Comtech EF Data OMS11 - Manual (Page 65)

Appendix B OMS11 1:1 Redundancy Switch
B-2 TM133 – Rev. 1.1
The stop bits, S1 and S2, are each a mark. Data flow remains in a hold mode until S2 is replaced
by a space. If S2 is followed by a space, it is considered a start bit for the data byte and not part
of the actual data (B
0
- B
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’, which 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:
- the message format header character, or ASCII sync character, that defines the
beginning of a message. The character value is always 16h.
- the 2 Byte Data Count is the number of bytes in the field, ranging
from 0 through 509.
- the Source Identifier defines the multi-drop address origin. Note that all nodes
on a given control bus has an unique address that must be defined.
- the Destination Identifier serves as a pointer to the multi-drop destination
device that indicates where the message is to be sent.
-the FSN is a tag with a value from O through 255 that is sent
with each message. It assures sequential information framing and correct equipment
acknowledgment and data transfers.
- the 2 Byte 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. 1 Byte
for the DMD5000 protocol and 2 Bytes for the DMD15 protocol.
- the Data field contains the binary, bi-directional data bytes associated with the
- The number of data bytes in this field is indicated by the value.
- the checksum is the modulo 256 sum of all preceding message bytes, excluding
the 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.
BYTE FIELD
DATA CONTENT RUNNING CHECKSUM
02h = 00000000b 00000000b
02h = 00000010b 00000010b
F0h = 11110000b 11110010b
2Ah = 00101010b 00011100b
09h = 00001001b 00100101b
03h = 00000011b 00101000b
(Byte 1) DFh = 11011111b 00000111b
(Byte 2) FEh = 11111110b 00000101b
Table 1. Checksum Calculation Example
Thus, the checksum is 00000101b; which is 05h or 5 decimal. Alternative methods of calculating
the checksum for the same message frame are: