Comtech EF Data SFC1800A - Manual (Page 46)

User Interfaces SFC1800A Synthesized Frequency Upconverter
4-22 TM111 - Rev. 1.0
<...DATA..> - The Data field contains the binary, data bytes associated with the
. The number of data bytes in this field is indicated by the value.
- The checksum is the modulo 256 sum of all preceding message bytes,
excluding the 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 below in Table 4-4.
Table 4-4. Checksum Calculation Example
BYTE FIELD DATA CONTENT RUNNING CHECKSUM
(Byte 1) 00h = 00000000b 00000000b
(Byte 2) 02h = 00000010b 00000010b
F0h = 11110000b 11110010b
2Ah = 00101010b 00011100b
09h = 00001001b 00100101b
(Byte 1) 00h = 00000000b 00101000b
(Byte 2) 03h = 00000011b 00101000b
(Byte 1) DFh = 11011111b 00000111b
(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 =
4.4.2.3 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
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.