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TimeHub 5500 User’s Guide
097-55501-01 Revision M – January 2009
Chapter 1 Description
Passthrough Mode
If, for some reason, the recently active input reference is not available (for
example, due to loss of signal), the selected passthrough reference is taken from
any other inputs that had been qualified just prior to the removal of the Clock
cards. The attempt begins with the lowest numbered input that had been
qualified. If any input in this group is not available (for example, due to loss of
signal), the next-higher numbered recently qualified input is attempted. If none of
these inputs are providing a signal, the Passthrough mode will not activate.
As of firmware version 3.2.2, an additional passthrough-related alarm is provided.
The PTNA (PassThrough Not Available) alarm provides an indication on a
per-channel basis if the Management card is not receiving adequate signal to use
that input channel for passthrough should the need arise. The AID for this alarm is
IMC-x, where x is the input that is not receiving signal. This alarm will only set if the
corresponding input channel is currently qualified on the active Clock card (in other
words, in a situation where that input would be expected to be able to support
passthrough). Therefore, if an input becomes disqualified (for any reason) on the
active Clock card, the PTNA would be expected to clear if it had been set. If no
timing input is providing a signal while in Passthrough, the outputs are disabled and
Passthrough mode ends. Unlike Clock cards, which have the ability to provide
holdover capability when all timing inputs are removed (and thus maintain output
timing), the Management card cannot provide holdover. Therefore, outputs are
disabled if there is no timing input when in Passthrough mode.
Passthrough Considerations
There are several restrictions and special considerations that apply to Passthrough
operation:
Brief phase hits on the outputs are likely when entering and exiting Passthrough
mode
You cannot configure the Clock or outputs while in Passthrough mode
Passthrough does not have the capability to reject inputs with excessive jitter or
that are significantly off frequency.
Passthrough does not filter input-to-output in the same way as a Clock card, so
out-of-mask timing (for example, TDEV and MTIE) is possible.
Firmware Versions and Passthrough Operation
Details of the Passthrough capability depend upon the firmware and hardware
versions of the Management (090-55542-01), Clock (090-5551x-xx), and Expansion
Controller (090-55545-01) cards.
If either the Management or Clock cards contain firmware prior to 4315B or have
hardware version A, then the passthrough capability is as follows:
Passthrough functions only if the shelf is configured as a master system.
Passthrough will not function if the shelf is configured as a remote system.