Comtech EF Data MD2401 - Manual (Page 49)

MD2401 L-Band Multi Demod Installation and Operation Manual User Interfaces
MN-MD2401 4–17
Revision 7
that a packet sent to it is in the correct sequence relative to the previously received packet, it
computes a local checksum on all information within the packet excluding the <SYNC> character
and the <CHECKSUM> fields. If this checksum matches the packet <CHECKSUM>, the receiver
processes the packet and responds to the packet sender with a valid response (acknowledgment)
packet. If the checksum values do not match, the receiver replies with a negative
acknowledgment (NAK) in its response frame.
The response packet is therefore either an acknowledgment that the message was received
correctly, or some form of a packetized NAK frame. If the sender receives a valid
acknowledgment (response) packet from the receiver, the <FSN> increments and the next packet
is transmitted as required by the sender. However, if a NAK response packet is returned, the
sender re-transmits the original information packet with the same embedded <FSN>.
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.
Table 4-4. Link Level Protocol Message Block
SYNC COUNT SOURCE
ADDRESS
DESTINATION
ADDRESS
FSN OPCODE DATA
BYTES
CHECKSUM
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)
<BYTE COUNT>
Number of bytes in the <DATA> field. (2 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. (2 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)