178 Rockwell Automation Publication 1766-UM001O-EN-P - September 2021
Appendix E Connect to Networks via RS-232/RS-485 Interface
Using Modems with MicroLogix Programmable Controllers
The types of modems you can use with MicroLogix controllers include the
following:
• Dial-up phone modems.
A MicroLogix controller, on the receiving end of the dial-up connection,
can be configured for DF1 Full-Duplex protocol with or without
handshaking. The modem connected to the MicroLogix controller should
support auto-answer. The MicroLogix 1400 supports ASCII out
communications. Therefore, it can cause a modem to initiate or
disconnect a phone call.
• Leased-line modems.
Leased-line modems are used with dedicated phone lines that are
typically leased from the local phone company. The dedicated lines may
be in a point-to-point topology supporting Full-Duplex communications
between two modems or in a multi-drop topology supporting Half-
Duplex communications between three or more modems.
• Radio modems.
Radio modems may be implemented in a point-to-point topology
supporting either Half-Duplex or Full-Duplex communications, or in a
multi-drop topology supporting Half-Duplex communications between
three or more modems. MicroLogix 1400 also supports DF1 Radio
Modem protocol.
• 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…4000 ft (8000 ft 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.
IMPORTANT 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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