User's Manual

Radio Operation
Crescendo UHF Half-Duplex User Manual Page 22 of 78
If a payload packet or ACK is lost, resulting in the base radio not receiving the ACK, the source radio
retransmits the payload packet. This continues until the number of retries for the packet has been exhausted.
5.2.1 Data Path
Internally, the Crescendo stores a set of payload frames that are waiting to be transmitted, and a set of
payload frames that have been received but not yet processed. Combining this with the serial port interface
described in section 4 on page 16, an overall picture of the data path in the Crescendo radio can be obtained,
shown in Figure 4.
4096 byte output
buffer
4096 byte input buffer
outgoing frame buffers
incoming frame buffers
Tx Packet
Rx Packet
Figure 4: Overall data path in the Crescendo radio modem
Due to the framed structure over the air, and the data path shown above, the Crescendo cannot be regarded
as a direct wire replacement. It will induce additional latency in the communications link, as well as
potentially causing changes in the timing between bytes.
In addition, when the link is bad due to interference or low signal level, bytes can be lost when the number
of retries are exhausted.
LATENCY
The Crescendo will introduce latency into the system. This latency is caused by the following factors:
 Serialisation delays: Serialisation delay is the time taken for the incoming RS-232 bit stream to be
converted back into bytes. The serialisation delay for each serial port can be calculated in
milliseconds using Equation 1.
baud
bits
t
serial
1000
Equation 1: Latency induced by serialisation delay
Where bits is the number of bits in a byte (including start, stop and parity bits), and baud is the baud
rate of the serial port.
Thus, for 9600 baud, 8N1 the serialisation delay is around 1ms per serial port.
 Switching delay: When the Crescendo is presented with data to send over the air, it switches from
receive to transmit mode and performs synchronisation tasks to prepare for the transmission of the
data. The time taken to do this is the switching delay.