Data Sheet
4-Level FSK Modem Data Pump Page 19 of 47 MX919B PRELIMINARY INFORMATION
©2001 MX•COM, INC. www.mxcom.com Tele: 800 638 5577 336 744 5050 Fax: 336 744 5054 Doc. # 20480170.003
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4.5.2.12 R4S: Read 4 Symbols
This task causes the modem to read the next 4 symbols and translate them directly (without de-interleaving or
FEC) to an 8-bit byte which is placed into the Data Block Buffer. The BFREE and IRQ bits of the Status
Register are then set to '1' to indicate that the µC may read the data byte from the Data Block Buffer and write
the next task to the Command Register.
This task is intended for special tests and channel monitoring - perhaps preceded by a SFS task.
Note: It is possible to construct message formats, which do not rely on the block formatting of the THB, TIB,
and TLB tasks. This can be accomplished by using T4S or T24S tasks to transmit and R4S to receive the
user's data. One should be aware, that the receive level and timing measurement circuits need to see a
reasonably 'random' distribution of all four possible symbols in the received signal to operate correctly.
Accordingly, binary data may benefit from scrambling before transmission if it is not reasonably 'random' to
start with.
4.5.2.13 T24S: Transmit 24 Symbols
This task, which is intended to facilitate the transmission of Symbol and Frame Sync patterns as well as
special test sequences, takes 6 bytes of data from the Data Block Buffer and transmits them as 24 4-level
symbols without any CRC or FEC.
Byte 0 of the Data Block Buffer is sent first, byte 5 last.
Once the modem has read the data bytes from the Data Block Buffer, the BFREE and IRQ bits of the Status
Register will be set to '1', indicating to the µC that it may write the data and command byte for the next task to
the modem.
The tables below show what data needs to be written to the Data Block Buffer to transmit the MX919B
Symbol and Frame Sync sequences:
Values written to Data Block Buffer
‘Symbol Sync’
Symbols
Binary Hex
+3 +3 -3 -3 Byte 0: 11110101 F5
+3 +3 -3 -3 Byte 1: 11110101 F5
+3 +3 -3 -3 Byte 2: 11110101 F5
+3 +3 -3 -3 Byte 3: 11110101 F5
+3 +3 -3 -3 Byte 4: 11110101 F5
+3 +3 -3 -3 Byte 5: 11110101 F5
Values written to Data Block Buffer
‘Frame Sync’
Symbols
Binary Hex
-1 +1 -1 +1 Byte 0: 00100010 22
-1 +3 -3 +3 Byte 1: 00110111 37
-3 -1 +1 -3 Byte 2: 01001001 49
+3 +3 -1 +1 Byte 3: 11110010 F2
-3 -3 +1 +3 Byte 4: 01011011 5B
-1 -3 +1 +3 Byte 5: 00011011 1B
4.5.2.14 THB: Transmit Header Block
This task takes 10 bytes of data (Address and Control) from the Data Block Buffer, calculates and appends
the 2-byte CRC1 checksum, translates the result to 4-level symbols (with FEC), interleaves the symbols, and
transmits the result as a formatted 'Header' Block.
Once the modem has read the data bytes from the Data Block Buffer, the BFREE and IRQ bits of the Status
Register will be set to '1'.