Pre-Phase A Report - Lisa - Nasa

132 Chapter 5 Payload Design
Units Number Total Mass Power per Notes
of Units (kg) Unit (W)
Processor 2 2.3 10.5
X-strap board 2 1.8 0.75 between A & B strings
1553 board 2 1.8 1 1553 bus controller
RS 422 board 4 1.8 2 Serial I/O for accels., etc
VME chassis 2 1.8 – composite material
Shielding – 1 –
Totals 10.5 kg 14.25 W
Dimensions of complete payload computer: 233×199×321 mm 3 =14.9 litres
Table 5.6 Payload computer mass, power, volume budgets.
5.9.2 Payload data interfaces
The data interfaces between the payload subsystems and the spacecraft are shown in
Figure 5.7 . The baseline approach is to use a MIL-STD-1553 data bus to link all of the
DFACS hardware elements to the payload processor, with the exception of the accelerometers
(including UV discharge system) and interferometer electronics which are directly
linked to the payload processor via an RS 422 interface. The payload processor is linked
to the spacecraft processor via the 1553 bus.
If it turns out to be feasible in terms of bus management and reliability, the 1553 interfaces
may be replaced throughout by RS 422 interfaces, with considerable cost benefit. For the
time being, however, the 1553 protocol is the baseline choice.
Bus control. The MIL-STD-1553 protocol requires no more than one bus controller.
Initially, the spacecraft processor is the bus controller for coarse attitude control. During
a transition phase, after the payload processor (nominally String A) is brought on line,
bus control is transferred from the spacecraft processor to the payload processor. This is
accomplished by a “dynamic bus control” command which is available within the framework
of the 1553 standard. (Alternatively, the spacecraft processor could retain control
of the bus and use remote-terminal-to-remote-terminal transfers.)
Payload command and data handling software. The payload command and data
handling (C&DH) software layer resides on the payload processor and includes the 1553
interface and data structures for normal payload operations, plus the RS 422 interface with
the accelerometers and interferometer electronics. The command handler accepts and
routes commands either directly from the 1553 interface, or as stored program commands
loaded into the payload processor memory. These program commands may be timetagged
with absolute or relative time, or may be conditional commands. The spacecraft
computer which controls the telemetry must be configured such as to accept/send data
packets from/to the payload processor as commanded by the payload C&DH layer. All
payload flight software will be developed using commercial compilers (C, C++, orAda).
3-3-1999 9:33 Corrected version 2.08