AC4868 868 MHz Transceiver
User’s Manual
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RF Packet Size – When the number of bytes in the transceiver transmit buffer equals RF Packet Size, those
bytes are sent out as a complete RF packet. It is much more efficient to send a few large packets rather than
several short packets as every packet the transceiver sends over the RF contains extra header bytes which are
not included in the RF Packet Size. However, if the RF Packet Size is set too large, the transceiver will not be
able to send any packets because the AC4868 requires the entire RF packet to be sent in the same hop period
(53 ms). The RF packet size if programmed in EEPROM automatically when Auto Config is enabled. It is
strongly recommended that Auto Config be left enabled to maximize the efficiency of the transceiver. RF
Packet Size must be set to a minimum of 6 in order to send the Enter AT command.
Flow Control
Flow control refers to the control of data flow between transceivers. It is the method used to handle data in
the transmit/receive buffer and determines how data flow between the transceivers is started and stopped.
Often, one transceiver is capable of sending data much faster than the other can receive and flow control
allows the slower device to tell the faster device when to pause and resume data transmission.
Engineer’s Tip: Can I implement a design using just Txd, Rxd and Gnd (Three-wire Interface)?
Yes. However, it is strongly recommended that your hardware monitor the CTS pin of the
radio. CTS is taken High by the radio when its interface buffer is getting full. Your hardware
should stop sending at this point to avoid a buffer overrun (and subsequent loss of data).
You can perform a successful design without monitoring CTS. However, you need to take
into account the amount of latency the radio adds to the system, any additional latency
caused by Transmit Retries or Broadcast Attempts, how often you send data, non-delivery
network timeouts and interface data rate. Polled type networks, where the Server host
requests data from the Client host and the Client host responds, are good candidates for
avoiding the use of CTS. This is because no one transceiver can monopolize the RF link.
Asynchronous type networks, where any radio can send to another radio at any point in
time, are much more difficult to implement without the use of CTS.
Half Duplex / Full Duplex
When Half Duplex communication is chosen, the AC4868-250 will send a packet out over the RF whenever it
can. This can cause packets sent by multiple transceivers at the same time to collide with each other over the
RF. To prevent this, Full Duplex communication can be chosen. Full Duplex shares the bandwidth intelligently
to enable two-way collision-free communication without any collision. This is done by calculating the amount
of time until the next hop to ensure that it has time to send the packet; if there is enough time, it will send the
packet and if not, it will wait until its next appropriate hop. The Server transmits during the even hops while
the Client(s) will transmit during the odd hops. Although there is technically only one frequency bin, the Server
still maintains a bin count for the purpose of handling Full Duplex mode. While the RF hardware is still
technically half duplex, the bandwidth sharing it makes the transceiver seem full duplex. Enabling Full Duplex
can cause overall throughputs to be cut in half.
