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User Interfaces
<CHECKSUM> - The checksum is the modulo 256 sum of all preceding message bytes,
excluding the <SYN> character (1 Byte). The checksum determines the presence or absence of
errors within the message. In a message block with the following parameters, the checksurn is
computed as shown in Table 4-4 below.
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:
0002h + F0h + 2Ah + 09h + 0003h + 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>
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-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 (such as the DMD2401 LB/ST) 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.
4-30
DMD2401 LB/ST L-Band Satellite Modem and ODU Driver
Table 4-4. Checksum Calculation Example
Data Content
02h = 00000010b
F0h = 11110000b
2Ah = 00101010b
09h = 00001001b
03h = 00000011b
DFh = 11011111b
FEh = 11111110b
Running Checksum
00000010b
11110010b
00011100b
00100101b
00101000b
00000111b
00000101b
TM075 – Rev. 1.3
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