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User Interfaces
If an acknowledgment (response) packet or a NAK packet is lost, corrupted, or not issued due to
an error and is thereby not returned to the sender, the sender re-transmits the original information
packet; but with the same <FSN>. When the intended receiver detects a duplicate packet, the
packet is acknowledged with a response packet and internally discarded to preclude undesired
repetitive executions. If the M&C computer sends a command packet and the corresponding
response packet is lost due to a system or internal error, the computer times out and re-transmits
the same command packet with the same <FSN> to the same receiver and waits once again for
an acknowledgment or a NAK packet.
To reiterate, the format of the message block is shown in Table B-4, Link Level Protocol Message
Block.
SYNC
COUNT
The RLLP Remote Port Packet structure is as follows:
<SYNC>
Message format header character that defines the beginning of a
message. The <SYNC> character value is always 0x16. (1 byte)
<COUNT>
Number of bytes in the <DATA> field. (two bytes)
<SOURCE ADDR>
Identifies the address of the equipment from where the message
originated. (1 byte)
<DEST ADDR>
Identifies the address of the equipment where the message is to be sent.
(1 byte)
<FSN>
Frame sequence number ensures correct packet acknowledgment and
data transfers. (1 byte)
<OPCODE>
This byte identifies the message type associated with the information data.
The equipment processes the data according to the value in this field.
Return error codes and acknowledgment are also included in this field.
(two bytes)
<...DATA...>
Information data. The number of data bytes in this field is indicated by the
<BYTE COUNT> value.
<CHECKSUM>
The modulo 256 sum of all preceding message bytes excluding the
<SYNC> character. (1 byte)
4.4.9 DMD2401 Opcode Command Set
The DMD2401 Opcode Command Set is listed below, separated by commands that control the
modulator only, the demodulator only, or the entire module.
4-44
DMD2401/DMD2401L/DMD2401 IBS/IDR Satellite Modem
Table 4-6. Link Level Protocol Message Block
SRC
DEST
ADDR
ADDR
FSN
OP
DATA
CODE
BYTES
CHECKSUM
TM065 – Rev. 3.3
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