Comtech EF Data DMD20 - Manual (Page 13)

DMD20/DMD50/DMD2050/DMD2050E/DMD1050/OM20 Remote Protocol Remote Operations
MN-DMDREMOTEOP Revision 9 1–3
All nodes on a given control bus have a unique address that must be defined.
<DESTINATION ID>: The Destination Identifier serves as a pointer to the multi-drop destination
device that indicates where the message is to be sent.
<FRAME SEQUENCE NUMBER>: The Frame Sequence Number (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. 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 (2 Bytes).
<DATA>: 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.
<CHECKSUM>: The checksum is the modulo 256 sum of all preceding message bytes,
excluding the <SYNC> 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 1-1.
BYTE FIELD DATA CONTENT RUNNING CHECKSUM
<BYTE COUNT> (BYTE 1) 00h = 00000000b 00000000b
<BYTE COUNT> (BYTE 2) 04h = 00000100b 00000100b
<SOURCE ID> FFh = 11111111b 00000011b
<DESTINATION ID> 20h = 00100000b 00100011b
<FSN> 09h = 00001001b 00101100b
<OPCODE> (BYTE 1) 2Ah = 00101010b 01010110b
<OPCODE> (BYTE 2) 01h = 00000001b 01010111b
<DATA> (Byte 1) 08h = 00001000b 01011111b
<DATA> (Byte 2) 58h = 01011000b 10110111b
<DATA> (Byte 3) 3Bh = 00111011b 11110010b
<DATA> (Byte 4) 00h = 00000000b 11110010b
Table 1-1: Checksum Calculation Example
Thus, the checksum is 11110010b; which is F2h or 242 decimal. Alternative methods of
calculating the checksum for the same message frame are:
00h + 04h + FFh + 20h + 09h + 2Ah + 01h + 08h + 58h + 3Bh + 00h = 1F2h.
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 F2h. For a decimal checksum calculation, the
equivalent values for each information field are:
0 + 4 + 255 + 32 + 9 + 42 + 1 + 8 + 88 + 59 + 0 = 498;
498/256 = 1 with a remainder of 242.