Comtech EF Data DD240XR Rev Е - Manual (Page 51)

DD240XR High-Speed Digital Demodulator User Interfaces
MN-DD240XR – Rev. E 4-31
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 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 a response message is not received by the packet originator. 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 Corporation 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.
4.3.4 Global Response Operational Codes
In acknowledgment 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
three global responses.
If a bad parameter or inconsistent value is sent in an RLLP Message, the reply packet will have an
operational code value of 00FFh and the unit will log an event. The operator should inspect the
event log to determine the reason for a message failure.
Table 4-2. Response OPCODES
RESPONSE OPCODE DESCRIPTION OPCODE
Good Message 000d = 0000h
Bad Parameter 255d = 00FFh
Bad Opcode 254d = 00FEh
4.3.5 Collision Avoidance
When properly implemented, the physical and logical devices and ID addressing scheme of the
COMMSPEC normally precludes message packet contention on the control bus. The importance
of designating unique IDs for each device during station configuration cannot be overemphasized.
One pitfall, which is often overlooked, concerns multi-drop override IDs. All too often, multiple
devices of the same type are assigned in a direct-linked ("single-thread") configuration accessible
to the M&C computer directly. For example, if two DD240 Demodulators with different addresses
(DESTINATION IDs) are linked to the same control bus at the same hierarchical level, both will
attempt to respond to the M&C computer when the computer generates a multi-drop override ID
of 23. If their actual setup parameters, status, or internal timing differs, they will both attempt to