Radyne DMD20 Installation And Operation Manual page 95

DMD20 Universal Satellite Modem
<OPCODE>:
<DATA >:
<CHECKSUM>:
BYTE FIELD
<BYTE COUNT>
<SOURCEID>
<DESTINATION ID>
<FSN>
<OPCODE>
<DATA> (Byte 1)
<DATA> (Byte 2)
Thus, the checksum is 00000101b; which is 05h or 5 decimal. Alternative methods of
calculating the checksum for the same message frame are:
02h + F0h + 2Ah + 09h + 03h + DFh + FEh = 305h.
Since the only concern is the modulo 256 (modulo 1 00h) 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:
2 + 240 + 42 + 9 + 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>
TM103 – Rev. 1.1
The Operation Code field contains a number that
identifies the message type associated with the data that
follows it. Equipment under MCS control recognizes this
byte via firmware identification and subsequently steers
the DATA accordingly to perform a specific function or
series of functions. Acknowledgment and error codes
are returned in this field. 1 Byte for the DMD20 protocol,
and 2 Bytes for the DMD20 protocol.
The Data field contains the binary, bi-directional data
bytes associated with the <OPCODE>. The number of
data bytes in this field is indicated by the
<BYTE COUNT> value.
The checksum is the modulo 256 sum of all preceding
message bytes, excluding the <SYN> character. The
checksum determines the presence or absence of errors
within the message. In a message block with the
following parameters, the checksum is computed as
shown in Table 4-7.
Table 4-7. Checksum Calculation Example
DATA CONTENT
02h = 00000010b
F0h = 11110000b
2Ah = 00101010b
09h = 00001001b
03h = 00000011b
DFh = 11011111b
FEh = 11111110b
User Interfaces
RUNNING CHECKSUM
00000010b
11110010b
00011100b
00100101b
00101000b
00000111b
00000101b
4-47