MA28140 Datasheet, PDF (10/72 Pages) Dynex Semiconductor – Packet Telecommand Decoder

MA28140
Standard Procedures Within the Layer
The Clean Frame Validation Process
On receiving a new frame, the Clean Frame Validation
process performs the following tasks:
- the number of octets in the frame is checked to be greater
than 7 octets,
- the transfer frame is assumed to be a version 1 frame,
- the frame length field is checked to be compliant with the
real number of octets of the frame,
- the number of fill octets is verified to be minimum zero and
maximum six,
- the fill octets are removed,
- the CRC error syndrome verification is carried out.
All candidate frames passing all the preceding validation
checks are declared clean and transferred immediately to the
Legal Frame Validation process. Frames failing any of the
preceding tests are declared dirty and are erased.
The Legal Frame Validation Process
On receiving a clean frame, the Legal Frame Validation
process performs the following validation checks:
- the version number is checked to be as defined in the
ROM,
- the reserved fields A and B are checked to be as
defined in the ROM,
- the value of the spacecraft ID is checked to be as
defined in the ROM,
- the value of the Virtual Channel ID is checked to be as
defined in the ROM and by the VCLSB input,
- the Bypass and Control Command flags must combine
legally,
- the BC frames must contain a valid control command
(either UNLOCK or SET V(R)),
- for a BC or BD frame the Frame Sequence Number field
must be set to all zeros.
The LSB of the VC ID is indirectly defined from a dedicated
pin VCLSB; it allows easy configuring of a pair of redundant TC
decoders.
- VCLSB = 1: The VC ID LSB read from the ROM is
inverted.
- VCLSB = 0: The VC ID LSB read from the ROM is not
inverted.
All candidate frames passing all the preceding validation
checks are declared legal and transferred immediately to the
FARM-1 process. Frames failing any of the preceding tests are
declared illegal and erased.
The FARM-1 Process
THE FARM-1 VARIABLES
The Frame Acceptance and Reporting mechanism
(FARM-1) is described by a finite state machine represented
by the FARM-1 state table. The FARM-1 maintains a set of
variables which are described below:
• The State. This may be one of the following:
- Open (S1)
- Wait (S2)
- Lockout (S3)
This variable represents the state of the FARM-1
automaton. In Open State, the FARM-1 accepts frames
and passes them to the above layer. In Wait State, there is
no buffer space available in which to place any further
received data of type AD. The protocols leaves the Wait
State upon receipt of a buffer release signal from the
Higher Layer. Lockout is entered if the protocol machine
detects an error. It is a safe state in that no user data (AD
frames) will be accepted or transferred to the Higher Layer.
The only accepted data frames are the BD frames, but
even in this case the protocol machine remains in lockout
state. The protocol machine leaves the Lockout State upon
receipt of an UNLOCK control command.
• The Lockout Flag. This is set to 1 whenever the protocol
is in the Lockout State.
• The Wait Flag. This is set to 1 whenever the protocol is
in Wait State.
• The Retransmit Flag. This is set to 1 whenever the
protocol machine knows that an AD
frame has been lost in transmission or has been
discarded because there was no buffer space available.
This flag is reset to 0 upon the successful receipt of a
frame with N(S)=V(R), the receipt of a SET V(R) control
command (unless in Lockout State) or receipt of an
UNLOCK control command.
• FARM B counter. This is incremented whenever
a valid BD or BC frame arrives. This counter is a 2 bit
wraparound counter.
• Receiver Frame Sequence Number V(R). This
records the value of N(S) expected to be
seen in the next AD frame.
• The buffer management variable. The PTD maintains a
flag indicating the number of the back end buffer. The
AUBUF output pin provides the value of this flag (the
back end and front end buffers are represented in Figure
6). The number of the TC channel on which the data
stored in the back-end buffer has been received is
provided on the output pins (SELTC2-0).
• FARM Sliding window variables. The purpose of these
are to protect FARM-1 against the unauthorised transfer
of a sequence of frames such that the Frame Sequence
Number N(S) of one or more of these frames will exceed
the current value of the V(R) counter. The FARM Sliding
Window concept applies only to AD frames.
The FARM Sliding Window is defined in terms of two
variables:
- the width of the positive part referred to as PW
- the width of the negative part referred to as NW
The FARM Positive window area starts with V(R) and
extends PW frames in the positive direction. The FARM
Negative window starts at V(R) - 1 and extends NW frames in
the negative direction.
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