1200/2400bps MSK Modem for Trunked Radio Systems
7
MX429A
4.1.3 Non MPT Application – Full-Duplex
The functions described in this section, to allow the MX429A modem to operate as a general purpose device,
are obtained using the commands and indications detailed in the “Register Instructions” pages.
Tx: When enabled the device transmits a “101010......10” preamble until data for transmission is loaded by
the host microprocessor. Transmits 6 bytes of the loaded data followed by a 2-byte checksum based on
that data. As long as Tx data is loaded the transmitter will transmit, the 2-byte checksum being produced
after every 6 bytes (8 byte packages). Automatic checksum generation can be inhibited by a software
command to allow transmission of continuous data streams.
Rx: When enabled requires the 16-bit SYNC or SYNT word (see notes) before outputting data bytes. The
modem receiver will then output continuous bytes of data, after every 6 bytes received a 2-byte
checksum word will be output and can be ignored or used for error checking.
4.1.4 Control Register (A1 = 1, A0 = 1, R/W = 0, Write Only)
The Control Register, when selected, directs the modem’s operation as described in Table 4
Bit
Description
Function
Bit 0
D0
TX Enable
Set: D0 enables the transmitter for operation. A '0 – 1' transition causes bit
synchronization and the start of 1010.........10 preamble pattern transmission. At least
one byte of preamble will be transmitted. If data is loaded into the Tx Data Buffer before
one byte has been sent then that data will follow, otherwise whole bytes of preamble will
continue until data is loaded.
Clear: The Transmitter Output pin is set to a high impedance and no transmitter
interrupts are produced.
Bit 1
D1
Tx Parity
Enable
Set: D1 indicates to the transmitter that 2–byte checksums are to be generated by the
modem. A '0 – 1' transition starts checksum generation on the next six bytes loaded
from the Tx Data Buffer into the Tx Data Register. Checksum generation continues for
every 6 bytes loaded until this bit is cleared. The transmitter will send the generated
checksum (2 bytes) after the last of each 6 bytes have been sent. If an underrun (no
more data loaded) condition occurs before 6 bytes have been loaded checksum
generation will abort, the transmission will cease after one 'hang' bit has been sent and
Bit 4 in the Status Register (Tx Idle) will be set. No checksum will be transmitted.
Clear: No checksum generation is carried out and the host may supply the checksum
bytes. The output is then “as written”.
Bit 2
D2
Rx Enable
Set: D2 enables the receiver for operation. No data is produced (i.e. No Rx Data
Ready interrupts) until a 'SYNC' or 'SYNT' word is found in the received bit stream.
Clear: The receiver is disabled and all interrupts caused by the receiver are inhibited.
Bit 3
D3
Rx Message
Format
Set: D3 is sampled after a checksum has been received and allows the host to control
the way the receiver handles the following data bits. If 'set' the receiver will assume that
the next 6 bytes are data and will start error checking accordingly.
Clear: The receiver will stop data transfer to the host after the 2 checksum bytes until
another 'SYNC' or 'SYNT' frame word is received.
Bit 4
D4
Timer LSB
Timer
Bit 5
D5
Reference Table 5
Bit 6
D6
Timer
Bit 7
D7
Timer MSB
If a new timer value is written to these inputs within 1 byte period of the last timer
interrupt then the next timer period will be correct without first having to reset the timer,
otherwise the timer must be reset to zero and then set to the new time.
Tx Enable
If using the internal Tx preamble generation facility, e.g. with the internal timer setting
the preamble length, the device may occasionally produce a Tx Ready interrupt
immediately after a Tx Enable command. User software should handle this occurrence
by either:
a. Detecting that the timer interrupt status bit is not set and that it is not appropriate to
load Tx data at this time.
b. Not using the timer. i.e. immediately after Tx Enable, reading the Status Register and
loading a byte of preamble. This resets any interrupt. The length of preamble
transmitted is now controlled by the number of bytes loaded.
© 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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