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MX429A

1200/2400bps MSK Modem for Trunked Radio Systems

MX-COM Inc  

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

Bit 0

D0

Description

TX Enable

Bit 1

D1

Tx Parity

Enable

Bit 2

D2

Bit 3

D3

Rx Enable

Rx Message

Format

Bit 4

D4

Bit 5

D5

Bit 6

D6

Bit 7

D7

Timer LSB

Timer

Timer

Timer MSB

Tx Enable

Function

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.

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”.

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.

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.

Reference Table 5

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.

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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