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DMD2401 LB/ST L-Band Satellite Modem and ODU Driver
User Interfaces
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 the transmit power
level of a DMD2401 LB/ST modulator), 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 ComStream, Inc. 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.
4.4.4 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 equipment in the same manner. These are summarized as follows (all
opcode values are expressed in decimal form):
TM075 - Rev. 1.3
4-31
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