Primary Modes - RDT RM96xx User Manual

Primary Modes of Operation
Asynchronous Mode
In Asynchronous Mode data arrives through either the RS-232 or RS-485 serial port and is placed in the data
buffer. As soon as data is detected in the buffer, the transceiver is switched to transmit mode. Once switched
to transmit there will be a short delay (10mS), while the synthesiser locks and the transmitter reaches operating
power. The data buffer is then inspected to determine the number of bytes available for transmission in this
data packet. A small amount of header information (used internally by the receiving RM96xx) and the data
bytes are then transmitted along with a 16 bit CRC. After this packet has completed transmission the data buffer
is inspected to see if more data has arrived. If more data is available then the transmission process is
repeated. When no more data is available the transceiver is switched to receive mode.
When an RM96xx header block containing the appropriate modem address, size of data packet and valid
CRC is received then the number of bytes specified in the header block will be read into the data buffer and
then output to the RS-232 or RS-485 serial port.
Point to point, multidrop and repeater configurations are feasible in this mode.
Synchronous Mode
In Synchronous Mode timing constraints are imposed on the serial data. This mode is designed to be used
in systems such as RTU MODBUS where the end of a message is determined by a gap in the serial data
stream of 3.5 characters or more.
Data arrives through either the RS-232 or RS-485 serial port and is placed in the data buffer until a gap of
3 1 / characters is detected in the serial byte stream. At this point, no more serial data can be accepted until
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the stored data has been transmitted. The transceiver is then switched to transmit and the contents of the
data buffer with a header block are sent as a single data packet. The transceiver is then switched to receive
mode.
Following reception of a valid header block for synchronous mode, the data packet received is placed into
the data buffer. When all the data is in the buffer it is output synchronously to the serial port to ensure no gaps
appear in the serial data stream.
Point to point and multidrop configurations are feasible in Synchronous Mode.
Modem Mode
In Modem Mode the transceiver is controlled with the CTS/RTS control line. To transmit, the CTS input must
be taken high. When the synthesiser has locked and the transmitter reached operating power the modem will
set RTS output high. This signals the user that the RM96xx is now available to accept serial data. Once
RTS is raised, the RM96xx will operate in accordance with Asynchronous mode with the exception that the
transceiver will stay in transmit until the user lowers CTS.
Note: The diagrams of pages 16 & 17 provide timing information on each of the above modes.
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