Comtech EF Data RCS11 - Manual (Page 96)

RCS11 1:1 Redundancy Switch RCS11 Remote Communications
MN-RCS11 – Revision 9 B–2
B.1.2 Protocol Wrapper
The Radyne COMMSPEC is byte-oriented, with the Least Significant Bit (LSB) issued first. Each
data byte is conveyed as mark/space information with two marks comprising the stop data.
When the last byte of data is transmitted, a hold comprises one steady mark (the last stop bit).
To begin or resume data transfer, a space (00h) substitutes this mark. This handling scheme is
controlled by the hardware and is transparent to the user. A pictorial representation of the data
and its surrounding overhead may be shown as follows:
S1
S2  B
0
 B
1
 B
2
 B
3
 B
4
 B
5
 B
6
 B
7
 S1  S2, etc.
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’, 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:
- 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 have 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