Comtech EF Data DMD20 - Manual (Page 12)

DMD20/DMD50/DMD2050/DMD2050E/DMD1050/OM20 Remote Protocol Remote Operations
MN-DMDREMOTEOP Revision 9 1–2
This specification is applicable to the DMD20/20LBST, DMD50, DMD2050,
DMD2050E, DMD1050 and OM20 Modems. Any reference to the DMD20 in this
document can be applicable to any one of these three modems.
For configuration setup, refer to the product manuals.
1.2.1 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 9600-baud 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.
1.2.2 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 Byte Count is the number of bytes in the field (2 Bytes).
: The Source Identifier defines the multi-drop address origin.