Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 211)

Publication AG-UM008C-EN-P - February 2005
Configuring MicroLogix 1000 Controllers 6-7
Line Drivers
Line drivers, also called short-haul modems, do not actually modulate
the serial data, but rather condition the electrical signals to operate
reliably over long transmission distances (up to several miles).
Allen-Bradley’s AIC+ Advanced Interface Converter is a line driver that
converts an RS-232 electrical signal into an RS-485 electrical signal,
increasing the signal transmission distance from 50 to 4000 feet. In a
point-to-point line driver topology, configure the MicroLogix
controller for DF1 full-duplex protocol (as long as the modems used
do not require DTR or RTS to be high in order to operate). In a
point-to-multipoint line driver topology, configure the MicroLogix
controllers for DF1 half-duplex slave protocol. If these line drivers
require RTS/CTS handshaking, configure the control line parameter to
Half-Duplex Modem.
Modem Control Line
Operation DF1 Full-Duplex Operation
DF1 Full-Duplex protocol (also referred to as DF1 point-to-point
protocol) is useful where RS-232 point-to-point communication is
required. This type of protocol supports simultaneous transmissions
between two devices in both directions. DF1 protocol controls
message flow, detects and signals errors, and retries if errors are
detected. Its implementation in the MicroLogix 1000 does not support
any modem control lines.
DF1 Half-Duplex Slave Operation
DF1 half-duplex slave protocol provides a multi-drop single
master/multiple slave network. In contrast to DF1 full duplex,
communication takes place in one direction at a time. You can use the
RS-232 port on the MicroLogix as both a half-duplex programming
port, as well as a half-duplex peer-to-peer messaging port.
The master device initiates all communication by polling each slave
device. The slave device may only transmit message packets when it
is polled by the master. It is the master’s responsibility to poll each
slave on a regular and sequential basis to allow slaves to send
message packets back to the master. During a polling sequence, the
master polls a slave either repeatedly until the slave indicates that it
has no more message packets to transmit or just one time per polling
sequence, depending on how the master is configured.