MSK Modem
12
MX809
6 Application
6.1 Checksum Generation and Checking
6.1.1 Generation
The checksum generator takes the 48 bits from the 6 bytes loaded into the TX Data Buffer and divides them
into modulo-2 by the generating polynomial:
x15 + x14 + x13 + x11 + x4 + x2 + x1
It then takes the 15-bit remainder from the polynomial divider, inverts the last bit and appends an EVEN parity
bit generated from the initial 48 bits and the 15 bit remainder (with the last bit inverted). This 16 bit word is
used as the “Checksum”.
6.1.2 Checking
The checksum checker does two things:
1. It takes the first 63 bits of a received message, inverts bit 63, and divides them modulo-2 by the
generating polynomial:
x15 + x14 + x13 + x11 + x4 + x2 + x1
2. The 15 bits remaining in the polynomial divider are checked to make sure that they are all zero.
3. It generates an even parity bit from the first 63 bits of a received message and compares this bit with the
received parity (bit 64).
If the 15 bits in the polynomial divider are all zero and the two parity bits are equal, then the RX
Checksum True (Status Register bit 1) is set.
6.2 Modem Performance
Figure 3: Bit Error Rate vs. Signal-to-Noise Ratio
ꢀ1998 MX-COM, Inc.
www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054
Doc. # 20480036.004
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
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