Comtech EF Data RCS20 - Manual (Page 153)

RCS20 M:N Redundancy Switch Revision 15
Remote Operations MN-RCS20 and CD-RCS20
A–11
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 message block is shown in Table A-5, Link Level Protocol Message Block.
Table A-5. Link Level Protocol Message Block
SYNC
1 byte
COUNT
2 bytes
SOURCE
ADDRESS
1 byte
DESTINATION
ADDRESS
1 byte
FSN
1 byte
OPCODE
2 bytes
DATA BYTES
(count) bytes
CHECKSUM
1 byte
A.2 Remote Port Packet Structure:
The RLLP Remote Port Packet structure is as follows:
= Message format header character that defines the beginning of a message. The
character value is always 0x16, (1 byte).
= 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).
= Frame sequence number ensures correct packet acknowledgment and data transfers, (1 byte).
= 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 COUNT> value.
= The modulo 256 sum of all preceding message bytes excluding the character,
(1 byte).