Comtech EF Data DM240XR - Manual (Page 53)

DM240XR High-Speed Digital Modulator User Interfaces
MN-DM240XR– Revision 13 4–31
Complete information on monitor and control software is contained in the following sections.
4.4.1 Protocol Structure
The Communications Specification (COMMSPEC) defines the interaction of computer resident
Monitor and Control software used in satellite earth station equipment such as modems,
redundancy switches, multiplexers, and other ancillary support gear. Communication is bi-
directional, and is normally established on one or more full-duplex multi-drop control buses that
conform to EIA Standard RS-485.
Each piece of earth station equipment on a control bus has a unique physical address, which is
assigned during station setup/configuration or prior to shipment. Valid decimal addresses on one
control bus range from 032 through 255 for a total of up to 224 devices per bus. Address 255 of
each control bus is usually reserved for the M&C computer.
4.4.2 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 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 Byte Count is the number of bytes in the field, ranging from 0
through TBD. This field is 2 bytes long for the DM240 protocol.
- the Source Identifier defines the message originator’s multidrop address.