MA28140 Datasheet, PDF (9/72 Pages) Dynex Semiconductor – Packet Telecommand Decoder

MA28140
4.2 TRANSFER LAYER
Overview of the Layer
The Transfer Layer implements the following sublayers:
- The Frame Error Control Sublayer which ensures that
only “clean” frames are transferred to the sublayer above by
using a CRC error syndrome verification.
- The Frame Header Sublayer verifies the conformity of
the relevant frame header fields by using the Legal Frame
Validation process before passing the frame to the FARM1.
- The “Frame Acceptance and Reporting Mechanism
One” or FARM1 ensures that frames are processed in the
correct sequence.
There are three types of TC transfer frames:
- two types for the Sequence-Controlled Service: AD and
BC frames
- one type for the Expedited Service: BD frames
The Sequence-Controlled Service is used for normal
spacecraft communications. It concerns essentially TC
Transfer Frames carrying TC segments: the AD frames. To
configure the AD machine, special control frames are used
called BC frames.
The Expedited Service is used for recovery in the absence
of the telemetry downlink or during unexpected situations. It is
only concerned with TC transfer frames carrying TC segments:
the BD frames.
Standard Data Structures Within the Layer
The major fields of the TC Transfer Frame are shown
below:
5 octets
Frame Header
1 to 249 octets
Frame Data Field
2 octets
Frame Error
Control Field
Frame Header
• Virtual channel identifier. It is used as a spacecraft sub-
identifier. It can provide an identification of the spacecraft
telecommand chain selected for operating the spacecraft.
• Frame length. This field specifies the number of octets
contained within the entire TC transfer frame:
Field Value = (Total number of octets) - 1
• Frame sequence number. This number is denoted as
N(S). It is set to different values:
- for AD frames it should be set to the Transmitter
Frame Sequence Number and it is compared to the
Receiver Frame Sequence Number V(R) stored in the
PTD, to control the transfer of a sequence of frames
(see the FARM-1 process)
- for BC and BD frames it should be set to all zeros.
Except for the bypass and control command flags, the
values of the first three header octets are programmed in the
external ROM.
In the abbreviations AD, BD and BC, A stands for
Acceptance check of N(S), B stands for Bypass of A, C
stands for Control and D stands for Data. AC is an illegal
combination because Control Commands cannot reliably use
a transfer service which they are meant to modify.
Frame Data Field
The frame data field is of variable length from a minimum of
1 octet to a maximum of 249 octets. When the frame is a data
frame (type AD or BD), it contains a TC segment. When the
frame is a BC frame, this field can contain 2 control commands
to configure the FARM-1 process:
• the UNLOCK command. The FARM-1 has a built in
mechanism which will go into a Lockout state whenever it
receives a type-AD frame containing a frame sequence
number N(S) outside the limits of the FARM-1 Sliding
Window. The UNLOCK command provides a mechanism to
reset the Lockout condition. The UNLOCK command is
encoded as a single octet with the value: 00000000.
The structure of the frame header is given below:
version
number
2
bypass
flag
1
2 octets
control
command
flag
1
reserved spacecraft
field A
ID
2
10
1 octet
virtual reserved
channel
field B
ID
6
2
1 octet
frame
length
8
1 octet
frame
sequence
number
8
A description of the fields of the frame header is given
below:
• Version number, Reserved field A and Reserved field B
should always be 00 (ref 1).
• Bypass flag and control command flags. Their values
are given in the next table:
Bypass Flag
0
0
1
1
Control Command Flag
0
1
0
1
Interpretation
AD frames
ILLEGAL
BD frames
BC frames
• Spacecraft identifier. This field provides the identification
of the spacecraft being commanded.
• The SET V(R) command. The SET V(R) command
allows V(R) to be preset to any desired value. The SET V(R)
command is encoded as three octets with the values:
10000010
00000000
XXXXXXXX
The value to be set into V(R) is stored in the third octet.
Frame Error Field
The frame error field is a mandatory 16-bit field which
occupies the two trailing octets of the TC Transfer Frame. It is
a cyclic redundant code (CRC) generated with the polynom
X¹⁶+X¹²+ X⁵+1 with the shift register being initialised to all ones
before processing each frame (refer to ref 2 for a complete
description of this field). The CRC is only used for error
detection by the frame and not for error correction.
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