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ARF868 Radio Modems - Guide utilisateur / User guide version V2.2.3
3.1. Packet protocol
3.1.1 Protocol description
ARF868 modem uses a packet oriented protocol on its RF interface. The data coming from the UART interface are accumulated in an internal fifo
in the module and then encapsulated in an RF frame. The maximum amount of data that can be transferred into a single radio packet can reach
1024 Bytes.
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The maximum packet size can be set up in S218 register from 1 to 1024 bytes. Each new packet introduces some latency in the transmission
delay caused by the RF protocol overhead. The RF protocols encapsulate the data payload with the following elements:
• A preamble pattern required for receiver startup time
• A bit synchronization pattern to synchronize the receiver on the RF frame
• Other protocol field such as source address and destination address, payload length, optional CRC and internal packet type field.
The incoming fifo may accumulate up to 1024 data byte. No more data has to be set in the fifo while a 1024 bytes block of data has not been
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released by the radio transmission layer. To prevent from input fifo overrun, the hardware flow control may be activated. In this case, the RTS
signal will be set when the incoming fifo is almost full to prevent the host controller from sending new data.
3.1.2 Non-secured Packet protocol
In non-secured mode, each packet is transmitted without acknowledgment; The transmitter does not know if the package has been received.
3.1.3 Secured Packet protocol
In Secured Mode, each packet is transmitted and acknowledged by the receiver. If a packet is not recognized, the module continues with two
further attempts.
Following this sequence, the transmitter sends '>' if the package has been received or '#' if it has not received confirmation after three attempts.
Important note: addressing mode must be enabled to use secured mode.
3.1.4 Transmission integrity control
RF protocol includes a 16 bit CRC. Each data extracted from an RF packet with an invalid CRC is silently discarded by the state machine module.
The CRC ensures that all data received are valid. It can be disabled by the user whose protocols already have a control mechanism integrity or
when some bug fixes user protocols are implemented.
3.1.5 Implementation
The packetized protocol is selected through register S222 .
Non-secured protocol : S222 = 0
Secured protocol: S222 = 2
3.1.6 Transmission data path
In the Packed protocol mode, the packet size is specified by registration S218.
The modem begins transmission of a complete packet when it has received the specified number of data S218. If the number of data is less a
comprehensive package be established when the timeout (specified in the registration S217) is reached..
3.2. Continuous asynchronous protocol
3.2.1 Description of the protocol
The continuous asynchronous protocol enables data transfer from the UART interface to the radio link with the lowest possible latency. It is
associated with radio modulations enabling a wide range of air data rate and ranges to be covered. This is the mode that will allow you to use
the ARF868 modem to the maximum of its possibilities.
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In transmission, data entering on the UART is immediately transmitted into the "air".
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In reception, data from the RF frames is transmitted progressively to the UART interface.
There is no buffering of complete radio frames before or after transmission on the serial link.
However, the product has buffering enabling a buffer to be performed if the radio and UART data rates are different. The UART interface does
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