27 Feb
2013
27 Feb
'13
1:59 p.m.
The Omnibus drivers, at least on most 8/E-vingtage cards, were Signetics N8881, which were
not trapezoidal. I have Omnibus cards in my possession that have clearly-labelled TI 7439
parts used instead of the 8881. Early Omnibus cards used selected 7401s. The DS series
parts were used on Unibus and later-production Omnibus cards. Undoubtedly, they were
preferable once they became available. Likewise, the Schmitt-trigger receivers that
replaced the Signetics Utilogic II (SP380, etc.) parts. DEC used different solutions over
the years, some probably better-behaved electrically than others.
--Bill
--- On Wed, 2/27/13, Guy Sotomayor <ggs at shiresoft.com> wrote:
From: Guy Sotomayor <ggs at shiresoft.com>
Subject: Re: PDP8 Omnibus to USB interface
To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at
classiccmp.org>
Date: Wednesday, February 27, 2013, 11:24 AM
On Feb 27, 2013, at 12:41 AM, Philipp Hachtmann wrote:
DEC8881 which is a
relabelled 7439. The 7438 I used has very
similar specifications.
Am 27.02.2013 00:03, schrieb Guy Sotomayor:
> I should point out that (at least from looking at a
couple of the
> photos) you're not using Unibus
transceivers which
are standard on
> Omnibus devices.? It'll probably work OK
on a
lightly loaded system
> but a PDP-8/e with a double backplane with
lots of
boards may have
issues.
Ah, really? Of course NOT! The original driver was As the DEC8881 and the 7438 are not available, I
have
chosen the 7438.
And it does not make a big difference if the
machine is
more or less loaded. The load on the drivers is always the
same. It's an open collector bus.
The 8881 is *not* a relabeled 7439.? Yes, it's open
collector but the current sinking capabilities are pretty
high (80ma?).? It's also what's called a "trapezoidal"
driver which has a controlled slew rate (rise/fall times) to
minimize reflections on the bus.? The drivers are
optimized to drive into a bus terminated by a 180ohm
resistor to +5 and a 390ohm resistor to ground.
The receivers are also specialized and are not quite at the
"standard" TTL levels for greater noise immunity and minimal
input leakage.
The Unibus interface chips (that were also used on Omnibus)
are:
DS8640??? ??? quad bus
receivers
DS8641??? ??? quad bus
transceivers
DS8837??? ??? hex bus
receivers
DS8881??? ??? quad bus
drivers
There were older versions but these are pretty much what
everyone used for Unibus and Omnibus systems.
As I said, you can probably get away with it for small
configurations but I would expect some issues with larger
systems/configurations.
TTFN - Guy