Comtech EF Data DMD2401/DMD2401L - Manual (Page 77)

User Interfaces DMD2401/DMD2401L/DMD2401 IBS/IDR Satellite Modem
4-38 TM065 – Rev. 3.3
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 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 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 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 with
the , sets the 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):
Table 4-5. Response Opcodes
Response Opcode Description Opcode
Good Message 00h
Bad Parameter FFh
Bad Opcode FEh
Bad Checksum FDh
Command Not Allowed in LOCAL Mode FCh
Command Not Allowed in AUTO Mode FBh
Bad Destination FAh
Unable to Process Command F9h
Packet Too Long F8h