Comtech EF Data RCS11 - Manual (Page 99)

RCS11 1:1 Redundancy Switch RCS11 Remote Communications
MN-RCS11 – Revision 9 B–5
B.1.5 Software Compatibility
CAUTION
The DMD15 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.
The COMMSPEC, operating in conjunction within the RLLP shell, provides for full forward and
backward software compatibility independent of the software version in use. New features are
appended to the end of the DATA field without OPCODE changes. Older software simply
discards the data as extraneous information without functional impairment for backward
compatibility.
If new device-resident or M&C software receives a message related to an old software version,
new information and processes are not damaged or affected by the omission of data.
The implementation of forward and backward software compatibility often, but not always,
requires the addition of new Opcodes. Each new function requires a new Opcode assignment if
forward and backward compatibility cannot be attained by other means.
B.1.6 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 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 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