WSN Manual
Page 20 of 34
Rev. G
– 4 March 2007
5.15
TRANSMIT & RECEIVE TIMING
The WSN only operates in a simplex or semi-duplex mode. In simplex mode the receive and
transmit frequencies are the same, whereas in the semi-duplex mode they are different.
In either mode data is only sent in one direction at a time as the radios do not have separate
synthesisers for transmit and receive. If full duplex mode is required (transmit & receive at the
same time) the ART product should be considered.
In simplex/semi-duplex mode, the radio synthesiser must be reloaded each time Receive or
Transmit is selected. Although relatively short, the synthesiser loading time must be taken into
account when looking at data transfer times.
In order to reduce adjacent channel interference in line with ETS300-113, the power output from
the transmitter has finite rise and fall times, a distant receiving radio will therefore see an
incoming signal later than a nearby one. The receiving radio also requires time for the carrier
detect circuit to operate and for the modem to lock on to the incoming audio signal.
When using the WSN, there are a few timing considerations to be taken into account. The main
one is the programmable “lead in delay”, which is required for the modem to lock on to the
incoming data stream and is dependant on the radio signal baud rate. Minimum timings are
given below:
Baud Rate
Lead in Delay (Minimum)
150
80ms
300
60ms
600
40ms
1200
40ms
2400
40ms
4800
20ms
9600
30ms
For simplex/semi-duplex operation, time is required for the transmit and receiver synthesiser to
be loaded and locked prior to transmission/reception. This timing constraint is important when
deciding how soon after receiving a message a reply may be sent. For simplex/semi-duplex
operation the WSN is ready to receive data approximately 25ms after transmission ceases. It is
therefore necessary to either wait this length of time after receiving a message before sending a
reply or to extend the lead in delay by the same amount to hold off transmission of the data.
For applications where power save is in use the lead in delay should be extended to allow the
receiving device to wake up. The time required can be calculated by adding the save on time to
the save off time and adding 10 percent, e.g. for a save on time setting of 800ms and a save off
time of 200ms the lead in delay should be 1100ms.
Care must be taken when replying to a previously transmitting WSN when RTS/CTS handshake
is not being used, in this mode the transmitting device will wait for two character times before
turning off its carrier and may therefore miss the beginning of a reply if it comes too soon, this
may be overcome either by imposing an additional two character delay in the controlling device
or by extending the lead in delay by that amount.
The WSN also has a facility for imposing a lead out delay, which is the time that the carrier
remains on after transmission of the message is complete. This delay can normally be left at
zero as it is only of use where a controller makes use of the DCD signal to suppress data
processing but suffers some delay in processing received data, or where there is a need to
delay any spurious squelch tail characters generated sufficiently that connected equipment
does not confuse them with part of the message.
