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2.4.4
TT - Time-tags
Selected events can be time-tagged with a resolution of better than 2 ns. All of the DSI’s ports use the same time
reference, so that relative time-tags between ports are consistent.
With option SO, Synchronized Outputs, it is also possible to accurately measure round-trip times for data.
Multiple EtherSpaceLink units can be linked so that their time-tags are synchronized to allow comparisons of
time-tags generated on multiple units.
DSI Version 1.0 introduced optional delta time-tags. When enabled, time-tags occurring shortly after another
time-tag (on the same link) are reported as delta times. Long intervals (>
∼
6.5us) result in the usual absolute time-
tag. Delta time-tags are coded using 2-bytes instead of the usual 8-bytes and thus considerably reduce traffic
when time-tags are used with high speed data.
2.4.5
SO - Synchronized Outputs
In order to test functions such as the arbitration in a routing switch, it is necessary to present inputs to the
device-under-test that are synchronized with each other. The Synchronized Outputs option allows
outputs from all or any sub-set of the ports to be synchronized at user specified points in their data streams. For
example, multiple packets may be scheduled to start their transmission at the same time, or one packet may be
started at the same time as another finishes.
From a single DSI, output synchronization is to within 2 ns.
Multiple units can be linked so that their outputs are synchronized between units as well as within.
2.4.6
CO - Controlled Outputs
It is possible to synchronise the transmission of any character in the transmit stream to an external clock
transition by using the CO option.
Low-to-high transitions on a rear-mounted SMA connector permit the transmission of traffic beyond each HOLD
character that is inserted in the transmit data stream.
Each port on the DSI has a transmit buffer of approximately 32 kbytes, which stores data characters, EOP tokens
and HOLD markers.
2.4.7
OD and OE - Output Disable and Output Enable
When testing delicate or valuable SpaceWire hardware, some users might prefer to disable all of the SpaceWire
signals from the DSI when their test system detects an abnormal condition.
The DSI’s Output Disable option (OD) responds to a high-level input signal on SMA connector pair J3-J4, and tri-
states the SpaceWire ports. When J3-J4 is not driven, the DSI behaves as it normally would.
Conversely, the DSI’s Output Enable option (OE) only enables its SpaceWire outputs when a high-level input signal
is present on SMA connector pair J3-J4, making it fail-safe. The drawback to this option is that the DSI will appear
inert unless J3-J4 is driven high, which is a non-standard behaviour.
The OD and OE options indicate “-3S-” on the DSI unit’s front-panel when its SpaceWire outputs are tri-stated.