AC4490 User Guide
Version 4.5
Laird Connectivity Solutions Support Center:
http://ews-support.lairdtech.com
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T
HEORY OF
O
PERATION
RF Architecture
The AC4490 utilizes a server-client network architecture where all clients synchronize their hopping to the
server. The server transmits a beacon during the first 20 milliseconds of every hop. The client transceivers
listen for this beacon and, upon hearing it, assert their In_Range Low and synchronize hops with the server.
Each network consists of only one server. There should never be two servers on the same RF channel number
in the same coverage area because the interference between the two servers will severely hinder RF
communications. For those applications requiring collocated servers, Laird recommends using the Sync-to-
Channel feature, further explained in
Modes of Operation
The AC4490 has three different operating modes:
If the transceiver is not communicating with another radio, it is in Receive mode actively listening for a
beacon from the server. If the client determines that the beacon is from a server operating on the same RF
channel and system ID, it responds by asserting
In_Range
Low. A transceiver enters Transmit or Command
mode when the OEM host sends data over the serial interface.
The state of the
CMD
Data pin (Pin 17) or
the data contents determine which of the two modes the transceiver enters.
Transmit Mode
All packets sent over the RF are either Addressed or Broadcast packets. Broadcast and Addressed delivery can
be controlled dynamically with the API Control feature set, which can be enabled in the EEPROM
configuration. To prohibit transceivers from receiving broadcast packets, Unicast Only can be enabled.
Addressed
Packets
When sending an addressed packet, the RF packet is sent only to the receiver specified in the
destination address. To increase the odds of successful delivery, the packet uses transmit
retries. Transparent to the OEM host, the sending radio sends the RF packet to the intended
receiver. If the receiver receives the packet error-free it returns an RF Acknowledgement in
the same 20 ms hop. If a Receive Acknowledgement is not received, the radio uses a
transmit retry to resend the packet. This continues until either an acknowledgement is
received or all transmit retries are used. The received packet is only sent to the OEM Host if
and when it is received free of errors.
Broadcast
Packets
When sending a broadcast packet, the RF packet is sent to every eligible transceiver on the
network. To increase the odds of successful delivery, it uses broadcast attempts. Transparent
to the OEM host, the sending radio sends the RF packet to the intended receiver(s).
Unlike Transmit Retries, all broadcast attempts are used, regardless of when the RF packet is
actually received. RF acknowledgments are not sent or received when a broadcast packet is
transmitted. If the packet is received on the first attempt, the receiver ignores the remaining
broadcast attempts. The received packet is only sent to the OEM host if and when it is
received free of errors.
