Comtech EF Data RCS20 - Manual (Page 145)

RCS20 M:N Redundancy Switch Revision 15
Remote Operations MN-RCS20 and CD-RCS20
A–3
-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, (1 byte).
- the Operation Code field contains a number that identifies the message type associated
with the data that follows it. Equipment under MCS control recognizes these bytes 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 command
indicated by the field. The number of bytes in this field is indicated by the
value.
- the checksum is the module 256 sum of all preceding message bytes excluding the
character, (1 byte). 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 A-1.
Table A-1. 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 00100101b
(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 + 00 + 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.