Comtech EF Data MD2401 - Manual (Page 43)

MD2401 L-Band Multi Demod Installation and Operation Manual User Interfaces
MN-MD2401 4–11
Revision 7
The COMMSPEC developed for use with the Radyne Link Level Protocol (RLLP) organizes the
actual monitor and control data within a shell, or ‘protocol wrapper’, that surrounds the data. The
format and structure of the COMMSPEC message exchanges are described herein. Decimal
numbers have no suffix; hexadecimal numbers end with a lower case h suffix and binary values
have a lower case b suffix. Thus, 22 = 16h = 000010110b. The principal elements of a data
frame, in order of occurrence, are summarized as follows:
<SYNC> - the message format header character, or ASCII sync character, that defines the beginning of a
message. The <SYNC> character value is always 16h (1 Byte).
<BYTE COUNT> - the Byte Count is the number of bytes in the <DATA> field (two bytes).
<SOURCE ID> - the Source Identifier defines the multi-drop address origin. Note that all nodes on a
given control bus have a unique address that must be defined (1 Byte).
<DESTINATION ID> - The Destination Identifier serves as a pointer to the multi-drop destination device
that indicates where the message is to be sent (1 Byte).
<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 this code 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 (two bytes).
<...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
<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 checksurn is
computed as shown in Table 4-4 below.
Table 4-2. 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: