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DMD15/DMD15L IBS/IDR Satellite Modem User Interfaces
TM051 - Rev. 5.8 4-55
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 Link Level Protocol Message Block is shown below.
SYNC COUNT SRC
ADDR
DEST
ADDR
FSN OPCODE DATA
BYTES
CHECKSUM
4.9.2 Remote Port Packet Structure:
The Modem protocol is an enhancement on the DMD5000 protocol. It also uses a packet
structure format. The 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>: The number of bytes in the <DATA> field (2 bytes).
<SOURCE ID>: Identifies the address of the equipment from where the
message originated (1 byte).
<DEST. ID>: 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).
The Modem RLLP is not software-compatible with the following previous Radyne
products: RCU5000 and DMD4500. These products may not occupy the same
bus while using this protocol as equipment malfunction and loss of data may
occur.
NOTE: When transmitting a packet at 9600 baud, the Remote M&C should
ensure that the timeout value between characters does not exceed the time it
takes to transmit 200 characters(
| 200 msec). If this timeout value is exceeded,
the equipment will timeout.