Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 32)

2–3 Understanding DF1 Protocol
Publication 1770Ć6.5.16 - October 1996
Using half-duplex protocol
When you use half-duplex protocol, the intended environment is a
multidrop link with all nodes interfaced through half-duplex
modems. Unless there is only one slave directly connected to a
master, you must use a modem. Your modems must support these
signals:
•
request-to-send (RTS)
•
clear-to-send (CTS)
•
data-carrier-detect (DCD)
•
data-set-ready (DSR)
•
data-terminal-ready (DTR)
If your modem does not support the DCD and DSR signals, you
must jumper DCD and DSR to DTR.
You designate one node as master to control which node has access
to the link. All other nodes are slaves, and must wait for permission
from the master before transmitting. Each slave node has a unique
node number between 0 and 254 (decimal).
The master can send and receive messages to and from each node on
the multidrop link and to and from every node on network links
connected to the multidrop link.
If the master is programmed to relay messages, then nodes on the
multidrop link can engage in virtual slave-to-slave communication.
This communication is transparent to the application.
Multiple masters are not allowed, except when one acts as a
backup to the other and does not communicate unless the primary is
shut down.
Slave-to-slave communication
In slave-to-slave communication, the master looks at the packet
received from the slave. If the packet is not for the master,
the master reassembles the packet as a master packet and sends the
packet to slave devices.
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