PDP-11/10 repair started

Noel Chiappa jnc at mercury.lcs.mit.edu
Mon Oct 5 08:46:27 CDT 2015

    >> From: Tony Duell 

    >> I am working from 2 Printsets, both from Bitsavers. One is the GT40 one
    >> (yet another backplane of course, but the same CPU, core memory, etc).

    > Ah, thanks for that pointer; I'll see if it shows the same board
    > versions as my 'early' hardcopy set.

It does seem to show _basically_ the same as my set; the print revs are
slightly different (slightly later), but it does have what I've called the
'early' boards. The differences with mine are minor - e.g. on the M7261,
there are two extra capacitors in the prints in the GT40 set.

    > isn't the switchable divider only present on later boards (the early
    > ones being pretty much 110 baud only)?

Ooh, right you are - another way to tell the early M7260 from later ones. If
your memory of a version with a crystal is correct, that does indeed make
three versions of that board. Can all -11/05 and -11/10 owners look at their
M7260, and see if they have one with a crystal? If so, we can institute a
search for the prints of that version.

    > This printset _does_ show the jumpers I mentioned. Look at page 75 of
    > the .pdf bottom, left-ish. Jumper W1 is described as disabling the
    > internal serial port when fitted.

Ah, right you are; maybe I am mis-remembering a long search through the
'early' printset for jumper W1?

    >> You have to tweak the trim pot to change from the 110/220/440/880/1760
    >> speed set to the 150/300/600/1200/2400! Ugly!!)

    > May be easier than finding the right crystal to change a DL11A-E to the
    > 'other' set of baud rates :-)

Well, today that's not so easy (although I did stumble on a pair of the 9600
baud crystals on eBay a while back), but back then, it was a lot easier!

    > The M9302 includes logic to assert SACK if a grant (any BG or NPG) gets
    > to it ... This causes problems with an open grant chain in that the CPU
    > sees the SACK, tries to deassert the grant (which it hasn't asserted in
    > the first place) and the bus is locked with SACK asserted and no grants.

So, how did the M9302 see a 'grant' to start the whole process? Noise on an
open input? Or maybe it powers up in that state?

    >> From: Johnny Billquist

    >> You most likely want to terminate the other end as well.

    > It may not be a perfect electrical match, but if all you have is the
    > CPU backplane .. I am certain a terminator at the CPU end only will get
    > the machine doing something

Yes, I think that in electrical terms it would be very similar to the typical
LSI-11, which works fine with termination at one end only. Yes, there will be
more noise on the bus due to the un-terminated end, but it will probably
still work OK.


More information about the cctalk mailing list