5 Oct
2015
5 Oct
'15
6:46 a.m.
> 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.
Noel