ADAM Dual Bus Expander
10
Failure Modes
In general, communications between two frames (called control) can occur when the “active” DBX card in the
first frame has a coaxial path to the destination frame, AND the “active” DBX card in the destination frame has
a coaxial path to the first frame.
Audio between frames will exist as long as there is a single valid link between the two frames. In redundant
systems, there are two links between each frame, so losing either one will not affect audio (aside from a small
glitch as the fault is recognized and corrected). In non-redundant systems, there is only one link between
each frame, so if it goes down, the audio between the two frames is lost.
Control in a frame (i.e. the ability to talk to keypanels and act on keypresses) will exist as long as there is a
coaxial path between the frame and the “active” DBX in the first frame, AND, there is a coaxial path between
the “active” DBX in local frame and the first frame.
If a frame loses contact with the first frame, the crosspoints that have already been made will stay. If contact is
restored before any critical messages need to be sent (such as crosspoints or key presses), the frame will
resume normal operation. If a critical message needed to be sent but couldn’t be delivered, the frame’s panels
will go to (****) and come back when contact is finally restored.
In a redundant system, it is possible to pull any one DBX card, or cut any one link (RX or TX or both), and the
system will continue to operate normally (other than a small glitch in audio as faults are detected and the
redundant resources kick in). It is even possible to pull other DBX cards, and/or cut other links without ad-
versely affecting system operation, as long as there is at least one link between each frame (to provide audio),
and as long as the “active” DBX cards in each slave frame have a valid coaxial path to the first frame, and the
“active” DBX card in the first frame has a valid coaxial path to each slave frame (to allow for control opera-
tions).
In a non-redundant system, the same rules for control apply, however every cut link causes the audio between
the affected frames to be lost, and every pulled DBX card loses the audio between that frame and the two
frames it was connected to. This is because every link in a non-redundant system carries audio.
If a frame was completely isolated because all its links were down, any crosspoints that already existed within
the frame would stay made, but no new crosspoint changes could occur.
Lastly, every frame needs at least one DBX card to operate because the DBX cards provide the clock to the
frame and, as with SBX systems, slave frames must have at least one link up that can trace its clock origin
back to the first frame in order for the audio in that frame to be synched to the audio in other frames.
Definitions
Redundancy: We talk about DBX systems as being either “Redundant Audio”, or “Non-Redundant Audio”
where we’ve defined redundant audio as meaning that it is possible to cut any one DBX coaxial link without
losing any audio between frames.
In order for redundant audio to exist, there must be two coaxial links between every pair of frames. In a three
frame system, that means two links between frames 1 and 2, two links between frames 2 and 3, and two links
between frames 1 and 3 (or six links in total). A non-redundant three frame system needs only one link
between each pair of frames, (1-2, 2-3, and 1-3, or three links in total).
When there are two links between a pair of frames, only one link is required to pass audio (although both carry
the audio between frames, the audio is only used from one link). So, if one link is cut, the other link can
immediately be used to provide the same audio.
Because a DBX card can connect to two other frames, both ports on a DBX card are only needed when there
are an odd number of frames. When there are an even number of frames, there will be one DBX card in each
frame that has a port that is unused. However, since our message passing scheme requires that all the DBX
cards be connected in a big loop, the unused ports on the DBX cards are connected to each other anyway
which leads to “Partial Redundancy” of audio in systems with an even number of frames.
