User's Manual

Serial Port Operation
Crescendo UHF Half-Duplex User Manual Page 18 of 78
4.3.2 Packetiser Timers
Many protocols delimit packets of data by silence on the communications line for a set period of time. A
common example of such a protocol is Modbus.
Due to the framed structure over the air when the radio is in packet driven mode, packetiser timers should be
used to support protocols and increase the efficiency of data transmission. Packetiser timers can be set
between 0 and 10000ms.
The RX packetiser timer is used to detect the end of each packet. The Crescendo will only begin transmitting
data once the end of the packet has been detected. The suggested RX packetiser timer values for each serial
baud rate when using the radio in packet driven mode is given in Table 7.
For protocols such as ModBus, DNP3, and TDE, the RX packetiser timer should be set greater than
maximum delay between characters in the same packet. Table 7 can also be used as a guide.
Baud
RX Timer
300
>= 35ms
600
>= 18ms
1200
>= 10ms
2400
>= 6ms
4800
>= 4ms
9600
>= 3ms
19200
>= 2ms
38400
>= 2ms
57600
>= 2ms
115200
>= 2ms
Table 7: Packetisation timers for different baud rates
If the packet size is greater than the RX serial buffer size (4096 bytes), then RX packetiser timers should not
be used as the internal buffers will overflow, and bytes will be lost. Tx packetiser timers should be used
instead, and set to the suggested value given in Table 8.
When using the data driven protocol, it is recommended to use TX packetiser timers and data timeout, rather
than the RX packetiser timers, to maintain a low end-to-end latency.
 Set the data timeout to the maximum delay between characters in the same packet. Suggested values
are given in Table 7. See section 6.2 for more information on the data timeout setting.
 Set the TX packetiser timer to the maximum delay between RF blocks.
Suggested TX packetiser timer values are given in Table 8.
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