RCS20 M:N Redundancy Switch
Revision 15
Remote Operations
MN-RCS20 and CD-RCS20
A–10
the packet is complete, the IOPT conveys the appropriate message to the RLLT and invokes an I/0 wait
state (wait for next <SYNC> character).
If the RLLT receives the packetized message from the sender before it times out, it checks for any error
messages appended by the IOPT. In the absence of errors, the RLLT processes the received command
sent via the transmitted packet and issues a "message out" system call to ultimately acknowledge the
received packet. This call generates the response packet conveyed to the sender. If the IOPT sensed
errors in the received packet and an RLLT timeout has not occurred, the RLLT causes the equipment to
issue the appropriate error message(s) in the pending equipment response frame.
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/0 wait state and searches for the next <SYNC> character.
When transmitting a packet, 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.
A.1.8
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 timeout 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 <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
