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RCS20 M:N Redundancy Switch Revision 15

Remote Operations MN-RCS20 and CD-RCS20

A–3

<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, (1 byte).

<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 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 <Opcode> field. The number of bytes in this field is indicated by the <BYTE COUNT>

value.

<CHECKSUM> - the checksum is the module 256 sum of all preceding message bytes excluding the

<SYNC> 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 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 00100101b

<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 + 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.

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