Rockwell Automation Publication 1766-UM001I-EN-P - June 2015 211
Connecting to Networks via RS-232/RS-485 Interface Appendix E
• 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). Line drivers are
available in Full-Duplex and Half-Duplex models. Allen-Bradley’s AIC+
Advanced Interface Converter is a Half-Duplex 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 (8000 feet when
bridged).
For point-to-point Full-Duplex modem connections that do not require any
modem handshaking signals to operate, use DF1 Full-Duplex protocol with no
handshaking. For point-to-point Full-Duplex modem connections that require
RTS/CTS handshaking, use DF1 Full-Duplex protocol with handshaking.
For radio modem connections, use DF1 Radio Modem protocol, especially if
store and forward capability is required.
For general multi-drop modem connections, or for point-to-point modem
connections that require RTS/CTS handshaking, use DF1 Half-Duplex slave
protocol. In this case, one (and only one) of the other devices must be configured
for DF1 Half-Duplex master protocol.
DH-485 Communication
Protocol
The DH-485 protocol defines the communication between multiple devices that
coexist on a single pair of wires. DH-485 protocol uses RS-485 Half-Duplex as its
physical interface. (RS-485 is a definition of electrical characteristics; it is not a
protocol.) RS-485 uses devices that are capable of co-existing on a common data
circuit, thus allowing data to be easily shared between devices.
The DH-485 network offers:
• interconnection of 32 devices
• multi-master (peer-to-peer) capability
• token passing access control
• the ability to add or remove nodes without disrupting the network
Never attempt to use DH-485 protocol through modems under any
circumstance.
All MicroLogix controllers support RTS/CTS modem handshaking
when configured for DF1 Full-Duplex protocol with the control line
parameter set to Full-Duplex Modem Handshaking or DF1 Half-Duplex
slave protocol with the control line parameter set to “Half-Duplex
Modem”.
MicroLogix 1400 controllers also support DCD (Data Carrier Detect)
line for DF1 Radio Modem protocol. For other protocols, you can only
access the DCD signal from your ladder logic. No other modem
handshaking lines (such as Data Set Ready and Data Terminal Ready)
are supported by MicroLogix 1400 controller.
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