Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 35)

Publication AG-UM008C-EN-P - February 2005
Designing Communication 1-17
Designing Communication
for DF1 Radio Modem
Protocol When designing communication using DF1 Radio Modem protocol,
you must consider the capabilities of both the controllers and radio
modems. The DF1 Radio Modem protocol can only be used with
contollers that support and are configured for this protocol.
Determining When to Use DF1 Radio Modem Protocol
If your radio modem can handle full-duplex data port buffering and
radio transmission collision avoidance, you can use peer-to-peer
message initiation capability in every node (i.e., the ladder logic in
any node can trigger a MSG instruction to any other node at any
time). For messaging between nodes that are outside of radio
transmission/reception range of each other, you may use either the
Store and Forward capability of the protocol or the repeater capability
of the radios.
If your radio modem cannot handle full-duplex data port buffering
and radio transmission collision avoidance, you can still use DF1
Radio Modem protocol in a Master/Slave configuration, with message
initiation limited to a single master node. If you still require slave
node message initiation, then you must use the DF1 Half-Duplex
protocol.
The primary advantage of using DF1 Radio Modem protocol for radio
modem networks is in the transmission efficiency. Each read/write
transaction (command and reply) requires only one transmission by
the initiator (to send the command) and one transmission by the
responder (to return the reply) as illustrated in Figure 1.9. The number
of transmissions is minimized, radio power is minimized, and
throughput is maximized. In contrast, DF1 Half-Duplex protocol
requires five transmissions for the DF1 Master to complete a
read/write transaction with a DF1 Slave as illustrated in Figure 1.7.
Figure 1.10 illustrates the DF1 Radio Modem protocol.
An efficiency trade-off exists in that the DF1 Radio Modem protocol
does not provide immediate feedback (ACK) to the initiator to indicate
that the responder successfully received the communications packet
without error.
The Store and Forward capability of the DF1 Radio Modem protocol
allows messages between nodes that are outside of radio
transmission/reception range of each other to be routed through
intermediary nodes that are within range. Each of the intermediary
nodes needs a Store and Forward table. The configuration needs to
indicate, based on the source and destination addresses in the
message packet, which packets to receive (store) and then
re-broadcast (forward). Figure 1.11 illustrates the Store and Forward
capability.