Comtech EF Data SDM-2020 - Manual (Page 111)

SDM-2020 Satellite Modulator Revision 7
Low Voltage Differential Signal (LVDS) MN/SDM2020M.IOM
8-4
8.1.4 R
ECEIVE
/T
RANSMIT
D
ATA
, J3/J4, DB25 F
EMALE
The receive transmit 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 bytes are added by the modulator to create a satellite frame
of 204 bytes. The 16 bytes are removed by the demodulator 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 (047 hex) 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.
Figure 6-1 and Figure 6-2 depict the 204-byte packetization for byte parallel and byte
serial formats at J3 and J4, showing all possible data fields and timing signal
relationships.
The 188-byte and no framing formats are subsets of the 204-byte format.
8.1.4.1 C
ONNECTOR
P
INOUTS
J3
The connector is a 25-pin sub-miniature D female, with threaded jack nuts. All signals for
this connector are outputs to the demodulator, except the signal pair on pins 2 and 15 (for
an exception; see 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 Configuration Interface menu. For DBS applications, the polarity of
signals may be inverted from the Configuration Interface menu.