Contents of this section
Together with modifying the IF input connectors, the designers of the
``geodetic'' VLBA rack had to find a way to connect the extra BBCs to
the 32 available bit stream input channels of the VLBA formatter. In
the original VLBA design one sampler box takes the 8 USB/LSB ``video''
channels of 4 BBCs and samples each with two bits, resulting in 16 bit
streams which are routed to the formatter in a 40-way ribbon cable as
differential ECL levels (two wires, a twisted pair per each bit
stream). Two sampler boxes together take care of all 8 BBCs and use
all 32 available formatter inputs.
The sampler boxes of a geodetic VLBA rack have six additional sign bit
samplers each: one for USB/LSB of BBCs 9--14. Since there is no place
for these bit streams in the original VLBA sampler output connectors,
an alternative ``MarkIII'' output connector was added to both sampler
boxes. These connector carry only outputs of sign bit samplers and
use 28 of the available 32 bit stream inputs of the VLBA formatter.
The FS currently supports the use of either kind of sampler output
connectors. However, every time one has to switch from VLBA
configuration to MarkIII and vice versa, the following happens:
- The field system must be stopped.
- The connectors must be moved to other positions. The
formatter clock will stop while the cables are disconnected since 1pps
is being input using this route.
- A control file of the FS (`/usr2/control/equip.ctl') must be
edited to reflect the change of rack type (`vlba'/`vlbag').
- FS needs to be restarted.
- The formatter clock has to be set with `sy=fmset'.
- FS time has to be updated with `sy=run setcl'.
Additionally, regular ribbon cable connectors are not rated for more
than tens, perhaps a few hundreds of insertions.
These cable swaps can be avoided if:
- Magnitude bit streams of two additional BBCs are made
available in the ``MarkIII'' compatible sampler output connectors.
These additional wires have been indicated with dashed lines in the
diagram in appendix. We have to use BBC05 instead of the ``natural''
BBC02, since there are only two vacant outputs in each sampler box and
BBC01 and BBC02 share the same box.
- The FS is updated to know about this new ``rack type''.
- The VSOP schedules will start to use BBC05 instead of BBC02,
or at least tolerate stations which substitute all references to BBC02
with BBC05 on their own.
- Alternatively the problem will eventually go away when there
is no demand for MarkIII mode A. MarkIII modes B, C, and E, all VLBA
modes, and all MarkIV modes possible for a VLBA rack can be recorded
using only BBCs 1--8. For astronomy purposes, a number of VLBA/MarkIV
modes offer improved bandwidth, better tape usage, and faster
correlation than MarkIII mode A, and still use only eight or fewer BBCs.
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