27 Feb
2013
27 Feb
'13
11:24 a.m.
On Feb 27, 2013, at 12:41 AM, Philipp Hachtmann wrote:
Am 27.02.2013 00:03, schrieb Guy Sotomayor:
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
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 DEC8881 which is a
relabelled 7439. The 7438 I used has very similar specifications.
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.