Comtech EF Data DD2401 VME - Manual (Page 32)

User Interfaces DD2401 VME L-Band Demodulator Card Installation & Operational Manual
4-8 MN-VME2401 – Rev. B
4.4.3 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-SYNC-character mode. The remaining message format
header information is then loaded, either by the M&C computer or by a subordinate piece of
equipment (such as the DD2401/DD2401L) 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 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 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, 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 Comtech EF Data 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