > not reliable.
      > > > >
      > > > > This 8800b of mine was flaky - it would occasionally be able to boot
      > Basic from disk. But normally it would fail with an 'M' error, indicating
      > that the monitor did not read back the same data that it wrote to memory.
      > > > >
      > > > > The most repeatable symptom that I saw was using TURMON. Using the 'M'
      > command, it would write successfully to memory for addresses 000000 through
      > 000003, and would normally fail (and crash) when it tried to write to
      > address 000004. Sometimes it would fail at other addresses. Occasionally, it
      > would work fine at address 000004. When I looked in memory (with the front
      > panel), the 'failed' write had actually written the correct data!
      > > > >
      > > > > The worst part was that nothing was repeatable - the bad behavior
      > seemed to morph as I chased the problem.
      > > > >
      > > > > Perhaps you have seen the same misbehavior on your 8800b...
      > > > >
      > > > > I have another (working) 8800b, so I was able to narrow the problem
      > down to the D/C Interface board by swapping boards.
      > > > >
      > > > > It seemed like the ribbon cables to the front panel were not
      > constantly making good contact, so I replaced them - no easy task! Then I
      > one-by-one replaced nearly every single chip on the board, with no
      > improvement. And none of these chips was socketed :-(
      > > > >
      > > > > Finally I noticed that IC 'R' on my good D/C Interface board was not a
      > 74LS04 as the assembly instructions indicate. Instead, it was a plain 7404.
      > So I substituted a 7404 onto the troublesome board, and the board suddenly
      > works perfectly.
      > > > >
      > > > > This was a bit puzzling to me, because none of the inverters in IC 'R'
      > are involved in *any* switching during normal operation. 2 of the inverters
      > are in the PROT (memory protect) circuitry, which doesn't do anything. One
      > is in the PRESET path. One drives Single Step onto the bus, which goes to
      > nothing. One inverts the -FRDY signal, which is used to stop/start the
      > processor when the front panel is in the STOP mode, and is always high when
      > in RUN mode.
      > > > >
      > > > > So I put the 74LS04 back, and set my 'scope to trigger at 1.25V, and
      > looked at the inputs to IC 'R' when running. And sure enough, the -FRDY
      > signal was occasionally experiencing a >1.25V glitch, induced by the
      > adjacent signal on the ribbon cable to the front panel.
      > > > >
      > > > > The adjacent ribbon cable signal is -DO5 - the only data-out signal
      > sent to the front panel. With the right combination of data bytes written by
      > the processor, the processor was getting a random -RDYIN glitch.
      > > > >
      > > > > Just for fun, I swapped out the 7404 on the good board for a 74LS04.
      > Sure enough - it failed too. This made me conclude that the problem is more
      > universal than just my bad boy 8800b - you might have seen the same problem.
      > > > >
      > > > > The 7404 works because it draws 4 times the current on its input as
      > does the 74LS04 - and so it "eats" the glitch, reducing the glitch amplitude
      > to around 0.75V.
      > > > >
      > > > > For extra margin, I replaced IC 'R' with a 7414, which has the higher
      > input current, but is also a Schmitt trigger, so that it could tolerate a
      > higher glitch voltage.
      > > > >
      > > > > So as I say, I recommend replacing IC 'R' on the D/C Interface board
      > with a 7414.
      > > > >
      > > > > -Martin E
      > > > >
      > > >
      > >
      >
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