Comtech EF Data SDM-2020 - Manual (Page 141)

SDM-2020 Satellite Demodulator Revision 4
LVDS Data Interface MN/SDM2020D.IOM
8–5
8.1.4 User Interfaces
The following paragraphs describe the electrical and physical interfaces, which support
terrestrial data and auxiliary information on the LVDS data interface module.
8.1.4.1 RX/TX Data, J3/J4, DB25 Female
The RX/TX data interfaces have the same formats. There are three general modes of
operation controlled via the front panel display/keypad or the remote port. The standard
frame formats supported are as follows:
• 188 Mode: The unit seeks a DVB/MPEG2 frame consisting of 1 sync byte
(047 hex) and 187 bytes of data. The frame structure is acquired, and 16 bytes of
Reed-Solomon check the modulator to create a satellite frame of 204 bytes adds
bytes. The demodulator removes the 16 bytes and the 188-byte frame returned to
the terrestrial circuit.
• 204 Mode: A 204-byte frame structure consisting of 1 sync byte (047 hex) plus
187 bytes of data and 16 bytes reserved for check bytes is expected. The
modulator acquires the frame structure and fills the Reed-Solomon bytes into the
16 reserved bytes, again resulting in a 204-byte satellite frame. The demodulator
returns the 204-byte frame to the terrestrial circuit.
• None: In this mode, no incoming frame structure is expected, and the modulator
creates one. A sync byte (047hex) is created, followed by 187 bytes of data plus
16 Reed-Solomon check bytes. The resulting satellite frame is 204 bytes. The
demodulator removes the 16 Reed-Solomon bytes and the sync byte and returns
the data to the terrestrial circuit.
The 188 byte and no framing formats are subsets of the 204-byte format used over the
satellite.
8.1.4.1.1 Connector Pinouts, J3
Note: Table 8-2 provides applicable data for the SDM-2020 Demodulator.
The connector is sub-miniature 25-pin D female, with threaded jack nuts.
All signals for this connector are outputs from the Demodulator, except the signal pair on
pins 2 and 15 (for an exception, see Figure 8-1, for jumper settings).
When used as a serial data port, Data 7 (pins 3 and 16) are the active data pins. For DVB
parallel operation, Data 7 through Data 8 are used.
Signal levels are as defined in TIA/EIA-644. Per TM1449, the differential sense of all
signal pairs is a logic “1” when “A” is positive with respect to “B”. This is the normal
selection in the Functional Select: Configuration: Interface menu. For DBS