PDP-11/10 repair started
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.
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