Guy Sotomayor wrote:
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.
Philipp Hachtmann wrote:
Ah, really? Of course NOT! The original driver was
DEC8881 which is a
relabelled 7439.
I think it must have been more than just relablled; the original 8881
was from a non-7400 Signetics family whose name escapes me at the
moment, and may have been binned. The characteristics were somewhat
similar to the 7439 but not identical. The edge rate and maximum
leakage current are different. They may have binned 7439 parts that met
the Omnibus specs, but there's no expectation that an ordinary 7439 (or
7438) would.
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.
The load isn't the same. Unlike modern CMOS parts, the receiver inputs
are not especially high-impedance. The receivers do present both DC and
AC loading on the bus lines. More receivers presents more loading. The
AC loading is often more of an issue than the DC.
The bigger problem with using parts that don't meet the DEC specs in
heavily loaded systems (Omnibus, Qbus, or Unibus, all of which are
similar electrically) is the receiver thresholds and the leakage
current. In large systems (multiple backplanes, or in the case of
Omnibus, even a full single 20-slot backplane), third-party modules that
used receivers with the wrong threshold (including all normal TTL parts)
do NOT work reliably, because in a heavily loaded bus asserted signals
don't always dip as low as the threshold of normal TTL parts, or don't
do so in a timely manner. This can easily be observed on such a system
using a 100 Mhz or better oscilliscope (with a slower scope you won't
see accurate signal edge waveforms).
On a lightly loaded system you can get away with using ordinary TTL
receivers and OC buffers.