Comtech EF Data SDM-2020 - Manual (Page 124)

SDM-2020 Satellite Modulator Revision 7
ECL/HSSI TX MN/SDM2020M.IOM
10-3
Table 10-2. Terrestrial Transport Protocols
Condition Description
TX Data Valid Input For all TX framing modes, the data valid input “DVALID” is ignored. Packet structures and
validity of data are based on the programmed frame format.
For all receive framing modes, the data valid output “DVALID” operates as defined in the
frame mode definitions.
TX Interface
Synchronization
Modes
The TX interface supports two synchronization modes:
1. Sync Byte Data Correlation Mode - Frame synchronization is correlated from the
packet input data, based on the embedded sync word (47 HEX). In this mode the
“SYNC,” input is ignored.
2.
External Sync Mode - Frame synchronization is based on the external “SYNC” input.
In this mode the data which occupies the sync byte data position(s) is over written
with a valid sync word (47 HEX) based on the occurrence of the “SYNC” input timing
by the Modem.
The frame synchronization mode is not directly selectable by the user from the front panel
interface. Synchronization mode is derived from the Modulator configuration interface sync
mode selection programmable from the front panel.
Serial Data Formats The processing order always starts from the MSB (i.e. “0”) of the sync word-byte
(i.e., 01000111).
187 Byte (no
framing) Serial
Format
The 187-byte format contains no framing information. Figure 10-1 shows the general packet
arrangement from the receiver. There is no sync, so the “SYNC” signal is always logic “0”.
Since data is always present, the data valid signal “DVALID” is always logic “1”. The SYNC
and DVALID signals into the Transmitter are not utilized.
188 Byte Serial
Format The 188 byte serial format consists of 1 sync byte (8 bits, MSB first), followed by 187 payload
bytes (1496 bits). Figure 10-2 shows the signal relationships for the 188 byte serial interface.
The DVALID signal into the Transmitter is not utilized.
Serial Format The 204 byte serial format consists of 1 sync byte (8 bits, MSB first), 187 payload bytes
(1496 bits), and 16 dummy bytes (128 bits for Reed-Solomon check-sum). Typically the
16 dummy bytes reserved for Reed-Solomon coding are set to zero (logic low), this is a
requirement at the receive interface. Figure 10-3 shows the signal relationships for the 204
byte serial format.
Note: Observe the “DVALID” signal is de-asserted during the last 16 bytes of the frame. The
DVALID signal into the TX is not utilized.
204 Byte DBS
Serial Format A special case of the 204-byte format is the Digital Broadcast via Satellite (DBS) mode. The
DBS mode is the same as the general 204-byte format except that the sync pulse is only
asserted during the first bit of the sync byte. Figure 10-4 shows the signal relationships for the
204-byte DBS serial format. The DVALID signal into the Transmitter is not utilized.