Rockwell Automation 1761-HHP-B30 MicroLogix 1000 with Hand-Held Programmer (HHP) - Manual (Page 434)

Appendix D
Understanding the Communication Protocols
D–8
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 point-to-multipoint topology supporting half-duplex
communications between three or more modems. In the point-to-point
topology, configure the MicroLogix 1000 controllers 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 the point-to-multipoint topology, configure the
MicroLogix 1000 controllers for DF1 half-duplex slave protocol with the
control line parameter set to “Half-Duplex Modem”.
Radio Modems
Radio modems may be implemented in a point-to-point topology supporting
either half-duplex or full-duplex communications, or in a point-to-multipoint
topology supporting half-duplex communications between three or more
modems. In the point-to-point topology using full-duplex radio modems,
configure the MicroLogix 1000 controllers 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 the point-to-point topology using half-duplex radio modems, or
point-to-multipoint topology using half-duplex radio modems, configure the
MicroLogix 1000 controllers for DF1 half-duplex slave protocol. If these
radio modems require RTS/CTS handshaking, configure the control line
parameter to “Half-Duplex Modem”.
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 1000 controller for DF1 full-duplex
protocol (as long as the line drivers do not require DTR or RTS to be high in
order to operate). In a point-to-multipoint line driver topology, configure the
MicroLgoix 1000 controllers for DF1 half-duplex slave protocol. If these
line drivers require RTS/CTS handshaking, configure the control line
parameter to “Half-Duplex Modem”.