Data Sheet

4-Level FSK Modem Data Pump Page 28 of 47 MX919B PRELIMINARY INFORMATION
©2001 MX•COM, INC. www.mxcom.com Tele: 800 638 5577 336 744 5050 Fax: [phone] Doc. # 20480170.003
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA All trademarks and service marks are held by their respective companies.
4.6.1.2 CRC2
This is a thirty-two-bit CRC check code contained in bytes 8 to 11 of the 'Last' Block, which provides error
detection coverage for the combined Intermediate Blocks and Last Block of a message. It is calculated by the
modem from all of the data and pad bytes in the Intermediate Blocks and in the first 8 bytes of the Last Block
using the generator polynomial:
x
32
+ x
26
+ x
23
+ x
22
+ x
16
+ x
12
+ x
11
+ x
10
+ x
8
+ x
7
+ x
5
+ x
4
+ x
2
+ x
1
+ 1
Note: In receive mode the CRC2 checksum circuits are initialized on completion of any task other than NULL
or RILB. In transmit mode the CRC2 checksum circuits are initialized on completion of any task other
than NULL, TIB, or TLB.
Command Register bit B5 (CRC) allows the user to select between two different forms of the CRC1
and CRC2 checksums. When this bit is set to '0', the CRC generators are initialized to 'all ones' for
calculations such as CCITT X25 CRC. When this bit is set to '1', the CRC generators are initialized to
'all zeros'.
4.6.1.3 Forward Error Correction
In transmit mode, the MX919B uses a Trellis Encoder to translate the 96 bits (12 bytes) of a 'Header',
'Intermediate' or 'Last' Block into a 66-symbol (132 bits) sequence which includes FEC information.
In receive mode, the MX919B decodes the received 66 symbols of a block into 96 bits of binary data using a
'Soft Decision' Viterbi algorithm to perform decoding and error correction.
4.6.1.4 Interleaving
The 66 symbols of a 'Header', 'Intermediate' or 'Last' block are interleaved by the modem before transmission
to provide protection against the effects of noise bursts and short fades.
In receive mode, the MX919B de-interleaves the received symbols prior to decoding.
4.7 Transmitted Symbol Shape
Bit 4 of the Command Register (TXIMP) selects the transmit baseband signal and the receive signal
equalization as follows:
If the TXIMP bit is '0', then the transmit baseband signal is generated by feeding full-symbol-time-width 4-level
symbols into the RRC lowpass filter. The receive signal equalization is optimized for this type of signal. With
this setting, the MX919B is compatible with the MX919A devices, another member of the MX919 device
family.
If the TXIMP bit is set to '1,' impulses, rather than full-symbol-time-width symbols are fed into the RRC filter
when in TX mode, and the receive signal equalization is suitably adjusted in RX mode.
1 symbol
time
+3
TXIMP = 0 TXIMP = 1
-1
+1
-3
1 symbol
time
+3
-1
+1
-3
Figure 16: Input Signal to RRC Filter in Tx Mode for TXIMP = 0 and 1