Table of Contents
Advertisement
RCS20 M:N Redundancy Switch
Remote Operations
with the OPCODE, and the checksum for the frame is calculated. If the anticipated checksum
does not match then a checksum error response is returned to the message frame originator.
The entire message frame is discarded and the wait-for-SYNC mode goes back into effect.
If the OPCODE resides within a command message, it defines the class of action that denotes an
instruction, which is specific to the device type, and is a prefix to the DATA field if data is
required. If the OPCODE resides within a query message packet, then it defines the query code,
and can serve as a prefix to query code DATA.
The Frame Sequence Number (FSN) is included in every message packet, and increments
sequentially. When the M&C computer or bus-linked equipment initiates a message, it assigns the
FSN as a tag for error control and handshaking. A different FSN is produced for each new
message from the FSN originator to a specific device on the control bus. If a command packet is
sent and not received at its intended destination, then an appropriate response message is not
received by the packet originator. The original command packet is then re-transmitted with the
same FSN. If the repeated message is received correctly at this point, it is considered a new
message and is executed and acknowledged as such.
If the command packet is received at its intended destination but the response message
(acknowledgment) is lost, then the message originator (usually the M&C computer) re-transmits
the original command packet with the same FSN. The destination device detects the same FSN
and recognizes that the message is a duplicate, so the associated commands within the packet are
not executed a second time. However, the response packet is again sent back to the source, as an
acknowledgment in order to preclude undesired multiple executions of the same command.
To reiterate, valid equipment responses to a message require the FSN tag in the command packet;
this serves as part of the handshake/acknowledge routine. If a valid response message is absent,
then the command is re-transmitted with the same FSN. For a repeat of the same command
involving iterative processes, the FSN is incremented after each message packet. When the FSN
value reaches 255, it overflows and begins again at zero. The FSN tag is a powerful tool that
assures sequential information framing, and is especially useful where commands require more
than one message packet. The full handshake/acknowledgment involves a reversal of source and
destination ID codes in the next message frame, followed by a response code in the <OPCODE>
field of the message packet from the equipment under control.
If a command packet is sent and not received at its intended destination, a timeout condition can
occur because the packet originator does not receive a response message. On receiving devices
slaved to an M&C computer, the timeout delay parameters may be programmed into the
equipment in accordance with site requirements by Radyne Corp. prior to shipment, or altered by
qualified personnel. The FSN handshake routines must account for timeout delays and be able to
introduce them as well.
A.1.4 Global Response Operational Codes
In acknowledgment (response) packets, the operational code <OPCODE> field of the message
packet is set to 0 by the receiving devices when the message intended for the device is evaluated
as valid. The device that receives the valid message then exchanges the <SOURCE ID> with the
<DESTINATION ID>, sets the <OPCODE> to zero in order to indicate that a good message was
received, and returns the packet to the originator. This "GOOD MESSAGE" Opcode is one of
nine global responses. Global response Opcodes are common responses issued to the M&C
computer or to another device that can originate from and are interpreted by all Radyne
A–4
Rev. 14
MN-RCS20
Advertisement
Table of Contents
