Operating instructions

RCS20 M:N Redundancy Switch User Interfaces
TM056 - Rev. 2.3 4-13
<DESTINATION ID> - The Destination Identifier specifies the multidrop address of the device(s) to
which the message is sent.
<FRAME SEQUENCE NUMBER> - The FSN is a tag with a value from 0 through 255 that is sent
with each message. It assures sequential information framing and correct equipment
acknowledgment and data transfers.
<OPCODE> - The Operation Code field contains a number that identifies the message type
associated with the data that follows it. Acknowledgment and error codes are returned in this field.
This field is 2 Bytes for the DM240 protocol.
<...DATA...> - The Data field contains the binary, data bytes associated with the
<OPCODE>. The number of data bytes in this field is indicated by the <BYTE COUNT> value.
<CHECKSUM> - 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
below in Table 4-4.
Table 4-4. Checksum Calculation Example
BYTE FIELD DATA CONTENT RUNNING CHECKSUM
<BYTE COUNT> (Byte 1) 00h = 00000000b 00000000b
<BYTE COUNT> (Byte 2) 02h = 00000010b 00000010b
<SOURCEID> F0h = 11110000b 11110010b
<DESTINATION ID> 2Ah = 00101010b 00011100b
<FSN> 09h = 00001001b 00100101b
<OPCODE> (Byte 1) 00h = 00000000b 00101000b
<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>