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User Interfaces
RCS20 M:N Redundancy Switch
4.7.3 Frame Description and Bus Handshaking
In a Monitor and Control environment, every message frame on a control bus port executes as a
packet in a loop beginning with a wait-for-SYN-character mode. The remaining message format
header information is then loaded, either by the M&C computer or by a subordinate piece of
equipment requesting access to the bus. Data is processed in accordance with the OPCODE, and
the checksum for the frame is calculated. If the anticipated checksum does not match then the
wait-for-SYN mode goes back into effect. If the OPCODE resides within a command message, it
defines the class of action that denotes an instruction that 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 (such as increasing or decreasing transmit power level), the FSN is
incremented after each message packet. When the FSN value reaches 255, it overflows and begins
again at zero.
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.
4.7.4 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 devi ce 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 ComStream, Inc.
equipment in the same manner. These are summarized as follows (all Opcode values are
expressed in decimal form):
4-14
TM056 - Rev. 2.3
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