7 DIGITAL MODES OF
OPERATION
7.1 DIGITAL MODES OF OPERATION
This section serves as a guide to the various ways the URT Series can transfer digital information via
its serial port in point to point links, point to multi-point (scanning telemetry) systems and networks
employing store and forward repeater nodes.
Due to the exceptionally large flash memory space available within the URT500, we are able to support
various PC selectable modes of operation to suit many different applications.
At the time of writing this manual, Transparent mode, MODBUS and RFT Routing Modes are
supported, with DNP3, IEC870 and MX25 modes under development. The basic modes of operation
of the radio modem are outlined below.
7.2 SERIAL INTERFACE & HANDSHAKING
The serial interface can be programmed either to use RTS/CTS handshaking to initiate transmission, or
to transmit whenever data is present at the serial input. In the latter mode CTS is still operated to
implement flow control but can be ignored unless message sizes exceed 1k byte and the serial port baud
rate is higher than the radio signal baud rate. These handshaking modes are compatible with the old
Communique CMD400 modes A, C and D. Mode B (byte stuffing mode) is not supported.
7.2.1
TRANSMISSION USING RTS/CTS HANDSHAKING
If handshaking is enabled transmission is started by operating RTS, CTS can then be monitored for
flow control purposes. In the idle state CTS is inactive, when RTS is operated CTS will become active
immediately and data may be input to the serial port, when all data has been loaded to the serial port
RTS should be dropped, transmission will continue until all data in the serial input buffer has been sent,
then CTS will become inactive and transmission will cease. During transmission the amount of data in
the serial buffer is checked by the radio, if the buffer becomes ¾ full CTS is dropped to request the host
to stop loading data, CTS is activated again when the buffer is reduced to ¼ full. To prevent timing
problems data will still be accepted into the buffer when CTS is de-activated due to buffer filling during
transmit, however any data received once CTS has dropped at the end of a transmission will be
discarded, this prevents such data from being prefixed to the beginning of the next message.
7.2.2
TRANSMISSION WITHOUT HARDWARE HANDSHAKE
If RTS/CTS handshaking is disabled the radio will start transmission as soon as data is received at the
serial port, transmission ceases as soon as the serial buffer has been emptied and a period equivalent to
two characters at the radio signal baud rate has elapsed. It is important to note that since transmission
ceases as soon as a two character delay in the incoming data stream is seen, data characters in a
message must be presented in a continuous back to back stream.
In this mode CTS is still used to indicate the serial buffer fill level in the same way as described in the
section on transmission using handshake, the difference is that in the idle state CTS is always active
indicating readiness to accept data. In most applications CTS can be ignored as messages are likely to
be smaller than the serial input buffer (1k byte), bear in mind also that if the radio baud rate and data
format is the same as that configured for the serial port the buffer is being emptied as fast as it is being
filled and so buffer overrun is unlikely.
7.2.3
DATA RECEPTION
Any data received by the radio is simply output to the serial port, the DCD line can be programmed to
operate in three different modes to assist the host. Firstly by indicating that a carrier is detected on the
radio channel, this is useful if a busy lockout function is required (although this can be dangerous if the
channel is susceptible to interference as well as wanted signals), secondly DCD can indicate presence of
a carrier and a valid data signal, data will normally be output under this circumstance, the third mode
behaves in the same way as the second except that DCD remains active until all data has been output to
the serial port after the signal has gone, this allows DCD to be used as a wake up signal.
URT500 Manual
Page 23 of 42
Rev. C – 6 August 2008
