Paradyne DMD15L operation manual Protocol Structure, Protocol Wrapper, Sync, Byte Count

User Interfaces DMD15/DMD15L IBS/IDR Satellite Modem

4.9 Modem Remote Communications

4.9.1 Host Computer Remote Communications

NOTE: This specification is applicable to the DMD15, DMD10 and DMD10L Modems. Any reference to the DMD15 in this document can be applicable to any one of these three modems.

Control and status messages are conveyed between the DMD15 and the subsidiary modems and the host computer using packetized message blocks in accordance with a proprietary communications specification. This communication is handled by the Radyne Link Level Protocol (RLLP), which serves as a protocol ‘wrapper’ for the RM&C data. Complete information on monitor and control software is contained in the following sections.

4.9.1.1 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.

4.9.1.2 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

B0

B1

B2

B3

B4

B5

B6

B7

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 (B0 - 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:

<SYNC>:	The message format header character, or ASCII sync
	character, that defines the beginning of a message. The
	<SYNC> character value is always 16h.
<BYTE COUNT>:	The Byte Count is the number of bytes in the <DATA>
	field, ranging from 0 through 255 for the DMD5000
	protocol (1 Byte), or ranging from 0 through 509 for the
	DMD15 protocol (2 Bytes).

4-44 TM051 – Rev. 5.8