9XTend
‐
NEMA™
RS
‐
232/485
RF
Modem
–
Product
Manual
v2.x4x
[2007.01.04]
©
2007
MaxStream,
Inc.,
Confidential
&
Proprietary
‐
All
Rights
Reserved
18
Channel initialization is the process of sending an RF initializer that synchronizes receiving
modems with the transmitting modem. During channel initialization, incoming serial data accumu-
lates in the DI buffer.
RF data, which includes the payload data, follows the RF initializer. The payload includes up to the
maximum packet size (PK Command) bytes. As the TX modem nears the end of the transmission,
it inspects the DI buffer to see if more data exists to be transmitted. This could be the case if more
than PK bytes were originally pending in the DI buffer or if more bytes arrived from the UART after
the transmission began. If more data is pending, the transmitting modem assembles a subsequent
packet for transmission.
Refer to the ‘RF Communication Modes’ section to view state diagrams that illustrate channel ini-
tialization and the sequence of events that follow.
RF Packet
Figure
3
‐
05. RF
Packet
Components
* When streaming multiple RF packets, the RF Initializer is only sent in front of the first packet.
RF Initializer
An RF initializer is sent each time a new connection sequence begins. The RF initializer contains
channel information that notifies receiving modems of information such as the hopping pattern
used by the transmitting modem. The first transmission always sends an RF initializer.
An RF initializer can be of various lengths depending on the amount of time determined to be
required to prepare a receiving modem. For example, a wake-up initializer is a type of RF initializer
used to wake remote modems from Sleep Mode (Refer to the FH, LH, HT and SM Commands for
more information). The length of the wake-up initializer should be longer than the length of time
remote modems are in cyclic sleep.
Header
The header contains network addressing information that filters incoming RF data. The receiving
modem checks for matching a Hopping Channel, VID and Destination Address. Data that does not
pass through all three network filter layers is discarded.
Refer to the ‘Addressing’ section of the “RF Communication Modes” chapter for more information.
CRC (Cyclic Redundancy Check)
To verify data integrity and provide built-in error checking, a 16-bit CRC (Cyclic Redundancy
Check) is computed for the transmitted data and attached to the end of each RF packet. On the
receiving end, the receiving modem computes the CRC on all incoming RF data. Received data that
has an invalid CRC is discarded [refer to the ‘Receive Mode’ section].
