Rockwell-automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command Uživatelský manuál stáhnout pdf (Strana 32)

2–3Understanding DF1 Protocol

Publication

17706.5.16 - 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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Rockwell-automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command Uživatelský manuál Strana 32