David Spain writes:

(J. Porter Clark) writes:

Since the ATU in the cupola shares an equipment bay with several
other electronics modules (such as the one that controls the station's
robotic arm, etc.) I was wondering if you could tell us if the cold
plates are shared between the modules in the cupola or if there are
separate water lines running to each, such as one for the ATU, another
for the robotic arm controller [I have no handy acronym for that :-)]
etc, or if they share a common cold plate which water cools for
everything in the cupola?

The ATUs use the smallest ("-5") coldplates in the inventory.
As far as I know, nothing else uses them or even has the
same bolt pattern. The three ATUs in the airlock share one
big coldplate. None of the other ATUs share coldplates with
anything else. I don't know much about the ATCS plumbing,
except that Node 2 is unusual in that it has two completely
separate water loops. I was under the impression that the
coldplates are generally connected in series in any given area.
If I get a chance, I'll actually look at some diagrams. What is
the reason for asking?

The acronym you're searching for is RWS, Robotic Work Station.

What is that connector at the top of the ATU, designed for immortals
;-), actually for?

It's a test connector for factory use. The Harris documentation
gives names to the pins, but that's about it.

As regards the rear cabling, I'm guessing:

1. Left-most multipin connector looks like it loops pins to
possibly +V or GND. I'm assuming this is the addressing connector
and the connections are unique per ATU? In practice does this
allow an ATU to be swapped with any other ATU in case you have
a failure and can afford to sacrific one less used? Has that
ever happened on the ISS?

That is the address connector. The addresses correspond to
locations. You can swap ATUs from one place to another. As far
as I can remember, we've never swapped any audio boxes on orbit,
but we did swap that one ATU in the Airlock with a "new" unit
twice. None of the other audio boxes have failed, although
there have been some unexplained hiccups that have been cleared
by power cycling.

2. The middle connector has the 3 fiber optic communications links.
So what is up with that 3rd fiber? From your caption it sounds like
it is dark? Does it not exist in the flight configuration?

The ATU has 4 fiber optic connections: 2 in, 2 out. Redundant
buses. A common connector for both buses violates separation
of redundant paths, but not too much. 8-) We couldn't figure
any good reason for an ATU cable to have exactly 3 fibers.
Admittedly, it was a test cable of uncertain origin.

3. The bottom right connector is power? Are you running a standard
voltage on the ISS? Curious to know if it's 48VDC?

Like most Freedom-era boxes, and a good many later ones, it runs
on 120 VDC. ISS using 120 VDC instead of 28 VDC or something
else is of long and tedious history.

Does the cold plate attach on the finned side of the ATU?

No, it goes on the bottom, which is smooth and unpainted. The
fins (really pins) are on the sides.

Yeah not much outside the telecom space and high end computers
used fiber optic in that time-frame. Any insight into what
caused those failures?

I'd have to do some digging; it's been long enough for my aging
memory to forget. IIRC, they were from parts failures inside
the units. Because the first two units we put into that one
location both failed, albeit in somewhat different ways, we
spent some time proving that the location wasn't the problem and
that we wouldn't be wasting time and money installing a third
ATU there.

The Airlock is the only element with 3 ATUs. The reason
the Airlock needs 3 is that 2 of them are used for hardline
connections to two EVA-suited crewmen. This was a cheaper
solution than the creation of a unique interface box just for
the suits.

That makes good sense. Your photo site seems to be depicting
interoperability testing between the Russian comm gear and
NASA's, among other things. Was that the case?

Yes. The "Joint" Airlock was designed to be used with the
Russian and U.S. EVA suits. We were testing the hardline and RF
communication with both types of suits. All of the voice comm
with these suits ultimately goes through the ISS (U.S.) audio
system, so we were doing a dev test to make sure that everything
worked together. We ran many tests like that from 1994 to 2001.

The disappointing thing about the Airlock Mockup testing was
that the program decided not to fly some hoses or cables that
are necessary to use the Orlan in the Airlock. Therefore, much
of that compatibility testing was ultimately wasted. I do not
know if this situation was ever corrected.

If Jordan has any further questions, I'll defer them to him.

It was great to get to actually write you here, many, many
thanks for getting back to us.

My pleasure.