1200/2400bps MSK Modem for Trunked Radio Systems
10
MX429A
4.2 Syndrome Word
This 16-bit word (both Low and High bytes) may be used to correct errors. Bits S1 to S15 are the 15 bits
remaining in the polynomial divider of the checksum checker at the end of 6 bytes of “ received message.”
For a correct message all 15 bits (S1 to S15) will be zero. The 2 Syndrome bytes are valid when the Rx Data
Ready bit (Status Register D0) is set for the second byte of the received checksum and should be read, if
required, before 8 byte periods.
4.2.1 Syndrome Low Byte (A1 = 0, A0 = 0, R/W = 1, Read Only)
D0
S1
D1
S2
D2
S3
D3
S4
D4
S5
D5
S6
D6
S7
D7
S8
4.2.2 Syndrome High Byte (A1 = 0, A0 = 1, R/W = 1, Read Only)
D0
S9
D1
D2
D3
D4
D5
D6
D7
Parity Error
S10 S11 S12 S13 S14 S15
D7: This is a “Parity Error Bit”. Indicating an error between the received parity bit and the parity bit internally
generated from the incoming message. Therefore a correctly received message all 16 bits of the Syndrome
Word
(S1 to S15 and Parity Error) will be zero.
4.3 Carrier Detect Time Constant
The value of the Carrier Detect capacitor, C5, determines the carrier detect time constant. A long time
constant (larger value C5) results in improved noise immunity but increased response time. C5 may be varied
to optimize noise immunity/response time.
1. With R2 = 1Mꢁ and C5 = 1µF as external components for the carrier detect function at 1200 baud
only.
2. By using C5 = 0.1µF and removing R2 completely the MX429A will operate at both 1200 and 2400
baud rates.
2 x 10-2
10-2
Ideal Coherent MSK
Characteristic
MX429A
Characteristic
10-3
10-4
8
6
4
3
2
10-5
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Signal to Noise Ratio (dB) [Bit-Rate Bandwidth]
(Linear Scale)
Figure 4: 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. # 20480128.007
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
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CMLMICRO [ CML MICROCIRCUITS ]
