7.3.3
TRANSMIT TO RECEIVE SWITCHING TIME
In full or semi-duplex operation transmit to receive switching time does not need to be considered as
the receive path is maintained during a transmission, in simplex operation some time must be allowed to
reload the transmitter synthesiser to stop it from interfering with the receiver. The diagram below
indicates the minimum time in which the radio is able to receive a signal after completing a
transmission.
symbol
Description
value
thold
Period for which carrier is held up after sending last data byte
2.5ms + LOD
trxrdy
Time to reload transmit synthesiser in simplex mode
6ms
During the time thold the radio transmits some padding bits to allow for propagation delays in the
receiving device before shutting off the carrier, this prevents possible chopping of the message tail. The
time thold is composed of a fixed 2.5ms period plus the programmable value LOD (lead out delay).
LOD is normally set to zero. After the time trxrdy has expired the radio is ready to receive a new
signal.
N.B. If RTS/CTS handshaking is not used the transmitter is turned on whenever data is received at the
serial port, the transmitter is left on until all buffered data has been transmitted and no data has been
input for a time equivalent to the length of two characters at the radio baud rate (refer to table C). In
general data transmitted by the radio is delayed with respect to its receipt at the serial port by the
receive to transmit switching time, if the
radio baud rate and serial port baud rate and both data formats are the same this delay remains
constant throughout the transmission. At the higher baud rates this delay is generally greater than the
length of two characters and so the procedure to stop transmission is started as soon as the last
character has been sent, at the lower baud rates however it is possible that the time thold is extended
while the radio waits for the two character timeout to expire, this can also happen if data characters
are not loaded back to back into the serial port.
URT500 Manual
Page 26 of 42
Rev. C – 6 August 2008
