1766-RM001A-EN-P MicroLogix 1400 Programmable Controllers ...

Protocol Configuration 593DF1 Half-DuplexProtocolDF1 Half-Duplex protocol provides a multi-drop single master/multipleslave network. In contrast to the DF1 Full-Duplex protocol,communication takes place in one direction at a time. You can use theRS-232 port on the MicroLogix controller as both a Half-Duplexprogramming port, and a Half-Duplex peer-to-peer messaging port.MicroLogix 1400controller supports Half-Duplex modems using RTS/CTShardware handshaking.DF1 Half-Duplex supports up to 255 devices (addresses 0…254, withaddress 255 reserved for master broadcasts). Note: When configuring amessage instruction, set the target node address to -1 for broadcastmessages.Broadcast messages are handled as follows:DF1 Half-Duplex Master Driver Broadcast MessagesA broadcast write command initiated by the DF1 half-duplex master isreceived and executed by all DF1 half-duplex slaves. A broadcast writecommand received by the DF1 half-duplex master after polling a DF1half-duplex slave is received, acknowledged and re-broadcast withoutbeing executed by the DF1 half-duplex master. It is treated like any otherslave-to-slave command, except that no acknowledgement is expectedafter re-broadcast.DF1 Half-Duplex Slave Driver Broadcast MessagesWhen a broadcast write command is initiated by a DF1 half-duplex slave,it is queued up just like any other MSG command until it receives a pollfrom the DF1 half-duplex master. After transmitting the broadcast writecommand, the DF1 half-duplex slave receives an acknowledgement thatthe DF1 half-duplex master received the packet without error. When theDF1 half-duplex master re-broadcasts the broadcast write command, theinitiating DF1 half-duplex slave receives and executes the command alongwith all of the other slave nodes receiving the broadcast packet. Noacknowledgement or reply is returned.Choosing a Polling Mode for DF1 Half-Duplex MasterA master station can be configured to communicate with slave stations ineither Message-based polling mode or Standard polling mode. The prosand cons of each polling mode are described below.Publication 1766-RM001A-EN-P - October 2008