Comtech EF Data DD240XR Rev Е - Manual (Page 50)

User Interfaces DD240XR High-Speed Digital Demodulator
4-30 MN-DD240XR – Rev. E
<DESTINATION ID> 2Ah = 00101010b 00011100b
<FSN> 09h = 00001001b 00100101b
<OPCODE> (Byte 1) 00h = 00000000b 00100101b
<OPCODE> (Byte 2) 03h = 00000011b 00101000b
<DATA> (Byte 1) DFh = 11011111b 00000111b
<DATA> (Byte 2) FEh = 11111110b 00000101b
Thus, the checksum is 00000101b; which is 05h or 5 decimal. Alternative methods of calculating
the checksum for the same message frame are:
00h + 02h + F0h + 2Ah + 09h + 00h + 03h + DFh + FEh = 305h.
Since the only concern is the modulo 256 (modulo 100h) equivalent (values that can be
represented by a single 8-bit byte), the checksum is 05h.
For a decimal checksum calculation, the equivalent values for each information field are:
0 + 2 + 240 + 42 + 9 + 0 + 3 + 223 + 254 = 773;
773/256 = 3 with a remainder of 5. This remainder is the checksum for the frame.
5 (decimal) = 05h = 0101b = <CHECKSUM>
4.3.3 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 a
checksum error response is returned to the message frame originator. The entire message frame
is discarded and 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.