MA28140 Datasheet, PDF (22/72 Pages) Dynex Semiconductor – Packet Telecommand Decoder

MA28140
4.5 COMMAND PULSE DISTRIBUTION UNIT
General Requirements
The CPDU is a simple unit that is solely accessible from
ground. The aim of this unit is to generate pulses to drive
certain actuators (e.g. relays). The CPDU is identified by the
Application Process Identifier placed in the TC Packet Header.
The Application Identifier of the CPDU is programmable in
ROM at addresses 006 and 007.
Functional Description
The CPDU receives TC segments, each segment
containing a complete TC Packet. TC segments having a MAP
equal to zero are carrying CPDU commands. It must be noted
that if the internal AU is enabled, MAP0 segments are always
authenticated. When a new segment carrying CPDU
commands has arrived, two cases are possible:
- the CPDU is still executing previous CPDU commands. In
this case, the incoming TC segment is ignored, whether it was
transferred in an AD or BD transfer frame.
- the CPDU is idle. The incoming TC segment is copied
from the back end buffer to the CPDU buffer for checking and
execution by the CPDU.
An important point must be noted: there is no packetisation
layer abort command associated with the CPDU. Once it has
accepted a TC Packet, the CPDU cannot release it until all
command instructions specified in that packet have been
executed.
The CPDU performs first the clean validation process
which verifies the complete packet (CRC, packet length,
segmentation flags). If the clean validation process is
successful, the CPDU performs the legal validation process,
which checks the content of the Packet Headers. The result of
the two previous verifications is reported in the 16 bits CPDU
status. For a dirty or illegal CPDU Packet, the CPDU buffer is
erased. The execution of the CPDU commands is possible
only if all the verifications succeed.
A short description of the fields of the CPDU Packet is
given below:
- version number: 3-bit field occupying the 3 MSBs of the
packet header. To be compliant with ref.1, these 3 bits should
be 000.
- type bit: this bit identifies if the Packet is telemetry type
(type bit = 0) or telecommand type (type bit = 1). To be
compliant with ref 1, this bit should be set to 1.
- data field header flag: this indicates the presence (data
field header flag = 1) or absence (data field header flag = 0) of
a data field header within the packet data field. To be
compliant with ref 1, this bit should be set to 0.
- application process identifier: this field identifies the
particular process to which the CPDU Packet is sent.
- sequence flags: this two-bit field indicates if the packet is
a first, last or intermediate component of a higher layer data
structure. For CPDU Packets, these two bits shall be equal to
11.
- packet sequence count: this 14-bit field allows a particular
TC Packet to be identified with respect to others occurring
within a telecommand session. This field is reported in the
CPDU status for clean and legal CPDU packets.
- packet length: this field specifies the number of octets
contained within the packet data field, by indicating the number
of octets in data field minus 1.
- packet data field: this field contains the CPDU commands
and the CRC for packet error control.
Checking the CPDU-Specific TC Packet
The CPDU Packet format is shown below:
PACKET HEADER (48 bits)
PACKET IDENTIFICATION
PACKET
SEQUENCE
CONTROL
PACKET
LENGTH
version
number
type
data field applicat- Sequence Packet
header
ion
Flags
Name or
flag
process
Sequence
ID
Count
3
1
1
11
2
14
16
16
16
PACKET DATA FIELD
(variable)
DATA APPLIC- PACKET
FIELD
ATION ERROR
HEADER DATA
CON-
TROL
variable variable
16
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