User's Manual

XTend™RFModule‐ProductManual
©2013DigiInternatonal,Inc. 53
The TT parameter (streaming limit) specifies the maximum number of bytes that the TX module
will send in one transmission event, which may consist of many packets and retries. If the TT
parameter is reached, the TX module will force a random delay of 1 to RN delay slots (exactly 1
delay slot if RN is zero). Each packet is counted only once toward TT, no matter how many times
the packet is retransmitted.
Subsequent packets in Acknowledged Mode are similar to those in Streaming Mode, with the
addition of an ACK between each packet, and the possibility of retransmissions. Subsequent
packets are sent without an RF initializer, as the RX modules are already synchronized to the TX
module from the preceding packet(s) and they remain synchronized for the duration of the
transmission event. Each retransmission of a packet includes an RF initializer.
Once the TX module has sent all pending data or has reached the TT limit, the acknowledged
transmission event is completed. The TX module will not transmit again for exactly RN delay slots,
if the local RN parameter is set to a non-zero value. The RX module will not transmit for a random
number of delay slots between 0 and (RN-1), if the local RN parameter is set to a non-zero value.
These delays are intended to lessen congestion following long bursts of packets from a single TX
module, during which several RX modules may have themselves become ready to transmit.
Polling Mode (Acknowledged)
NOTE: Polling Mode (Acknowledged) and Polling Mode (Basic) [p51] operate in the same way. The
difference between the two modes is in their means of achieving reliable delivery of data. In Poll-
ing Mode (Acknowledged), reliable delivery is achieved using retries and acknowledgements.
Attributes:Utilizes high percentage of available network bandwidth
Eliminates collisions
Works with reliable delivery (RR or MT parameters)
Supports binary data transfers
Base module requests packets from remote module by polling a sequential 
range of addresses
Base module is configured to specify the range of addresses being polled
Uses inter-character delay to create RF packet lengths aligned with protocol 
packet lengths up to 2048 bytes long.
Required Parameter Values (Base): MD (RF Mode) = 3, PB (Polling Begin Address), PE (Polling
End Address)
Required Parameter Values (Remote): MD (RF Mode) = 4
Related Commands: Networking (RR, PD, DT, MY, AM)
Constraints: The minimum time interval between polling cycles is configurable. However, if the
remote modules cannot all be processed within that time interval, the polling cycle is ineffective
(i.e. it will impose no additional delay). In order to ensure a pause between polling cycles, PD
must be set to a value which is large enough to accommodate the pause.
Recommended Use: Use for point-to-multipoint applications that require Reliable Delivery of
data. Use this mode when it is critical that a base module be able to discern data coming from
multiple modules.
Theory of Operation
A ‘Polling Base’ module will cycle through a sequential range of addresses. The ‘Polling Base’ will
poll each ‘Polling Remote’ module, wait for a response, then poll the next remote address in the
sequence. Each ‘Polling Remote’ will respond by sending the data from its DI (Data In) buffer
following the RB (Packetization Threshold) & RO (Packetization Timeout) parameters. When there
is no eligible data to send, the ‘Polling Remote’ will not respond. The ‘Polling Base’ will poll the next
address in the polling sequence after a short delay.