Comtech EF Data DMD20 - Manual (Page 20)

DMD20/DMD50/DMD2050/DMD2050E/DMD1050/OM20 Remote Protocol Remote Operations
MN-DMDREMOTEOP Revision 9 1–10
To maintain frame synchronization, the IOPT keeps track of error-laden packets and packets
intended for other equipment for the duration of each received packet. Once the packet is
complete, the IOPT invokes an I/O wait state and searches for the next character.
1.2.8 RLLP
Summary:
The RLLP is a simple send-and-wait protocol that automatically re-transmits a packet whenever
an error is detected, or when an acknowledgment (response) packet is absent.
During transmission, the protocol wrapper surrounds the actual data to form information packets.
Each transmitted packet is subject to time out and frame sequence control parameters, after
which the packet sender waits for the receiver to convey its response. Once a receiver verifies
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 character
and the fields. If this checksum matches the packet , 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 an acknowledgment either 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 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 .
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 . 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 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
Byte
Byte
COUNT
SOURCE
ADDRESS
DESTINATION
ADDRESS
FSN OPCODE
DATA
BYTES
CHECKSUM
1.3
Remote Port Packet Structure:
The Modem protocol is an enhancement on the DMD20 protocol. It also uses a packet structure
format. The structure is as follows:
: Message format header character that defines the beginning of a message. The
character value is always 0x16 (1 byte).
: The number of bytes in the field (2 bytes).
: Identifies the address of the equipment from where the message originated (1
byte).
: Identifies the address of the equipment where the message is to be sent (1
byte).